DESCRIPTION

This is the epic story of the universe and our place in it, from 13.8 billion years ago to the remote future. How did we get from the Big Bang to today’s staggering complexity, in which seven billion humans are connected into networks powerful enough to transform the planet? David Christian gives the answers in a mind-expanding cosmological detective story told on the grandest possible scale. He traces how, during eight key thresholds, the right conditions have allowed new forms of complexity to arise, from stars to galaxies, Earth to homo sapiens, agriculture to fossil fuels. This global origin story is one that we could only begin to tell recently, thanks to the underlying unity of modern knowledge. Panoramic in scope and thrillingly told, Origin Story reveals what we learn about human existence when we consider it from a universal scale.

TIMELINE (file not included in this release)

Review

Handwrite First

Use this section with the book and notes closed. The goal is retrieval, not polish.

1. Before Reading

Prime your mind before reading.

• Prediction:
• Question I want answered:
• What I already think about this topic:
• What I expect to disagree with:

2. Closed-Book Recall Dump

Close the book and notes. Reconstruct what you read from memory.

• Main points I remember:

• Key examples, evidence, stories, or cases I remember:

• Terms, people, dates, or concepts I remember:

• What feels fuzzy or incomplete:

3. Core Argument From Memory

• The book/section in one sentence:
• Main claim:
• Problem the author is responding to:
• Why the author thinks this is true:
• Best evidence/example I remember:
• Conclusion the author wants the reader to accept:

4. Argument Map From Memory

• Claim:
• Because:

• Evidence:

• Hidden assumptions:
• Therefore:

5. Feynman Check

Explain the idea simply, as if teaching it to someone unfamiliar with the topic.

• Simple explanation:
• Analogy or concrete example:
• Where someone might misunderstand this:

6. Pushback From Memory

• Strongest objection or weakness:
• What evidence would weaken the author’s argument?
• What a rival thinker/book might say:
• My current judgment:

7. Personal Takeaway

• What changed, clarified, or challenged my thinking?
• What am I less certain about now?
• One way I could apply this:
• One question I need to keep investigating:

Type Into Obsidian

Use this section after handwriting. The goal is long-term storage, search, linking, and spaced review.

1. Correction After Checking Notes

Reopen the book/notes. Compare them against your handwritten recall.

• I forgot:
• I misunderstood:
• I overemphasized:
• I underemphasized:
• This needs review:

2. Clean Core Argument

• The book/section in one sentence:
• Main claim:
• Problem the author is responding to:
• Why the author thinks this is true:
• Best evidence/example:
• Conclusion the author wants the reader to accept:

3. What Actually Matters?

• Most important idea:
• Best quote or passage:
• Term/concept worth remembering:
• Most useful example:
• Most surprising point:
• One sentence I want to remember:

4. Connections

• Connects to:
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• Supports:
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• Challenges:
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• Reminds me of:
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• Possible future note:
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5. Clean Pushback

• Strongest objection or weakness:
• What evidence would weaken the author’s argument?
• What the author might say in response:
• What a rival thinker/book would say:
• My current judgment:

Retrieval Questions

Answer these without looking during future reviews.

Basic Recall

• What is the author’s main thesis?
• What problem is the author responding to?
• What are the 3–5 main claims?
• What are the key terms or concepts?
• What is the best example or piece of evidence?

Argument Reconstruction

• How does the author move from evidence to conclusion?
• What assumptions does the argument depend on?
• What would the argument look like in premise-conclusion form?
• What does the author want the reader to reject?

Comparison

• How does this book/section compare with [[]]?
• How does this support or challenge [[]]?
• Which thinker would agree with this?
• Which thinker would disagree with this?

Criticism

• What is the strongest objection?
• What evidence would weaken the argument?
• What does the author ignore, minimize, or assume?
• Is the author’s conclusion stronger than the evidence allows?

Synthesis

• How does this change my larger framework?
• Where does this fit in the broader historical/philosophical debate?
• What question should I ask next?
• What would I write about using this idea?

Flashcards / Atomic Recall

Use only for facts, definitions, distinctions, names, dates, and compact claims.

• Q:
  • A:
• Q:
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  • A:

Retention Check

24 Hours Later

• I can explain the main argument without notes.
• I remember the best example or evidence.
• I can define the key concept.
• I can state one objection.

24-hour recall notes:

• What I remembered:
• What I forgot:
• What needs another review:

1 Week Later

• I can summarize this in 5 sentences.
• I can reconstruct the argument.
• I can connect it to another book, thinker, or historical period.
• I can state whether I agree, disagree, or remain undecided.

1-week recall notes:

• What I remembered:
• What I forgot:
• New connection:
• Revised judgment:

1 Month Later

• I still remember the core argument.
• I can explain why this matters.
• I know whether this changed my larger framework.
• I know whether this is worth rereading, citing, or building into a permanent note.

1-month recall notes:

• What endured:
• What disappeared:
• What this changed:
• Next action:

Scratchpad

Unfiltered thoughts, questions, comparisons, argument trees, future writing ideas, or possible links.

Simplified rule

Handwrite

Use handwriting for:

- Before-reading predictions
- Closed-book recall dump
- Core argument from memory
- Argument map from memory
- Feynman explanation
- Initial pushback
- Personal takeaway
- Confusion list

These are the sections where the benefit comes from struggling to reconstruct the idea.

Type in Obsidian

Use Obsidian for:

- Corrections after checking notes
- Clean one-sentence thesis
- Clean argument summary
- Important quotes
- Key concepts
- Links to other notes
- Retrieval questions
- Flashcards
- Retention checks
- Review logs
- Scratchpad for future essays or synthesis

These are the sections where the benefit comes from searchability, linking, spaced review, and long-term organization.

Best workflow

Use this order:

1. Handwrite from memory.
2. Check the book/notes.
3. Type only the corrected, useful version into Obsidian.
4. Create retrieval questions.
5. Review those questions later without looking.

Do not type all your handwritten notes into Obsidian. That turns the system into transcription work. Only transfer what is worth preserving, linking, or reviewing.

Notes

Book Overview

Origin Story reconstructs the history of the universe as a single, connected process extending from the big bang to the possible futures of humanity. David Christian’s central claim is not simply that cosmic, biological, and human history can be placed on one timeline. It is that they can be understood through a common analytical framework: increasing complexity emerges at rare thresholds when existing materials are rearranged under suitable Goldilocks conditions, producing new structures with emergent properties.

The story begins with an extremely simple universe dominated by undifferentiated energy. As the universe expanded and cooled, its operating rules allowed energy and matter to form stable structures. Gravity assembled matter into stars and galaxies. Stars generated the chemical elements from which planets were built. On at least one chemically rich planet, Earth, complex molecular reactions generated living cells. Life introduced a new kind of complexity because organisms actively obtain energy, process information, preserve themselves, reproduce, and evolve. Over billions of years, life transformed Earth into an interconnected biosphere.

Humans are part of this biological history, but Christian argues that our species crossed another major threshold through collective learning. Language allows humans to share information with unusual precision, preserve it beyond individual lifetimes, and accumulate it across generations. Genetic evolution therefore acquired a faster cultural counterpart. Collective learning enabled humans to colonize almost every terrestrial environment, develop increasingly powerful technologies, and capture growing shares of the biosphere’s energy.

Agriculture marked a further threshold because it allowed communities to redirect biological productivity toward selected plants and animals. The resulting energy surpluses supported larger populations, cities, specialists, states, armies, religions, writing systems, and long-distance exchange networks. Yet most of the agricultural energy bonanza was consumed by population growth, social infrastructure, ruling elites, and what Christian calls the complexity taxes required to maintain large societies. For most ordinary people, farming did not initially produce dramatically better lives.

Beginning around 1500, previously separated human world zones became linked into a single exchange system. Global trade, imperial expansion, commercial competition, capitalism, and modern science intensified collective learning. The decisive transformation came when humans began exploiting fossil fuels: immense stores of ancient sunlight accumulated over hundreds of millions of years. Fossil fuels broke through the energy constraints of agrarian societies and generated the industrial revolution, modern transportation, mass production, electrical systems, computing, modern states, unprecedented population growth, and large improvements in health and material welfare.

These developments carried humanity across threshold 8 and into the Anthropocene, an era in which one species has become a dominant planetary force. Christian distinguishes a Good Anthropocene—longer lives, reduced child mortality, greater consumption, mass education, and expanding scientific knowledge—from a Bad Anthropocene characterized by severe inequality, ecological destruction, climate change, mass-extinction pressures, and weapons capable of destroying civilization.

The book therefore changes from a descriptive history into a guardedly normative argument. Humans have acquired enough power to manage much of the biosphere without yet acquiring institutions, incentives, or knowledge adequate to that responsibility. Our immediate task is to preserve the benefits of the Good Anthropocene, distribute them more equitably, and keep the biosphere within habitable limits. Success might constitute a ninth threshold: the emergence of a sustainable planetary civilization capable of consciously regulating its relationship with the biosphere.

The book’s larger significance lies in its attempt to provide a scientifically grounded origin story for a global age. Christian believes origin stories orient individuals, connect fields of knowledge, and situate communities within a larger whole. A shared modern origin story cannot supply meaning from outside humanity, but it can show that all humans possess a common history, inhabit one interdependent biosphere, and face a common future. Origin Story - David Christian.pdfPDF

Central Argument in Brief

1. Humans need origin stories to orient themselves. Traditional origin stories link knowledge, identity, value, and community, but modern globalization has weakened their authority and exposed conflicts among local traditions.
2. Modern science makes a global origin story possible. Discoveries across cosmology, geology, chemistry, biology, anthropology, and history can now be assembled into one provisional but evidence-based map of the past.
3. The universe began extremely simple. Its default condition remains cold, empty, and relatively structureless, while entropy tends to dissolve the organized forms that emerge within it.
4. Complexity appears only under rare Goldilocks conditions. Suitable materials, controlled flows of free energy, and stabilizing rules occasionally allow existing components to form new arrangements with emergent properties.
5. Cosmic thresholds created the prerequisites for life. The big bang produced matter and physical forces; gravity formed stars; stars manufactured new elements; chemically enriched matter formed planets and moons.
6. Life introduced adaptive, information-processing complexity. Living organisms obtain energy, maintain homeostasis, reproduce, and evolve through natural selection, gradually transforming Earth into a biosphere.
7. Human language created collective learning. Humans can accumulate information across communities and generations, allowing cultural change to proceed much faster than genetic evolution.
8. Agriculture intensified human control over energy. Farming redirected biological productivity toward human purposes, supporting population growth, specialization, hierarchy, states, and agrarian civilizations.
9. Global interconnection accelerated collective learning. From approximately 1500, exchange among previously separated world zones intensified commerce, science, imperialism, capitalism, and technological innovation.
10. Fossil fuels shattered agrarian energy limits. Access to ancient stores of solar energy produced industrialization, the Great Acceleration, modern prosperity, and unprecedented human power.
11. Human power has become planetary in scale. The Anthropocene contains both dramatic improvements in human welfare and destabilizing effects on climate, biodiversity, and biogeochemical cycles.
12. The next threshold depends on conscious collective action. Humanity must build a more equitable and sustainable civilization or risk destroying the complex systems upon which modern life depends.

Book Structure at a Glance

Preface and Introduction

The opening sections explain why Christian believes a modern origin story is necessary. The Preface presents the moral and intellectual motivation for big history: national histories can reinforce tribal divisions, whereas a history of humanity must situate all humans within a common cosmic and biological inheritance. The Introduction establishes the analytical vocabulary—complexity, thresholds, Goldilocks conditions, emergence, energy, information, and entropy—that organizes the rest of the book.

Part I: Cosmos

The first part establishes the physical prerequisites of all later history. Chapter 1 explains the origin and early structure of the universe. Chapter 2 shows how gravity formed stars and how dying stars created the periodic table. Chapter 3 explains how chemically enriched matter formed planets, including an unusually dynamic Earth. The part moves from simple energy to environments capable of sustaining elaborate chemistry.

Part II: Biosphere

The second part introduces life as a qualitatively different form of complexity. Chapter 4 examines the origin and basic machinery of living cells. Chapter 5 follows the long age of microorganisms and the reciprocal transformation of life and geology. Chapter 6 explains the rise of multicellular organisms, larger bodies, nervous systems, brains, and mammals. This part establishes the evolutionary inheritance from which humanity emerged.

Part III: Us

The third and longest part traces the successive accelerations of human history. Chapter 7 identifies collective learning as the defining human adaptation. Chapter 8 explains the transition to farming. Chapter 9 shows how agricultural surpluses produced cities, states, hierarchies, and agrarian civilizations. Chapter 10 follows the global linking of human societies and the fossil-fuel revolution. Chapter 11 analyzes the Anthropocene, in which human activity becomes a geological and biospheric force.

Part IV: The Future

Chapter 12 turns the historical argument toward possible futures. It distinguishes relatively predictable cosmological futures from highly contingent human ones and frames the coming century as a collective quest. The question is whether humans can use collective learning to create a sustainable threshold 9 or whether their complexity will collapse under ecological, political, and energetic pressures.

Preface

Purpose and Argument

Christian presents big history as a response to the fragmentation of both knowledge and human identity. His earlier work in Russian and Soviet history made him uneasy with the implicit message that humanity is fundamentally divided into rival nations and civilizations. Memories of the Cuban missile crisis reinforced the danger of teaching histories that magnify tribal differences in a world possessing nuclear weapons.

A genuinely unified human history cannot begin with states, civilizations, or written records. It must include Paleolithic ancestors, early farmers, and the biological and cosmic processes that made humanity possible. Human history therefore expands logically into the history of life, Earth, the solar system, stars, and the universe.

Christian associates this project with William McNeill’s call for histories of humanity as a whole and H. G. Wells’s attempt to develop common historical ideas after World War I. Big history is both an intellectual synthesis and a civic project: it seeks a narrative capacious enough to connect different peoples and disciplines without erasing cultural diversity.

Essential Reasoning and Evidence

• Traditional national and imperial histories can strengthen solidarity within groups while deepening divisions between them.
• All origin stories unify knowledge by showing how individuals and communities fit within a larger order.
• Globalization has weakened confidence in many traditional origin stories by placing incompatible narratives in sustained contact.
• The result can be anomie: a loss of direction, meaning, and confidence in one’s place in the world.
• Modern science offers the materials for a new origin story constructed collectively by researchers from many cultures and disciplines.
• This modern story remains incomplete and may need to recover older traditions’ insights into sustainable living.
• Its principal advantage is that it describes a shared inheritance that belongs to humanity as a whole rather than to one nation or civilization.

The Preface does not claim that scientific history can simply replace religion or supply an objective cosmic purpose. Its more limited claim is that a shared account of the past can provide intellectual orientation and encourage recognition of humanity’s common interests.

Introduction

Purpose and Argument

The Introduction establishes the book as a deliberately compressed map rather than an exhaustive description. Like a transit map, it simplifies reality so that readers can perceive connections otherwise obscured by specialization. Its central question is how a universe that began in an extremely simple state generated stars, planets, life, humans, and modern global society.

Christian begins with humanity’s capacity to create mental maps. Other organisms also model their surroundings, but human language allows individual maps to be shared, tested, combined, and transmitted. This process of collective learning has gradually enabled one part of the universe to examine the rest of it.

The Introduction also explains why origin stories matter. Christian imagines ancestral communities around Lake Mungo using shared stories to connect stars, landscapes, animals, ancestors, obligations, and identity. Such stories did not merely explain physical origins; they taught people what kind of world they inhabited and how they should live within it.

Modern education possesses vastly more specialized knowledge but often lacks an integrating framework. Christian offers big history as a global, scientific, and still-developing origin story that can reconnect knowledge without claiming finality.

A Modern Origin Story

The modern origin story differs from many traditional stories in several respects:

• It was constructed through global scientific collaboration rather than inherited from a single community.
• It does not require a creator deity.
• It treats the universe as dynamic, evolving, and much larger than traditional cosmologies generally imagined.
• It does not locate meaning in the structure of the universe itself; meaning is generated by living and especially human agents.
• Its claims are provisional and subject to revision as evidence changes.
• It reflects the circumstances of modern capitalist and scientific societies and therefore has characteristic blind spots, including an insufficient sensitivity to ecological limits.

These limitations do not make the story dispensable. They make it an unfinished collective project.

Increasing Complexity

The book’s organizing problem is the emergence of complexity within a universe dominated by simplicity and entropy. Complexity means more components, more relationships among them, and more intricate internal organization. It is not synonymous with moral value, superiority, or progress.

Complex structures arise when particular environments provide Goldilocks conditions: combinations of temperature, pressure, chemical materials, energy flows, and stability suitable for new arrangements. At major turning points, these arrangements acquire emergent properties that could not be found in their components separately.

Christian calls the most important turning points thresholds. The threshold model gives narrative form to an otherwise overwhelming history. Each threshold:

1. depends on structures created earlier;
2. requires specific Goldilocks conditions;
3. produces something with novel properties;
4. creates new possibilities for later complexity;
5. remains vulnerable to entropy and breakdown.

The apparent contradiction between increasing complexity and entropy is resolved by energy. Complex entities survive only by using controlled flows of free energy and exporting disorder into their surroundings. Stars, organisms, cities, and modern societies all maintain internal organization by consuming energy and producing waste heat or other forms of disorder.

Transition to Part I

The Introduction gives the reader the book’s basic conceptual map. The story can now begin with threshold 1: the appearance of a universe containing the energy, matter, and operating rules from which all later complexity would emerge.

Part I: Cosmos

Function Within the Book

Part I establishes that human history rests on a long sequence of physical preconditions. Before life or collective learning could exist, there had to be matter, stable atoms, stars, chemical elements, planets, and environments with controlled energy flows.

The part also develops the recurring pattern used throughout the book: existing structures enter new conditions; energy reorganizes them; emergent properties appear; and the resulting complexity survives only by paying energetic costs. The same framework later explains cells, organisms, societies, and the Anthropocene.

Chapter 1: In the Beginning: Threshold 1

Chapter Objective and Contribution

Chapter 1 asks how the modern origin story can begin when science has no verified account of what preceded the universe. Christian distinguishes the well-supported history of the universe after its first moments from the unresolved metaphysical problem of ultimate origins.

The chapter’s main contribution is to explain threshold 1: the appearance of a universe with energy, matter, physical forces, stable particles, atoms, and operating rules. It also introduces the tension that structures the entire book. The second law of thermodynamics favors disorder, yet patterns and organized entities repeatedly appear. Christian explains that physical laws eliminate many possible states while chance selects among the possibilities that remain. Structure emerges through a collaboration between necessity and contingency.

The chapter concludes by showing why big-bang cosmology is not merely speculative. Expansion, redshift, the age of astronomical objects, and especially cosmic microwave background radiation provide converging evidence that the observable universe originated approximately 13.8 billion years ago.

Argument Progression

1. The problem of absolute beginnings remains logically and scientifically unresolved.
2. Big-bang theory nevertheless provides a well-supported account beginning immediately after the universe appeared.
3. Expansion and cooling produced phase changes that differentiated energy, matter, forces, and particles.
4. Physical laws constrained possibilities, while chance generated particular arrangements.
5. Stable particles and atoms emerged despite the general tendency toward disorder.
6. Astronomical observations eventually confirmed the central predictions of an expanding-universe model.
7. Threshold 1 created the basic materials and rules required for all later complexity.

Section-by-Section Argument

Jump-Starting an Origin Story

Christian reviews several ways origin stories confront the problem of beginnings. Some posit an eternal universe; others introduce a creator or first cause. Both approaches displace rather than necessarily solve the problem. An eternal universe removes the need for a beginning, while a creator raises the further question of the creator’s origin and complexity.

Modern cosmology also lacks a conclusive answer. Multiverse models, cyclic universes, and geometries in which time lacks a conventional starting edge remain possibilities, but no verified evidence reveals what existed before the observable big bang. Christian therefore preserves uncertainty: science can reconstruct very early cosmic history without explaining why there is a universe at all.

Threshold 1: Quantum Bootstrapping a Universe

The widely accepted account begins with the universe compressed into an extraordinarily small, hot, energetic state. Rapid expansion dispersed its energy and lowered its temperature. As conditions changed, new structures became possible.

The key principle is emergence. Novel properties do not arrive independently of prior reality; they appear when existing components are arranged differently. Hydrogen and oxygen atoms, for example, acquire the new collective property of water when bonded in a particular configuration. Cosmic history repeatedly follows the same pattern.

The earliest phase changes separated energy into gravity, electromagnetism, and the strong and weak nuclear forces. Matter also emerged as compressed energy. Matter and antimatter largely annihilated each other, but a minute surplus of matter survived. That imbalance supplied the material from which stars, planets, organisms, and humans were eventually constructed.

The First Structures

The existence of any organized structures requires explanation because entropy makes disorder statistically more probable than order. Christian personifies entropy as the persistent force that eventually dissolves every arrangement.

Complexity does not defeat entropy. It survives temporarily by using energy and exporting disorder. This requirement becomes the book’s recurring complexity tax. Maintaining an organized structure produces waste, heat, or other forms of increased disorder elsewhere.

Physical laws created a minimum of order by ruling out impossible states. Chance operated within those constraints. The first particles and forces therefore emerged from an interaction between:

• Necessity: the filtering effects of physical laws.
• Chance: the particular arrangements realized among permitted possibilities.

This pattern later reappears in natural selection, where environmental rules filter random biological variations.

The First Atoms

Within the first minutes, protons and neutrons combined into the nuclei of hydrogen and helium. Expansion cooled the universe too quickly for many heavier nuclei to form, explaining why hydrogen and helium still dominate ordinary matter.

Approximately 380,000 years later, temperatures fell sufficiently for electrons to attach to atomic nuclei. Electrically neutral atoms formed, photons could travel freely, and the universe became transparent. The released photons remain detectable as cosmic microwave background radiation.

Threshold 1 had now produced:

• stable matter;
• several fundamental forces;
• operating laws;
• hydrogen and helium atoms;
• an expanding environment in which gravity could create larger structures.

What’s the Evidence?

The first major clue was the observation that distant galaxies are generally redshifted and that more distant galaxies recede faster. Edwin Hubble’s work, building on techniques developed by Henrietta Leavitt and others, implied that the universe was expanding. Reversing that expansion pointed toward an earlier compressed state.

The strongest confirming evidence was the cosmic microwave background. George Gamow and Ralph Alpher predicted that a hot early universe should have left a faint field of cooled radiation. Arno Penzias and Robert Wilson detected that radiation accidentally in 1964. Its existence and uniformity could be explained by big-bang cosmology more successfully than by the rival steady-state model.

The evidence does not explain the ultimate cause of the big bang, but it firmly establishes the broad history of an expanding and cooling universe.

Chapter Conclusion and Transition

Chapter 1 establishes the materials, forces, and operating rules of cosmic history. The early universe contains simple atoms but little large-scale differentiation. Chapter 2 asks how gravity transformed this relatively uniform material into stars and galaxies and how stars generated the chemical diversity required for planets and life.

Chapter 2: Stars and Galaxies: Thresholds 2 and 3

Chapter Objective and Contribution

Chapter 2 explains two linked thresholds. Threshold 2 is the formation of stars and galaxies; threshold 3 is the creation of chemical elements heavier than hydrogen and helium.

The chapter’s conceptual contribution is its explanation of free energy. After the extreme energies of the big bang dissipated, complex structures could no longer be locked permanently into place. They had to be constructed and maintained by controlled, nonrandom flows of energy capable of doing work. Stars are held in dynamic equilibrium by energy from nuclear fusion; living cells and societies later follow the same basic principle.

Gravity supplied the activation energy for star formation by concentrating matter until nuclear fusion began. Fusion then stabilized stars against gravitational collapse. When stars eventually exhausted their fuel, renewed gravitational collapse created the high temperatures needed to manufacture heavier elements. Stellar death therefore became the condition for later chemical complexity.

Argument Progression

1. Post–big bang complexity requires sustained, structured flows of energy.
2. Gravity amplifies small differences in the distribution of matter.
3. Dense clouds heat until proton fusion begins and stars are born.
4. Fusion temporarily balances gravity and stabilizes stellar structures.
5. Stars and galaxies create new gradients of temperature, density, and radiation.
6. Aging and dying stars manufacture heavier chemical elements.
7. Stellar explosions distribute those elements, preparing the universe for planets and complex chemistry.

Section-by-Section Argument

Free Energy: The Driver of Complexity

Christian distinguishes free energy, which possesses enough direction or structure to perform work, from heat energy, whose motion is more random. A waterfall contains free energy because water moves coherently down a gradient and can power a turbine. When the water crashes below, much of that coherent movement becomes disordered heat.

Energy itself is conserved, but its capacity to do useful work declines. Complex structures therefore depend on temporary gradients—differences in temperature, density, pressure, or chemical potential—down which energy can flow.

The relationship between complexity and energy is not accidental. The more intricate a system becomes, the more energy it generally needs to maintain its components and relationships. This pattern links stars to cells and cities.

Galaxies and Stars: Threshold 2

The early universe was extraordinarily uniform, but it was not perfectly so. Slightly denser regions exerted stronger gravitational attraction, drawing in more material and becoming still denser. Gravity magnified minute differences into large clouds and filaments.

Compression raised temperatures. When a cloud’s center became hot enough, protons overcame their electromagnetic repulsion and fused into helium. Fusion converted a small amount of matter into enormous quantities of energy. That energy pushed outward against gravity, creating a stable dynamic equilibrium: a star.

Galaxies emerged as vast gravitational systems containing billions of stars. Matter and energy were no longer distributed evenly. The universe now contained dense, hot, radiant regions surrounded by cold, sparse space. These differences produced abundant energy gradients and new opportunities for complexity.

A Universe with Galaxies and Stars

Stars introduced a form of long-lived but conditional stability. Their internal structure depends on a balance between:

• gravitational contraction;
• heat and radiation generated by fusion;
• continued access to nuclear fuel.

A star’s mass largely determines its history. Massive stars burn hotter and faster; smaller stars consume their fuel slowly. All must pay the energetic costs of maintaining their organization.

Christian uses Eric Chaisson’s estimates of energy-rate density to suggest that more complex systems generally require denser flows of energy per unit of mass. Human society channels far denser energy flows than a star, even though the star’s total output is vastly greater.

The important conclusion is that local complexity accelerates rather than reverses universal entropy. Stars maintain their own structure while radiating energy into space, helping degrade free energy into less usable forms.

New Elements and Increasing Chemical Complexity: Threshold 3

The early universe contained almost nothing but hydrogen and helium. Larger atomic nuclei required the extreme temperatures found inside aging and collapsing stars.

When a star exhausts hydrogen in its core, gravity compresses it and raises its temperature. Sufficiently massive stars begin fusing helium into carbon and oxygen and then manufacture progressively heavier elements. Fusion ceases to yield energy once iron dominates the core.

The final collapse of massive stars produces supernovas, neutron stars, or black holes. Supernova explosions and neutron-star mergers generate or disperse many of the heaviest elements. Dying stars therefore fertilize surrounding space with carbon, oxygen, silicon, iron, gold, uranium, and other materials.

The Hertzsprung–Russell diagram provides an observational framework for reconstructing stellar life cycles by comparing stars of different luminosities and temperatures. Spectroscopy reveals their chemical composition. Christian uses these techniques to illustrate how scientists infer processes occurring over timescales too long for direct observation.

Chapter Conclusion and Transition

Thresholds 2 and 3 transformed a chemically simple universe into one containing stars, galaxies, energy gradients, and a wide range of elements. The next step requires gentler environments where those elements can combine into molecules and eventually form planets with the chemical richness needed for life.

Chapter 3: Molecules and Moons: Threshold 4

Chapter Objective and Contribution

Chapter 3 moves from the large-scale work of gravity to the nanoscale work of electromagnetism. Its central claim is that chemically complex structures require environments neither as hot as stars nor as empty and cold as deep space. Regions near stars provide controlled flows of energy in which atoms can form molecules.

Threshold 4 occurs when chemically enriched stellar debris forms planets, moons, asteroids, and solar systems. These bodies are cooler and chemically more diverse than stars. Rocky planets in particular create multiple environments in which solids, liquids, gases, heat, pressure, and diverse elements can interact.

The chapter narrows the story to Earth. Earth’s location, internal heat, differentiated structure, magnetic field, atmosphere, oceans, and geological dynamism generated an unusually rich arena for chemical experimentation. The chapter also explains how seismology and radiometric dating made it possible to reconstruct planetary history.

Argument Progression

1. Heavy elements from dying stars allow atoms to form chemically diverse molecules.
2. Electromagnetic interactions determine how atoms bond and what properties molecules acquire.
3. Chemically enriched dust clouds collapse into stars and orbiting protoplanetary disks.
4. Accretion and chemical sorting produce planets, moons, and asteroids.
5. Earth differentiates into a core, mantle, crust, atmosphere, and oceans.
6. Internal and external energy flows create a dynamic chemical environment.
7. Modern dating and observational techniques establish the chronology required for the history of life.

Section-by-Section Argument

From Stardust to Molecules

Rich chemistry cannot occur in stellar interiors because extreme heat breaks molecular bonds. It is also restricted in deep space because atoms rarely encounter one another and activation energy is scarce.

The most favorable regions lie near stars but outside their interiors. There, electromagnetic energy can rearrange atoms without destroying the resulting molecules. Galactic dust clouds contain not only hydrogen and helium but small quantities of heavier elements produced by previous generations of stars. Spectroscopy reveals water, silicates, amino acids, and other molecules within these clouds.

Chemical Trysts: How Atoms Combine

Chemical behavior is governed primarily by electrons. Atoms form bonds as electrons are transferred, shared, or distributed among neighboring nuclei.

Christian emphasizes several consequential patterns:

• Ionic bonds form when electrons move between atoms.
• Covalent bonds form when atoms share electrons.
• Weak hydrogen bonds give water many of its unusual properties and later help stabilize DNA.
• Metallic bonds allow electrons to move through large groups of atoms.
• Carbon’s four available bonding positions permit chains, rings, and highly elaborate molecular structures.

Carbon’s flexibility makes it exceptionally important to life. The universal rules of chemistry also imply that organic building blocks need not be unique to Earth.

Threshold 4: From Molecules to Moons, Planets, and Solar Systems

Planetary systems emerge as by-products of star formation. A contracting cloud of enriched gas and dust spins faster and flattens into a disk. Most material gathers into the central star, while the remainder accretes into orbiting bodies.

In the early solar system, the young sun’s radiation pushed lighter elements away from the inner regions. Consequently:

• Outer planets retained large amounts of hydrogen and helium.
• Inner planets became relatively enriched in oxygen, silicon, iron, carbon, and other heavier elements.

Accretion was violent. Dust formed rocks; rocks became planetesimals; planetesimals collided and merged into planets. Some collisions shattered bodies or altered their orbits. The moon probably formed after a Mars-sized body struck the young Earth and sent debris into orbit.

The discovery of exoplanets indicates that threshold 4 has been crossed many times. Planetary systems are common, although their arrangements vary considerably.

Planet Earth

Earth lies in a relatively favorable galactic and solar location. As the young planet grew, impacts, pressure, and radioactive decay heated it until much of it melted. Materials then separated by density:

• iron and nickel sank to form the core;
• semimolten rock formed the mantle;
• lighter rocks formed the crust;
• volcanic gases helped produce an atmosphere and oceans.

Movement in the metallic core generated a magnetic field that protected the surface from charged solar particles. Heat rising from the interior kept the mantle and crust active. Asteroids and comets added water, gases, minerals, and organic molecules.

Earth was therefore not simply a rock at the correct distance from the sun. It was a layered, internally heated, chemically heterogeneous system with continuous flows of matter and energy.

Studying Earth: Seismographs and Radiometric Dating

Humans cannot observe Earth’s deep interior directly. Seismic waves function as geological probes because different waves move differently through solids and liquids. Their paths reveal the planet’s layered structure.

Determining Earth’s age required radiometric dating. Radioactive isotopes decay at regular statistical rates, allowing minerals and organic materials to function as clocks. Clair Patterson’s measurement of uranium and lead in meteorites established an age of approximately 4.5 billion years for the solar system.

Radiometric dating, tree rings, genetic divergence, and astronomical measurements together provide the chronological framework of the modern origin story. The ability to assign reliable dates to events billions of years old is itself a recent achievement of collective learning.

Chapter Conclusion and Transition

Chapter 3 produces the setting for threshold 5: a chemically rich, liquid-water planet with diverse energy gradients and long periods of relative stability. Part II asks how chemistry in such an environment became life and how life subsequently transformed the planet.

Part II: Biosphere

Function Within the Book

Part II explains the emergence of complex adaptive systems. Stars and planets possess structure, but living organisms actively maintain themselves, interpret information, reproduce, and evolve. Life therefore introduces a different relationship among energy, information, and complexity.

The part also dissolves any simple distinction between organisms and environment. Life changes Earth’s atmosphere, rocks, oceans, and climate, while geological changes reshape the conditions of evolution. Together they form the biosphere.

Chapter 4: Life: Threshold 5

Chapter Objective and Contribution

Chapter 4 identifies the properties that distinguish life from nonlife and reconstructs the probable transition from complex chemistry to the first cells. Christian’s central claim is that life represents a new level of complexity because it combines metabolism, information processing, reproduction, and adaptation.

Living organisms do not passively retain a fixed structure. They continually draw energy and material from their surroundings, repair themselves, respond to changing conditions, and reproduce before entropy destroys them. Genetic information allows the organizational pattern of an organism to persist after individual bodies die.

The chapter does not claim that science has fully explained life’s origin. Instead, it identifies plausible Goldilocks conditions, intermediate chemical processes, and molecular mechanisms that could connect nonliving chemistry to early organisms.

Argument Progression

1. Life differs from earlier structures by actively maintaining and reproducing itself.
2. Energy makes biological work possible, while information directs that work.
3. Cells integrate metabolism, homeostasis, reproduction, and variation.
4. Natural selection preserves variations suited to local environments.
5. Earth supplied unusually favorable conditions for chemical evolution.
6. Autocatalytic reactions and early genetic molecules may have preceded complete cells.
7. Membranes, ATP-based metabolism, enzymes, and DNA produced autonomous prokaryotic life.

Section-by-Section Argument

Life and Information: A New Type of Complexity

Living organisms behave as though they have purposes: remaining alive and reproducing. Christian is careful not to attribute cosmic purpose to the universe, but purpose-like behavior is a legitimate description of systems that consistently favor some outcomes over others.

Life maintains organization by capturing free energy. A human body, for example, converts the chemical energy of food into movement, thought, maintenance, and heat. When energy flows stop, biological structure deteriorates.

Life also depends on information. Energy determines whether change can occur; information helps determine which change occurs. Organisms must detect local conditions, interpret them, and respond. All living things are therefore “informavores” that consume information about their environments.

Defining Life

Christian identifies five overlapping characteristics:

1. organization into cells bounded by semipermeable membranes;
2. metabolism that obtains and uses free energy;
3. homeostasis that adjusts internal conditions;
4. reproduction based on genetic information;
5. variation that allows populations to evolve.

No single property produces an absolute boundary between life and nonlife. Viruses and dormant organisms occupy ambiguous regions. Life should therefore be understood as an integrated cluster of capacities rather than a simple substance.

Natural selection combines random variation with environmental filtering. Beneficial variations tend to reproduce more successfully, while damaging variations disappear. Like the laws of physics, natural selection is a ratchet that preserves some nonrandom structures, but its rules are local and change with environments.

The Goldilocks Conditions for Life

Chemical evolution requires:

• a suitable position within a galaxy;
• an orbit providing moderate temperatures;
• liquid solvents, especially water;
• chemical diversity;
• sustained but non-destructive energy flows;
• environments stable enough for complex molecules to persist.

Earth satisfied these conditions unusually well. Experiments such as Stanley Miller’s demonstrated that amino acids and other organic compounds can form from simpler materials under plausible early-Earth conditions. Meteorites also contain organic molecules, suggesting that some ingredients arrived from space.

The greater difficulty is assembling long polymers such as proteins and nucleic acids. Alkaline hydrothermal vents offer a plausible environment because they contain mineral surfaces, chemical gradients, microscopic pores, and steady supplies of energy.

From Rich Chemistry to Life: Luca, the Last Universal Common Ancestor

The earliest known life appeared relatively soon after Earth became habitable, although direct evidence is sparse and disputed. Biologists infer a last universal common ancestor—LUCA—from genes and molecular mechanisms shared by modern organisms.

LUCA was probably not a fully independent modern cell. Christian presents it as a possible intermediate system living in mineral pores near alkaline vents. Natural proton and chemical gradients could have powered its reactions. It may already have used RNA, proteins, and ATP-related mechanisms while depending on its geological environment for a membrane and energy system.

LUCA is therefore an analytical reconstruction, not a discovered fossil. It represents the minimum shared biological machinery from which later life descended.

From Luca to Prokaryotes

Several innovations transformed LUCA-like systems into independent cells:

• phospholipid membranes created portable boundaries;
• enzymes accelerated and regulated reactions;
• chemiosmosis converted proton gradients into usable energy;
• ATP transported energy within cells;
• DNA provided a more stable archive than RNA;
• ribosomes translated genetic instructions into proteins.

DNA separates information storage from most chemical work, protecting genetic instructions inside a double helix. RNA helps copy and interpret those instructions, while proteins perform much of the cell’s labor.

Once cells possessed membranes, autonomous metabolism, and stable genetic systems, they could leave hydrothermal environments and spread through the oceans. Threshold 5 had been crossed.

Chapter Conclusion and Transition

Life combines matter, controlled energy flows, and encoded information in systems capable of reproduction and evolution. Chapter 5 follows the long age of single-celled organisms and shows that apparently “little” life transformed the entire planet before large organisms appeared.

Chapter 5: Little Life and the Biosphere

Chapter Objective and Contribution

Chapter 5 corrects the tendency to treat microbial history as a preliminary to the supposedly more important age of animals. Prokaryotes dominated most of the history of life and invented many of the biochemical mechanisms later organisms inherited.

The chapter also presents the biosphere as a coupled system. Geological forces create environments for life, while organisms alter atmospheric chemistry, climate, minerals, and nutrient cycles. The rise of photosynthesis and atmospheric oxygen transformed both biology and geology, producing crises for existing organisms while opening pathways toward larger and more energetic life.

Argument Progression

1. Earth’s internal heat and plate tectonics create a dynamic geological stage.
2. Geological carbon cycling helps regulate surface temperatures.
3. All modern organisms share molecular evidence of common ancestry.
4. Prokaryotes diversify and develop sophisticated ways of sensing and exploiting environments.
5. Photosynthesis grants life access to a vast new energy source.
6. Oxygenic photosynthesis restructures the atmosphere and causes ecological crisis.
7. Eukaryotic cells evolve through symbiosis and use oxygen to support greater complexity.

Section-by-Section Argument

Geology: How Planet Earth Works

Heat from Earth’s formation and radioactive elements drives convection in the mantle. Moving material carries tectonic plates, producing mountains, volcanoes, earthquakes, seafloor spreading, and subduction.

Plate tectonics also regulates atmospheric carbon dioxide. Rain and erosion remove carbon from the atmosphere and transport it into ocean sediments. Subduction carries some of this carbon into the mantle, while volcanism eventually releases it again. Temperature influences the speed of erosion, creating a rough negative-feedback system.

This geological thermostat helped keep Earth within a life-supporting temperature range despite the gradual brightening of the sun. Venus and Mars illustrate alternative outcomes: runaway heating on one and atmospheric loss and freezing on the other.

The Unity of Life

DNA comparisons reveal that all living organisms share common ancestry. Despite visible differences, organisms use many of the same molecular systems for energy transfer, information storage, protein production, and reproduction.

Christian follows the classification of life into three domains:

• Archaea;
• Eubacteria;
• Eukarya.

The first two consist of prokaryotes. Eukarya contains more structurally complex cells and all multicellular plants, animals, and fungi.

Prokaryotes: A World of Single-Celled Organisms

Prokaryotes are small but highly capable. They sense chemicals, light, temperature, acidity, and physical obstacles. Their responses already exhibit the basic structure of biological information processing:

1. input from sensors;
2. internal processing;
3. behavioral output.

Many prokaryotes exchange genetic material horizontally, making their evolutionary history less tree-like than that of many multicellular organisms. They developed diverse metabolisms based on geological chemicals, other organisms, and eventually sunlight.

Their continued abundance in modern microbiomes demonstrates that large size is not equivalent to evolutionary success.

Photosynthesis: An Energy Bonanza and a Revolution

Photosynthesis allowed organisms to capture energy from sunlight. Early forms used materials such as hydrogen sulfide or iron. Cyanobacteria later evolved a more productive system using water and carbon dioxide and releasing oxygen as waste.

This innovation produced a biological energy bonanza, but it also destabilized the existing biosphere. Oxygen reacted aggressively with surrounding materials and was toxic to many anaerobic organisms. The Great Oxygenation Event therefore caused an “oxygen holocaust” while simultaneously:

• creating new minerals;
• reducing atmospheric methane;
• helping generate global cooling;
• forming an ozone layer;
• opening previously inaccessible environments.

Photosynthesis illustrates a recurring principle: major innovations create opportunities while destroying systems adapted to earlier conditions.

Eukaryotes to the Rescue

Some organisms learned to use oxygen through respiration, a highly efficient method of extracting energy from food. The first eukaryotic cells probably emerged through endosymbiosis: one cell incorporated others that eventually became mitochondria and, in photosynthetic lineages, chloroplasts.

Eukaryotes possessed:

• nuclei protecting genetic material;
• internal organelles;
• larger genomes;
• more energy per gene;
• greater potential for specialization;
• sexual reproduction that accelerated genetic recombination.

This transition depended on cooperation as well as competition. Formerly independent organisms became mutually dependent components of a larger cell.

Chapter Conclusion and Transition

Microorganisms constructed the chemical foundations of the modern biosphere. Oxygen, respiration, eukaryotic cells, and new forms of genetic exchange created the prerequisites for multicellular organisms. Chapter 6 examines how these prerequisites produced large bodies, nervous systems, brains, and eventually mammals.

Chapter 6: Big Life and the Biosphere

Chapter Objective and Contribution

Chapter 6 explains why large multicellular life appeared so late and how its evolution transformed both organisms and environments. Multicellularity requires more than cells sticking together. Cells must communicate, specialize, coordinate development, distribute nutrients, and sometimes sacrifice themselves for the whole organism.

The chapter follows the emergence of large organisms through the Ediacaran and Cambrian periods, the repeated disruptions of mass extinctions, the colonization of land, and the evolution of larger bodies and brains. It culminates with the asteroid impact that eliminated non-avian dinosaurs and opened ecological space for mammalian diversification.

Its contribution is to establish the biological and ecological path leading from microbial complexity to primates and hominins.

Argument Progression

1. Multicellular organisms require elaborate systems of coordination and cellular specialization.
2. Rising oxygen and genetic toolkits make large bodies energetically and developmentally possible.
3. The Ediacaran and Cambrian radiations produce diverse body plans and ecological interactions.
4. Mass extinctions repeatedly destroy established ecosystems and open new evolutionary possibilities.
5. Plants and animals colonize land and transform terrestrial environments.
6. Nervous systems and brains expand organisms’ ability to process information.
7. The extinction of dinosaurs enables mammals and primates to diversify.

Section-by-Section Argument

Big Life

Large organisms are cooperative communities of cells. Their survival depends on division of labor among cells that may never reproduce independently. Multicellular organization therefore requires mechanisms suppressing cellular defection, including programmed cell death and immune responses against uncontrolled growth.

The transition also requires enough environmental oxygen to support energy-intensive bodies. Large life was delayed not because evolution lacked creativity but because its molecular, energetic, and atmospheric prerequisites accumulated slowly.

The Molecular Gadgets That Made Big Life Possible

Multicellular organisms rely on ancient molecular systems for:

• adhesion;
• intercellular communication;
• control of gene expression;
• developmental sequencing;
• identification of cellular position;
• regulation of growth;
• programmed cell death.

Genetic toolkits such as Hox genes allow similar regulatory instructions to build different structures depending on where and when they are activated. Evolution could therefore generate major anatomical diversity partly by modifying regulatory patterns rather than inventing entirely new genes.

Big Life Takes Off: The Ediacaran and Cambrian Periods

The Ediacaran period contains evidence of some of the earliest large organisms. During the Cambrian radiation, many major animal body plans appeared in the fossil record.

Christian treats this not as instantaneous creation but as an acceleration made possible by accumulated prerequisites:

• higher oxygen levels;
• predation and ecological competition;
• developmental genes;
• hard shells and skeletons;
• improved senses and movement;
• richer ecological networks.

Predation created evolutionary arms races in protection, speed, perception, and attack.

Evolutionary Ups and Downs: Mass Extinctions and Evolution’s Roller Coaster

Biological history is not a smooth ascent. Mass extinctions remove large portions of existing diversity, often through rapid environmental change. The Permian extinction was especially destructive.

Extinctions demonstrate that adaptation is local and temporary. Traits suited to one environment may become fatal when conditions shift. Yet ecological destruction also empties niches, allowing surviving groups to diversify. Evolution proceeds through alternating periods of elaboration, collapse, and reconstruction.

Greening the Land and Oxygenating the Atmosphere

Plants and fungi gradually colonized land, helped create soils, accelerated erosion, and altered carbon cycling. Terrestrial vegetation provided food and habitats for animals and changed atmospheric composition.

Land ecosystems increasingly integrated geological, atmospheric, and biological processes. The biosphere became more productive and structurally elaborate, though still vulnerable to changes in temperature, oxygen, sea level, and tectonic arrangement.

Long Trends: Larger Bodies and Bigger Brains

Over time, some lineages evolved larger bodies, more sophisticated senses, nervous systems, and brains. Brains offer selective advantages because they integrate information, model environments, guide movement, and allow flexible behavior.

Christian presents consciousness as an advanced form of focused biological information processing rather than as a complete break from earlier life. Perception, memory, emotion, and reasoning developed incrementally from simpler capacities.

An Asteroid Lands—A Lucky Break for Mammals

Approximately sixty-five million years ago, a large asteroid struck near the Yucatán Peninsula. Evidence includes a global iridium-rich layer and the Chicxulub crater. The impact generated fires, atmospheric debris, climatic disruption, and the collapse of food chains.

The event eliminated non-avian dinosaurs and many other organisms. It was catastrophic for existing ecosystems but advantageous to surviving mammals. Human existence therefore depends partly on a highly contingent astronomical event.

After the Asteroid: A Mammalian Adaptive Radiation

Mammals diversified into ecological roles previously occupied by dinosaurs. Some returned to the oceans; others became large grazers or predators; still others evolved arboreal adaptations.

Primates developed grasping hands, forward-facing eyes, and relatively large brains. Later climate cooling and environmental instability reshaped primate habitats and created the circumstances in which hominins emerged.

Chapter Conclusion and Transition

Part II ends with a biosphere containing complex organisms, sophisticated nervous systems, mammals, and primates. Part III narrows the scale to one primate lineage whose distinctive form of information sharing would transform the entire planet.

Part III: Us

Function Within the Book

Part III explains how cultural evolution accelerated the processes already visible in biological history. Human societies remain complex adaptive systems dependent on energy and information, but collective learning enables innovation to accumulate much faster than genetic changes.

The chapters trace three linked energy transitions: foraging, farming, and fossil fuels. Each supports larger populations and more complex networks, but each also generates new inequalities, risks, and complexity taxes. The part culminates in the Anthropocene, where human collective learning becomes a planetary force.

Chapter 7: Humans: Threshold 6

Chapter Objective and Contribution

Chapter 7 asks what distinguishes humans from other intelligent and social animals. Christian rejects explanations based on a single anatomical trait. Bipedalism, hands, fire, toolmaking, large brains, cooperation, and social learning all contributed to human evolution, but the decisive threshold was language powerful enough to support collective learning.

Collective learning allows information to accumulate rather than repeatedly disappearing with individual organisms. Communities preserve innovations, combine discoveries made by different people, and transmit knowledge across generations. Cultural change therefore becomes cumulative.

This mechanism explains how a physically unimpressive primate spread throughout the world, adapted to radically different environments, and eventually gained unprecedented control over energy and matter.

Argument Progression

1. Climate change and fragmented habitats reshape primate evolution.
2. Hominins develop bipedalism, dexterous hands, tools, fire use, and larger brains.
3. Homo sapiens emerges as one member of a diverse hominin family.
4. Enhanced language permits unusually precise sharing of abstract information.
5. Collective learning creates cumulative cultural evolution.
6. Paleolithic communities colonize most of the planet and modify local ecosystems.
7. Population growth and expanding exchange networks prepare the conditions for farming.

Section-by-Section Argument

Primate Evolution in a Cooling World

Long-term global cooling altered forests and expanded more open environments. Some primates adapted to life on the ground. Bipedalism freed hands for carrying, manipulating objects, and making tools, though no single adaptation made early hominins fully human.

Environmental instability favored behavioral flexibility. Organisms able to learn, cooperate, and exploit multiple resources could survive changing conditions more effectively than narrowly specialized species.

Early Hominin History: When Did the First Humans Appear?

The human lineage diverged from chimpanzee ancestors several million years ago. Early hominins combined ape-like and human-like traits. Their increasing diversity makes any sharp boundary between “human” and “nonhuman” artificial.

Stone tools indicate greater manipulative ability and learned behavior, but toolmaking itself was not unique. The chapter treats human emergence as a mosaic process involving changes in anatomy, diet, sociality, and cognition.

Later Hominin History: The Past Two Million Years

Homo erectus possessed larger brains, more human-like bodies, and the ability to migrate beyond Africa. Fire and cooking may have increased available nutrition, reduced digestive costs, extended the day, and created social gathering places.

Later hominins included Neanderthals, Denisovans, and Homo sapiens. Genetic evidence indicates interbreeding rather than a perfectly clean replacement. Modern humans inherited a mixed biological history.

What Makes Us Different? Crossing Threshold 6

Many animals communicate and learn socially. Human language differs in its precision, range, and ability to refer to absent, hypothetical, or abstract things.

Language connects individual brains into information networks. An innovation no longer has to be rediscovered in every generation. Knowledge becomes cumulative, allowing communities to:

• preserve technical skills;
• compare experiences;
• coordinate large projects;
• construct shared maps and stories;
• adapt culturally within generations.

Collective learning is the human equivalent of a new evolutionary ratchet. It does not replace genetic evolution, but it operates far more rapidly.

Living in the Paleolithic

Most human history occurred in small foraging communities. These groups possessed detailed environmental knowledge and often maintained flexible, relatively egalitarian social arrangements.

Their small size limited specialization and material accumulation, but periodic gatherings and marriage networks connected them into larger information systems. Kinship, ritual, exchange, and storytelling helped preserve knowledge across wide regions.

Settling the Biosphere: Humans Migrate Around the World

Humans spread from Africa through Eurasia, Australia, the Americas, and remote islands. Each migration required new knowledge about climates, animals, plants, clothing, shelters, navigation, and food preparation.

Human settlement sometimes coincided with megafaunal extinctions and ecological change. Techniques such as deliberate burning altered landscapes long before farming. Paleolithic humans were therefore already significant ecological agents, although their total impact remained limited by low population and modest energy use.

Increasing Complexity in the Paleolithic

Population growth was extremely slow, but over tens of thousands of years it increased the total number of minds participating in collective learning. Denser populations and broader exchange networks increased the rate at which innovations could arise and spread.

Most additional energy supported more people rather than dramatically higher living standards. This pattern—energy bonanzas being absorbed by population and complexity—reappears in agrarian history.

The Earliest Era of Human History

By the end of the last ice age, humans had occupied most habitable regions and accumulated extensive ecological knowledge. They had not yet become farmers, but some communities managed plants, animals, waterways, and landscapes intensively.

The warming and increasing climatic stability of the Holocene would allow these practices to become more systematic.

Chapter Conclusion and Transition

Threshold 6 gives human communities a cumulative information system capable of accelerating innovation. Chapter 8 examines why, after hundreds of thousands of years of foraging, some communities began using that capacity to domesticate plants and animals.

Chapter 8: Farming: Threshold 7

Chapter Objective and Contribution

Chapter 8 explains agriculture as a major intensification in human control over biological energy. Farmers do not create new energy; they redirect a larger share of recent photosynthesis toward selected organisms and human consumption.

Christian rejects the idea that farming was an obvious improvement voluntarily adopted as soon as humans understood plants. Early farming often required more labor, reduced dietary variety, increased disease, and produced less secure lives. Agriculture emerged when favorable climate, accumulated knowledge, population pressure, and local ecological opportunities converged.

Its long-term importance lies not in an immediate rise in individual welfare but in its capacity to support denser populations, permanent settlements, larger information networks, and eventually agrarian states.

Argument Progression

1. Agriculture reorganizes ecosystems to favor useful species.
2. Foragers already possessed much of the knowledge required for cultivation.
3. Stable Holocene climates made systematic domestication more viable.
4. Population pressure reduced the attractiveness of continued mobile foraging.
5. Farming arose independently in several regions with suitable plants and animals.
6. Agricultural societies expanded because they supported higher population densities.
7. Surpluses and settlement created the conditions for states and civilizations.

Section-by-Section Argument

What Is Agriculture?

Agriculture is a symbiotic relationship in which humans assist selected species while those species supply food, fiber, labor, or other resources. Domestication changes both partners. Humans alter environments and reproductive patterns, while domesticated species reshape human settlement, diet, labor, and social organization.

Farming increases the share of an ecosystem’s productivity available to humans by removing competitors, preparing soils, controlling water, protecting crops, and breeding useful traits.

The History and Geography of Early Farming

Agriculture appeared independently in several world regions. Different environments produced different combinations of crops, animals, technologies, and settlement systems.

Geography mattered because domesticable species were unevenly distributed. Afro-Eurasia possessed several large animals suitable for transport and labor, while other regions relied more heavily on human labor or different ecological adaptations.

The independent appearance of farming suggests that it became increasingly probable once human knowledge, population, and Holocene conditions reached suitable levels.

Why Did Humans Take Up Farming? Crossing Threshold 7

The transition was usually gradual and reluctant rather than revolutionary. Communities experimented with harvesting, tending, transplanting, irrigating, and protecting wild resources long before full dependence on domesticated foods.

Evidence from Natufian communities and sites such as Abu Hureyra suggests a combination of:

• sedentary settlement;
• population growth;
• environmental disruption;
• declining access to preferred wild foods;
• detailed knowledge of local species.

The Younger Dryas cooling event placed stress on communities already using plants intensively. Cultivation became a way to preserve food supplies under pressure.

Early farmers often became shorter, less healthy, and more vulnerable to famine and infectious disease. Farming succeeded demographically because it could feed more people per unit of land, not because every farmer lived better.

The Early Agrarian Era: Farming Spreads Around the World

Agriculture spread through migration, imitation, exchange, and the expansion of farming populations. Farmers frequently displaced or absorbed foraging communities because their populations grew more rapidly.

Pastoralists developed mobile systems based on domesticated animals in regions unsuitable for intensive cultivation. Irrigation agriculture supported especially dense populations but required coordinated labor and increasingly complex rules.

Agricultural landscapes became human-managed biological machines. Forests were cleared, soils altered, rivers redirected, and species redistributed.

How Farming Transformed Human History

Farming produced an energy bonanza from recent photosynthesis. Most of the additional energy was allocated to:

• larger populations;
• children and reproduction;
• permanent buildings;
• food storage;
• irrigation and transport;
• specialists and rulers;
• defense and warfare;
• losses from waste, disease, and ecological damage.

Dense settlements increased collective learning by placing more people in regular contact. At the same time, they intensified the “Four Horsemen” of famine, disease, warfare, and death.

Chapter Conclusion and Transition

Threshold 7 generated larger and more productive communities without guaranteeing better individual lives. Chapter 9 follows the political and social consequences of agricultural surplus: specialization, hierarchy, cities, taxation, states, and agrarian civilizations.

Chapter 9: Agrarian Civilizations

Chapter Objective and Contribution

Chapter 9 explains how farming communities developed into cities and states. The critical intermediary is surplus: the production of food beyond the immediate needs of cultivators. Surpluses support nonfarm specialists, but they also create resources that can be appropriated, stored, taxed, defended, and redistributed.

Agrarian states are treated as complex adaptive systems. They require energy from farmers, information about populations and resources, rules for coordinating behavior, and institutions capable of enforcing decisions. Writing, accounting, administration, religion, armies, and taxation are therefore not independent achievements; they are interlocking components of large-scale mobilization.

The chapter also emphasizes inequality. Agrarian civilizations generated remarkable cultural and technological achievements, but much of their surplus sustained elites and state infrastructure while most peasants remained near subsistence.

Argument Progression

1. Agricultural surpluses support specialization and exchange.
2. Specialization creates interdependence and more elaborate divisions of labor.
3. Surpluses can be appropriated by emerging elites and institutions.
4. Towns and cities become centers for storage, administration, religion, and trade.
5. States use information, coercion, and legitimacy to mobilize resources.
6. States expand into empires and larger exchange networks.
7. Most agrarian growth increases population and complexity rather than average welfare.

Section-by-Section Argument

Surpluses, Hierarchies, and a Division of Labor

Surplus production allows some people to become potters, builders, soldiers, priests, scribes, merchants, and rulers. Specialization raises productivity and knowledge but makes individuals dependent on systems of exchange.

Differences in control over land, storage, military force, and religious authority become durable inequalities. Wealth can be inherited, enabling social hierarchies to reproduce themselves across generations.

The emergence of elites is not explained solely by superior ability. It depends on institutional positions that permit some groups to control flows of resources and information.

From Towns to Cities and Rulers: Mobilization and a New Trophic Level

Cities concentrate people, goods, skills, and administrative functions. They also require large inflows of food and energy from surrounding rural areas. Urban elites occupy a new trophic level, drawing sustenance from producers much as predators draw energy from prey.

States mobilize resources through taxes, tribute, rent, corvée labor, and military service. To do this effectively, rulers need:

• records of people and land;
• standardized weights and measures;
• storage systems;
• roads and communications;
• legal rules;
• officials and scribes;
• military and religious support.

Writing initially serves these managerial needs. It expands collective learning while also increasing the state’s capacity to monitor and extract.

Religion contributes legitimacy by embedding political authority within a cosmic or moral order. Coercion and persuasion operate together.

The Spread of Agrarian States

States appeared independently in several regions and expanded through conquest, colonization, and imitation. Larger empires linked previously separated communities and supported wider exchange networks.

The Silk Roads and other routes circulated goods, technologies, religions, crops, and pathogens. Greater connectivity accelerated collective learning but also transmitted disease and warfare.

Agrarian states repeatedly expanded and collapsed. Their stability depended on the continued flow of surplus, effective administration, environmental conditions, and the ability to control internal and external rivals.

Measuring Change in the Agrarian Era of Human History

Christian uses population and energy estimates to compare historical regimes. From the beginnings of agriculture to the appearance of large agrarian civilizations, both human population and total energy use increased dramatically.

Most additional energy was consumed by:

• reproduction and larger populations;
• infrastructure and complexity taxes;
• ruling elites;
• armies and warfare;
• waste and inefficiency.

Average energy consumption per person rose only modestly. Agrarian civilization transformed social organization and collective knowledge more than it transformed the daily material circumstances of most people.

Chapter Conclusion and Transition

Agrarian civilizations created powerful institutions and vast networks but remained constrained by annual flows of solar energy captured through plants and animals. Chapter 10 explains how global interconnection intensified collective learning and how fossil fuels eventually broke through those energetic limits.

Chapter 10: On the Verge of Today’s World

Chapter Objective and Contribution

Chapter 10 identifies two developments that prepared the modern world: the linking of previously separated human world zones and the exploitation of fossil fuels. Global connection enlarged the scale at which collective learning could operate. Fossil fuels supplied the energy required to convert rapidly expanding knowledge into unprecedented material power.

Christian does not portray European dominance as the straightforward result of intrinsic civilizational superiority. Europe occupied a relatively marginal position within Afro-Eurasian networks but gained a gateway advantage when Atlantic voyages connected the Americas to existing exchange systems. Commercial competition, state support, imperial violence, and scientific experimentation reinforced one another.

The chapter’s decisive transition is from agrarian societies limited by recent photosynthesis to industrial societies drawing on hundreds of millions of years of stored solar energy.

Argument Progression

1. Around 1400, major human world zones remained incompletely connected.
2. European maritime expansion joined these zones into one global system.
3. Biological exchange, conquest, and coerced labor reorganized populations and environments.
4. Global commerce, printing, capitalism, and science accelerated information exchange.
5. Agrarian societies approached the limits of available land and recent solar energy.
6. Coal-powered steam engines unlocked ancient stores of energy.
7. Industrial feedback loops spread through production, transport, communications, and warfare.

Section-by-Section Argument

The World Six Hundred Years Ago

The major agrarian regions possessed large cities, sophisticated states, commercial networks, and extensive knowledge. China, India, the Islamic world, and other older hubs were generally wealthier and more developed than western Europe.

Nevertheless, the Americas, Afro-Eurasia, Australasia, and Pacific societies remained separated by major barriers. Knowledge and biological materials circulated within world zones more readily than between them.

Creating a Single World System

European Atlantic voyages breached these barriers. The resulting Columbian Exchange transferred crops, animals, pathogens, and people between ecosystems that had developed separately for millennia.

The consequences were asymmetric. Eurasian diseases devastated many Indigenous American populations. European conquerors exploited political divisions and the epidemiological collapse. Enslaved Africans were transported across the Atlantic to supply labor for plantations and mines.

Global trade moved American silver through Europe and across the Pacific to Asian markets. Ports, shipping routes, currencies, and commercial institutions increasingly formed a single world economy.

Printing accelerated the circulation and correction of information. Reports from voyages challenged inherited textual authorities, while scientific communities developed experimental and mathematical methods for testing claims.

Capitalism strengthened the incentives for innovation. Merchants could not generally rely on direct coercion as fully as states could; they sought information about prices, goods, routes, and production methods. States, in turn, funded navies, protected commerce, borrowed from financial institutions, and taxed profitable trade.

This alliance was productive but frequently violent. Slavery, plantation agriculture, colonial extraction, and military conquest were integral to the emerging global system.

Fossil Fuels: A Mega-Innovation

Agrarian energy came primarily from crops, wood, animal labor, wind, and water—all forms of recent solar energy. By the eighteenth century, densely populated European regions were encountering land and timber constraints.

Coal contained energy captured by ancient organisms hundreds of millions of years earlier. England possessed accessible coal near navigable waterways and growing urban markets. As mines deepened, flooding created demand for improved pumps.

The Newcomen engine used steam to pump mines but consumed large amounts of coal. James Watt’s improvements sharply increased efficiency. Better engines allowed deeper mining; more coal made engines cheaper to operate; cheaper energy encouraged further industrial applications.

This feedback loop converted a local mining problem into a general-purpose energy revolution.

Early Industrialization

Steam power transformed manufacturing, mining, transport, and metallurgy. Railways and steamships lowered transportation costs and expanded markets. Demand for tracks, engines, and hulls stimulated iron and steel production.

Electricity provided a flexible means of distributing power. Telegraphs, telephones, and radios accelerated information exchange. Industrial chemistry, textiles, and mechanized agriculture developed through linked innovations.

Industrial energy also transformed warfare. Railways moved armies; steamships projected power; explosives and machine guns multiplied destructive force. European states used these advantages to conquer or dominate much of the world.

Oil initiated a second fossil-fuel expansion. Internal combustion engines eventually powered automobiles, aircraft, ships, and machinery. Industrialization became a self-reinforcing system of energy, innovation, commerce, state power, and military competition.

Chapter Conclusion and Transition

Global exchange turned collective learning into a planetary process, while fossil fuels removed the principal energy constraint of agrarian civilization. Chapter 11 examines the resulting Great Acceleration and the emergence of humans as a force capable of transforming the entire biosphere.

Chapter 11: The Anthropocene: Threshold 8

Chapter Objective and Contribution

Chapter 11 argues that the fossil-fuel revolution carried humanity across threshold 8. Humans did not merely construct more complex societies; they became a dominant driver of geological, atmospheric, and biological change.

The Anthropocene contains a profound duality. Cheap energy and scientific knowledge made it possible to support billions of people, raise life expectancy, reduce some forms of violence, expand education, and create a large middle class. The same systems destabilize climate, reduce biodiversity, produce extreme inequalities, and create weapons capable of global destruction.

The chapter’s contribution is to convert the book’s history of increasing complexity into a diagnosis of the present. Human societies now consume enough energy and process enough information to manage the biosphere, but their institutions remain fragmented and poorly adapted to planetary responsibility.

Argument Progression

1. Industrialization initially creates a large power gap between fossil-fuel states and agrarian societies.
2. Industrial states use their advantage for empire and then turn their weapons against one another.
3. After World War II, energy use, production, trade, population, and consumption accelerate globally.
4. Science and technology intensify feedback loops among knowledge, energy, wealth, and power.
5. Wage labor and industrial interdependence transform states and everyday life.
6. Human activity begins altering climate, nutrient cycles, biodiversity, and geological processes.
7. The Anthropocene produces both unprecedented welfare and systemic planetary danger.

Section-by-Section Argument

The Great Acceleration

Christian divides the Anthropocene into three broad acts:

1. Industrial divergence: Britain and other early industrializers gain overwhelming economic and military advantages.
2. Industrial warfare: competing powers mobilize fossil-fuel technologies in the world wars, culminating in nuclear weapons.
3. Global acceleration: after 1945, industrialization, trade, consumption, and population growth spread through much of the world.

During the Great Acceleration, oil, gas, electricity, machinery, and mass production dramatically increased productivity. International trade recovered, global consumption expanded, and older industrial inequalities between regions began to narrow.

The Anthropocene is dated especially to the mid-twentieth century because this is when multiple indicators of human influence—energy use, population, production, emissions, and resource extraction—rise almost simultaneously.

Transforming the World: Technologies and Science

Cheap energy drives innovation, while innovation creates new gradients of wealth and power that stimulate further competition.

Important examples include:

• Haber-Bosch nitrogen fixation, which converts fossil-fuel energy into fertilizer and food;
• internal combustion engines;
• aviation and container shipping;
• electronics and transistors;
• computers and global communication networks;
• nuclear fission and fusion weapons;
• modern genetics and biotechnology.

Modern science intensifies collective learning by organizing research through universities, laboratories, governments, and corporations. Computers expand the capacity to store, analyze, and distribute information. The noösphere becomes a major force within the biosphere.

Transforming the World: Governance and Society

Industrial societies consist increasingly of specialized wage earners who cannot survive independently of markets, currencies, employers, transportation systems, and public infrastructure. Governments therefore become deeply involved in daily economic and social life.

Modern states develop:

• national education systems;
• censuses and detailed records;
• economic regulation;
• mass armies;
• welfare and public-health institutions;
• transportation and communications infrastructure;
• systems of political participation and nationalism.

Government expenditure grows as a share of national production. International organizations, corporations, and nongovernmental institutions also create embryonic forms of global governance.

New Ways of Living and Being

Urban-industrial life differs radically from peasant life. Specialized knowledge replaces many generalized household skills. People become embedded in long chains of dependence involving food, energy, medicine, employment, finance, and transportation.

Major transformations include:

• urbanization;
• mass literacy and prolonged education;
• longer life expectancy;
• reduced child mortality;
• consumer society;
• declining fertility;
• changes in gender roles;
• reduced social acceptance of slavery, corporal punishment, and interpersonal violence.

The demographic transition occurs when mortality falls before fertility, producing rapid population growth followed by slowing growth as families have fewer children.

Greater interdependence also increases fragility. Modern societies can function only while complex infrastructures and institutions remain operational.

Transforming the Biosphere

Human activities now alter major planetary systems:

• Fossil-fuel combustion returns ancient carbon to the atmosphere.
• Methane emissions rise through agriculture and livestock.
• Industrial fertilizer transforms the nitrogen cycle.
• Farming and settlement reduce wild-animal biomass.
• Mining, construction, and road building move immense quantities of earth.
• Groundwater is extracted faster than natural replacement.
• Plastics, concrete, aluminum, and other human-made materials accumulate.
• Extinction rates rise far above long-term background levels.

Human weaponry represents another biospheric danger. Nuclear arsenals possess destructive power comparable in consequence to major natural catastrophes.

Measuring Change in the Anthropocene

Statistical comparisons reveal the scale of the transformation. In two centuries, population rose several times over, but total energy consumption rose even faster. Per-capita energy availability increased dramatically after remaining nearly flat through most of agrarian history.

The energy bonanza was allocated among:

• population growth;
• infrastructure;
• waste and pollution;
• warfare;
• elite consumption;
• rising mass consumption and welfare.

Unlike the agrarian energy bonanza, fossil fuels left enough surplus to improve the living standards of a large global middle class. Life expectancy increased, nutrition and medicine improved, and many people escaped subsistence.

Yet benefits remain highly unequal. Christian therefore distinguishes:

• Good Anthropocene: health, longevity, education, technological capability, and material abundance.
• Bad Anthropocene: poverty amid abundance, inequality, ecological destabilization, mass extinction, climate change, and nuclear danger.

Chapter Conclusion and Transition

The Anthropocene gives humanity both the capacity and the obligation to manage planetary systems. The problem passed to Chapter 12 is whether collective learning can produce institutions and values capable of preserving human welfare without destroying the biosphere that supports it.

Part IV: The Future

Function Within the Book

Part IV turns explanation into orientation. The past reveals the mechanisms that created modern power, but it cannot determine what humans will do with that power.

Christian distinguishes three temporal scales. The near human future is difficult but practically urgent. The next several millennia are highly contingent and resist reliable prediction. The distant cosmological future is easier to model because it involves simpler physical systems, but it offers little practical guidance.

The book has no separately titled conclusion; Chapter 12 performs the concluding function.

Chapter 12: Where Is It All Going?

Chapter Objective and Contribution

Chapter 12 asks what kinds of futures can be inferred from the modern origin story. Christian rejects both confident prediction and passive fatalism. Complex adaptive systems are contingent, and human choices can redirect historical trajectories.

The immediate future is framed as a quest with three goals:

1. avoid civilizational and ecological collapse;
2. extend the benefits of the Good Anthropocene to all humans;
3. preserve a flourishing biosphere.

Success would require moving from explosive growth toward a mature Anthropocene based on controlled energy use, reduced inequality, ecological restraint, and more conscious global regulation. Such a transition might constitute threshold 9.

The chapter then widens its scale to possible posthuman and cosmic futures, returning finally to entropy. Human agency matters profoundly at human scales even though all structures remain temporary in cosmological time.

Argument Progression

1. Humans possess exceptional capacity to model futures through collective learning.
2. Prediction becomes less reliable as systems grow more complex and timescales lengthen.
3. The next century is unusually consequential because present decisions can alter conditions for millennia.
4. Collapse remains possible if modern complexity outruns human capacity to manage it.
5. A successful quest requires equity, sustainability, and preservation of planetary boundaries.
6. A mature Anthropocene could replace endless material growth with dynamic stability.
7. Threshold 9 would represent conscious regulation of the biosphere through the noösphere.
8. Distant biological and technological futures remain speculative, while cosmic entropy is comparatively predictable.

Section-by-Section Argument

Future Games

Brains model possible futures in order to guide action. Human collective learning combines billions of individual models and enhances them through science, data, and computers.

Predictability varies by scale:

• Near-term human futures can be modeled imperfectly but usefully.
• Millennial futures involve too many contingencies for confident prediction.
• Cosmological futures depend on fewer variables and can sometimes be projected more confidently.

Quantum uncertainty, biological mutation, political decisions, pandemics, impacts, and other contingencies prevent history from being deterministic.

The Human Future: The Quest

Humanity already manages the biosphere whether it acknowledges that role or not. Failure could involve declining productivity, political collapse, famine, conflict, disease, and the loss of modern welfare. If climate stability is seriously damaged, even agriculture could become unreliable.

Christian presents three virtues required for the quest:

• Alertness: noticing evidence and warning signs.
• Determination: sustaining political and institutional action.
• Hope: maintaining confidence that successful transition remains possible.

The UN Sustainable Development Goals and Paris climate agreement represent early attempts to define common planetary objectives. Their existence demonstrates growing awareness, but declarations do not eliminate conflicts among development, equality, consumption, and ecological restraint.

Christian questions the assumption that economic growth must continue indefinitely. Fossil fuels may have functioned as an activation energy that launched industrial society. Once modern infrastructures and knowledge exist, it may be possible to maintain high welfare with smaller, more carefully managed energy flows.

A mature Anthropocene would likely include:

• stabilization or reduction of population;
• elimination of extreme poverty;
• limits on extreme wealth concentration;
• reduced dependence on fossil fuels;
• renewable energy;
• greater energy and material efficiency;
• recycling and lower resource extraction;
• protection of biodiversity and ecosystems;
• higher valuation of leisure, education, culture, and quality of life;
• stronger forms of global coordination.

Christian invokes John Stuart Mill’s stationary state to distinguish the end of material expansion from the end of cultural or moral improvement. Stability need not mean stagnation.

Planetary boundaries provide rough indicators of biospheric limits. Climate change and biodiversity loss are especially dangerous because crossing critical points may trigger self-reinforcing feedbacks.

The greatest obstacle is not necessarily technical knowledge but political determination. National competition, short electoral cycles, poverty, immediate economic pressures, and profit incentives make long-term global action difficult.

If sustainability becomes profitable and politically compelling, capitalism and governments might redirect their innovative power toward the quest. Success would reveal thresholds 6 through 9 as stages of one larger human revolution: collective learning leading eventually to conscious planetary regulation. Origin Story - David Christian.pdfPDF

Beyond Humans: Millennial and Cosmological Futures

Christian treats intermediate futures cautiously. Possibilities include:

• stronger global institutions;
• fusion energy;
• settlement beyond Earth;
• asteroid mining;
• engineered organisms;
• nanotechnology;
• artificial intelligence;
• biological or technological human enhancement;
• a more unified global noösphere.

These are extrapolations rather than predictions. They could end recognizable human history by transforming Homo sapiens into something else.

Cosmological futures are more constrained. Continents will continue moving, perhaps forming another supercontinent. The sun will eventually become a red giant, sterilize or consume Earth, and end as a white dwarf. The Milky Way and Andromeda will merge.

If cosmic expansion continues accelerating, galaxies will become increasingly isolated. Star formation will cease; stellar remnants and black holes will dominate; black holes will eventually evaporate; and organized structures will disappear. Entropy will ultimately prevail within our universe.

Final Synthesis

Chapter 12 returns the book to the problem of meaning. The universe does not guarantee human survival or supply humanity with a predetermined purpose. Nevertheless, purpose exists within living agents. Humans can formulate goals, evaluate possible futures, and act collectively.

The modern origin story therefore provides orientation without providence. It shows that human societies are fragile arrangements dependent on billions of years of prior complexity, but it also shows that cumulative information can generate genuinely new possibilities. The book ends neither with confidence in inevitable progress nor with resignation to entropy. Its conclusion is conditional: humanity may be capable of crossing another threshold, but only if knowledge is converted into sustained collective action.

Whole-Book Synthesis

How the Argument Develops

The book’s argument depends on a sequence of conceptual expansions.

First, Christian establishes that complexity requires explanation. The universe begins in a simple state, and entropy favors the dissolution of organized forms. Any account of stars, organisms, or societies must therefore explain both how organization arises and how it is maintained.

Second, the cosmic chapters identify the basic mechanism. Goldilocks conditions permit structured energy flows to rearrange existing materials. Gravity forms stars; fusion stabilizes them; stellar death creates new elements; and chemically enriched matter forms planets. These chapters supply the vocabulary later applied to life and society.

Third, the biological chapters add information and adaptation. Living organisms do not merely persist through passive physical stability. They monitor environments, use encoded instructions, reproduce, and evolve. Natural selection retains successful structures without foresight or design.

Fourth, the human chapters introduce collective learning. Language allows information to accumulate across generations, creating a cultural evolutionary system much faster than genetic evolution. This is the indispensable explanation for the extraordinary speed of human expansion and innovation.

Fifth, Christian interprets farming and fossil fuels as energy transitions produced by collective learning. Agriculture captures a larger share of current biological production. Fossil fuels release ancient biological production. Each energy bonanza supports greater population and complexity, but each also produces waste, inequality, conflict, and environmental disruption.

Sixth, the history of the Anthropocene shows that the human threshold remains incomplete. Humanity has acquired global power without fully developing global regulation. Human societies are simultaneously more capable and more vulnerable than any previous social systems.

The final transition is normative but grounded in the preceding history. Because human activity already regulates significant parts of the biosphere, nonintervention is impossible. Humanity must either manage its power more consciously or allow fragmented incentives to continue destabilizing the systems on which civilization depends.

The conclusion available at the end—but not at the beginning—is that human history may represent one phase of a still-unfolding threshold. Collective learning created agriculture, states, global networks, science, and industrialization. Its culminating achievement would not be unlimited growth but the stabilization of a sustainable planetary civilization.

Major Themes and Recurring Problems

1. Complexity Against the Background of Entropy

Every part of the book asks how organized structures emerge and persist despite entropy. Stars, cells, organisms, states, and cities all require continuous energy flows. Complexity is therefore temporary, costly, and vulnerable.

The theme deepens as the book progresses. In the cosmic chapters it is primarily physical; in biological chapters it involves metabolism and reproduction; in human history it becomes institutional and ecological. Modern civilization is extraordinarily complex because it depends on immense infrastructures, information systems, and energy flows. That complexity creates capability but also fragility.

2. Energy and Information as Complementary Drivers

Energy makes change possible; information directs it. Physical laws guide cosmic change, genetic information guides biological organization, and collectively accumulated knowledge guides human action.

Later thresholds require increasingly dense and carefully managed energy flows and increasingly elaborate information systems. Modern society combines fossil-fuel energy with science, bureaucracy, computers, and global communications. Its crisis arises partly because information about planetary danger does not automatically produce coordinated action.

3. Thresholds and Emergent Properties

Thresholds organize the narrative around qualitative transformation rather than uniform chronology. Stars are not simply larger atoms; life is not simply more elaborate chemistry; human culture is not simply another genetic adaptation.

The model emphasizes dependency. Each threshold uses structures created before it and generates possibilities unavailable at earlier stages. At the same time, the boundaries are heuristic rather than perfectly sharp. The emergence of life, humanity, farming, and the Anthropocene unfolds through long transitional processes.

4. Collective Learning and the Noösphere

Collective learning is the central mechanism of human history. It explains migration, adaptation, technology, agriculture, state formation, science, industry, and global coordination.

The noösphere expands from small Paleolithic exchange networks to a globally connected system of research institutions and computers. Yet cumulative knowledge does not imply collective wisdom. The same process produces medicine and nuclear weapons, renewable energy and industrial pollution.

5. Energy Bonanzas and Their Distribution

Photosynthesis, agriculture, and fossil fuels are transformative because they release or redirect major energy flows. Each bonanza increases the total complexity a system can support.

The benefits are never distributed automatically. Agricultural surplus supports elites and states while peasants remain poor. Fossil fuels improve mass welfare but also produce extreme inequalities. Energy availability is therefore distinct from social justice.

6. Creative Destruction

Major innovations undermine structures adapted to earlier conditions:

• oxygen photosynthesis poisons anaerobic life;
• mass extinctions destroy dominant ecosystems;
• agriculture displaces foraging systems;
• global trade devastates Indigenous populations;
• industrial production destroys artisan economies;
• fossil fuels destabilize the Holocene climate.

Creative destruction is neither wholly progressive nor wholly regressive. It generates new possibilities by disrupting existing arrangements, but its costs are real and frequently borne by those with the least power.

7. Planetary Feedbacks and Fragility

Earth’s habitability depends on feedback mechanisms connecting geology, atmosphere, oceans, and organisms. Life flourishes within a range of conditions that is broad enough for resilience but not unlimited.

Humanity is now interfering with carbon, nitrogen, water, and biodiversity systems without knowing all relevant thresholds. The same interconnectedness that stabilizes the biosphere can generate dangerous positive feedbacks after critical limits are crossed.

8. Shared History and Global Responsibility

The narrative continually expands the circle of identity: individual, family, species, biosphere, and cosmos. Christian presents this widening not merely as an intellectual exercise but as a response to nationalism and global fragmentation.

A planetary civilization needs a history adequate to the scale of its problems. The modern origin story is intended to cultivate awareness that different societies participate in one human project and depend on one biosphere.

Key Concepts and Distinctions

• Modern origin story: A global, scientifically grounded account connecting cosmic, biological, and human history.
  Function in the argument: Provides the overarching framework and civic purpose of the book.
  Development: Begins as an answer to fragmented knowledge and ends as a guide for planetary responsibility.
• Big history: Historical inquiry conducted across multiple disciplines and timescales, from the big bang to the present.
  Function in the argument: Allows human history to be explained through its cosmic and biological preconditions.
  Development: Moves from intellectual synthesis to a practical framework for thinking about the future.
• Complexity: Organization involving numerous components and intricate relationships.
  Function in the argument: Supplies the common object studied across cosmology, biology, and history.
  Development: Changes from physical structure to adaptive, informational, social, and planetary organization.
• Threshold: A major transition at which new structures and emergent properties appear.
  Function in the argument: Organizes an immense chronology around qualitative turning points.
  Development: Early thresholds describe completed physical transitions; threshold 9 remains conditional and future-oriented.
• Emergent property: A quality produced by a particular arrangement of components but absent from the components individually.
  Function in the argument: Explains novelty without requiring creation from nothing.
  Development: Applies successively to atoms, water, stars, life, consciousness, and social systems.
• Goldilocks conditions: A rare combination of materials, energy, stability, temperature, and other circumstances suitable for a new complexity.
  Function in the argument: Explains why thresholds occur in particular environments rather than everywhere.
  Development: Used first for physical and chemical environments, then for historical conditions such as the Holocene and global commerce.
• Free energy: Structured energy capable of performing work.
  Function in the argument: Explains how complex structures are built and maintained.
  Development: Appears as gravity, stellar radiation, metabolism, food, agricultural production, and fossil-fuel power.
• Entropy: The statistical tendency of organized energy and matter to become more disordered.
  Function in the argument: Establishes the universal constraint against which complexity must operate.
  Development: Begins as a thermodynamic law and becomes an analogy for the maintenance costs and fragility of societies.
• Complexity tax: The energy and resources required to preserve an organized system and dispose of its waste.
  Function in the argument: Explains why larger, more intricate systems require greater energy flows.
  Development: Extends from stars and organisms to governments, cities, infrastructure, administration, and pollution.
• Information: Differences, rules, or encoded instructions that influence outcomes.
  Function in the argument: Directs the changes that energy makes possible.
  Development: Moves from physical laws to genes, nervous systems, language, writing, science, and digital networks.
• Complex adaptive system: A system whose interacting components respond to one another and to changing environments.
  Function in the argument: Distinguishes organisms and societies from simpler physical structures.
  Development: Applies to cells, ecosystems, states, corporations, and global civilization.
• Natural selection: Environmental filtering of heritable variation.
  Function in the argument: Explains how biological complexity accumulates without foresight.
  Development: Provides a model for understanding cumulative processes while remaining distinct from intentional human learning.
• Collective learning: The accumulation and transmission of information across individuals and generations.
  Function in the argument: Explains the speed and power of human cultural evolution.
  Development: Expands from oral communities to writing, global science, computing, and the noösphere.
• Noösphere: The interconnected sphere of human thought, knowledge, and information.
  Function in the argument: Names the informational system through which humans increasingly influence the biosphere.
  Development: Begins with small linguistic networks and becomes global and electronically integrated.
• Anthropocene: The period in which human activity becomes a dominant planetary force.
  Function in the argument: Defines threshold 8 and the central problem of the present.
  Development: Divides into a Good Anthropocene of welfare gains and a Bad Anthropocene of inequality and ecological danger.

Essential Evidence and Examples

1. Expansion of the universe and cosmic microwave background radiation
   These are foundational evidence for big-bang cosmology. The background radiation is especially important because it was predicted before its discovery and is difficult for rival models to explain. The argument substantially depends on this evidence.
2. Stellar spectra and the Hertzsprung–Russell diagram
   Spectroscopy identifies the elements in stars, while comparisons of luminosity and temperature reconstruct stellar life cycles. This evidence supports the claim that stars manufacture and distribute heavier elements.
3. Radiometric dating of meteorites and terrestrial materials
   Radioactive decay provides the chronology of Earth and the solar system. Without these techniques, the book’s deep-time narrative could not be dated with confidence. This is structural rather than merely illustrative evidence.
4. Shared biological machinery and genetic sequences
   Common genetic codes, ribosomes, ATP systems, and other molecular features support the unity of life and the reconstruction of LUCA. The precise scenario for life’s origin remains inferential, but common ancestry is central to the argument.
5. The Great Oxygenation Event
   Geological evidence for rising oxygen demonstrates that microorganisms transformed planetary chemistry. It is indispensable to Christian’s claim that life is a geological force long before humans.
6. Plate tectonics and the geological carbon thermostat
   Seafloor spreading, paleomagnetism, subduction, and volcanic cycling explain Earth’s long-term dynamism and partial climate regulation. This evidence links geology to the conditions required for life.
7. Mass-extinction evidence and the Chicxulub impact
   The iridium layer and impact crater support the explanation for the disappearance of non-avian dinosaurs. The case demonstrates the importance of contingency: mammalian and human history depended on an external catastrophe.
8. Archaeological and genetic evidence of human dispersal
   Tools, fossils, settlement remains, and genomes show humans moving into diverse environments and interacting with other hominins. This evidence supports the practical power of collective learning.
9. Natufian and Abu Hureyra evidence for agricultural transition
   Settlement patterns, plant remains, skeletal health, and climatic change reveal farming as a gradual and often costly adaptation rather than an uncomplicated advance. The cases substantially support Christian’s interpretation of threshold 7.
10. Writing, accounting, taxation, and urban storage in early states
    Administrative records show that information systems developed partly to mobilize agricultural surplus. These examples connect material production with state complexity.
11. Steam engines, coal production, and industrial feedback loops
    The relationship between deeper mines, pumping technology, cheaper coal, transportation, and industrial production demonstrates how an energy innovation can reorganize an entire social system.
12. Great Acceleration indicators
    Population, energy use, atmospheric carbon dioxide, fertilizer production, urbanization, and biodiversity loss all increase sharply after the mid-twentieth century. These measurements are indispensable to the claim that humanity has become a planetary force.

Major Causal Relationships

• Slight density differences → gravitational concentration → stars and galaxies
  Evidence: Cosmic structure, gravitational theory, and observations of star formation.
  Qualification: Gravity requires initial variations and sufficient matter; it does not produce identical structures everywhere.
• Stellar fusion and collapse → nucleosynthesis → chemical diversity
  Evidence: Stellar spectra, supernova observations, and models of stellar evolution.
  Qualification: Different elements arise through different processes, including neutron-star mergers.
• Chemical richness + liquid water + energy gradients → chemical evolution → early life
  Evidence: Organic molecules in experiments and meteorites, shared molecular machinery, and plausible hydrothermal environments.
  Qualification: The exact path from chemistry to life remains unresolved.
• Variation + inheritance + environmental filtering → natural selection → biological adaptation
  Evidence: Genetic relationships, observed adaptation, and the fossil record.
  Qualification: Selection is not goal-directed and does not guarantee greater complexity.
• Oxygenic photosynthesis → atmospheric oxygen → new metabolisms and ecological restructuring
  Evidence: Geological oxidation signatures and atmospheric history.
  Qualification: Oxygen initially caused widespread destruction and required a long accumulation process.
• Enhanced language → collective learning → cumulative cultural adaptation
  Evidence: Human technological expansion, symbolic culture, migration, and intergenerational knowledge.
  Qualification: Language was one element in a larger package of biological and social adaptations.
• Holocene stability + ecological knowledge + population pressure → cultivation and domestication → agriculture
  Evidence: Independent agricultural origins and transitional archaeological sites.
  Qualification: Causes varied by region; there was no single universal agricultural revolution.
• Agricultural surplus → specialization and appropriation → cities, hierarchy, and states
  Evidence: Storage, monumental construction, administrative writing, taxation, and urban settlement.
  Qualification: Surplus was necessary but not sufficient; institutions and coercive arrangements differed greatly.
• Global interconnection + commercial competition + scientific exchange → accelerated innovation → modern world economy
  Evidence: Printing, global trade, imperial expansion, finance, and scientific institutions.
  Qualification: Innovation was inseparable from conquest, slavery, and unequal exchange.
• Fossil-fuel access → massive energy bonanza → industrialization and population growth
  Evidence: Coal and oil production, mechanization, transportation, and energy statistics.
  Qualification: Institutions, geography, capital, knowledge, and state power shaped where industrialization began.
• Industrial growth → rising welfare and consumption + biospheric disruption → Anthropocene duality
  Evidence: Life expectancy, production, emissions, biodiversity, and resource-use trends.
  Qualification: Benefits and damages are distributed unequally across regions, classes, and generations.

Major Comparisons and Contrasts

Traditional and Modern Origin Stories

Both integrate knowledge and locate humans within a larger order. Traditional stories are generally rooted in particular communities and often connect cosmic explanation with sacred authority. The modern story is global, scientific, provisional, and without a creator or externally supplied purpose.

The comparison supports Christian’s claim that scientific history can perform some orienting functions of older stories while remaining epistemically different from them.

Physical Structures and Complex Adaptive Systems

Stars maintain equilibrium but do not interpret changing local information in the biological sense. Organisms actively sense, respond, repair, and reproduce.

The distinction marks threshold 5 and explains why information becomes increasingly important after life appears.

Genetic Evolution and Collective Learning

Genetic evolution stores information biologically and changes over generations. Collective learning stores information culturally and can change within a lifetime.

Human history accelerates because cultural adaptations do not require corresponding genetic mutations. Yet cultural information can spread harmful as well as beneficial practices.

Foraging and Farming

Foraging generally supports small, mobile populations with broad ecological knowledge. Farming supports denser, more settled populations and larger surpluses but often demands more labor and creates greater exposure to disease and hierarchy.

The comparison undermines any simple identification of agriculture with immediate human progress.

Agrarian and Industrial Energy Regimes

Agrarian societies depend on recent photosynthesis and are constrained by land. Industrial societies tap ancient photosynthesis stored in fossil fuels and can mobilize vastly greater energy.

The contrast explains why industrialization produced not merely larger agrarian civilizations but a qualitatively different social and planetary system.

Good and Bad Anthropocene

The Good Anthropocene consists of health, knowledge, longevity, and abundance. The Bad Anthropocene consists of inequality, ecological degradation, and existential risks.

They are not separate historical periods. They are simultaneous consequences of the same energy-intensive system.

Endless Growth and Dynamic Stability

The dominant industrial model equates improvement with continuous material growth. Christian’s mature Anthropocene would stabilize population and material throughput while allowing cultural, scientific, moral, and experiential development to continue.

This comparison is central to the possible transition from threshold 8 to threshold 9.

Method and Use of Evidence

Christian uses an explicitly interdisciplinary method. The narrative combines:

• cosmology and particle physics;
• astronomy and chemistry;
• geology and Earth-system science;
• evolutionary biology and genetics;
• archaeology and anthropology;
• economic, political, and social history;
• energy and population statistics;
• future modeling.

The book moves between several forms of reasoning. In the natural sciences, Christian relies on established paradigms such as big-bang cosmology, plate tectonics, and natural selection. He explains how these paradigms gained support through prediction, measurement, and convergence among independent evidence.

For events that cannot be observed directly, he reconstructs processes from surviving traces: radiation, spectra, isotopic decay, fossils, genetic divergence, archaeological remains, and written records. Representative cases illustrate larger mechanisms, while quantitative comparisons reveal differences in scale.

The threshold framework is a synthetic device rather than an empirical discovery of a single discipline. It makes structural similarities visible across very different fields, especially relationships among energy, information, environment, and complexity.

Christian generally signals major uncertainties. Ultimate cosmic origins, the precise origin of life, the exact development of language, and future threshold 9 remain unresolved. The broader the chronological and disciplinary sweep, however, the more selective the evidence must become. The book depends on authoritative synthesis rather than original research in each field.

Qualifications, Tensions, and Limitations

1. Complexity is not moral progress. More complex systems may be exploitative, fragile, destructive, or short-lived. Christian repeatedly warns against treating complexity as automatic improvement.
2. Thresholds simplify gradual processes. Star formation, life, humanity, agriculture, and industrialization do not occur at perfectly discrete moments. Thresholds are maps of transition rather than sharp natural borders.
3. The origin of life remains incomplete. Hydrothermal vents, RNA systems, and LUCA provide plausible components, not a complete experimentally verified sequence.
4. Energy-rate comparisons are approximate. The association between complexity and denser energy flows is illuminating but does not provide a single precise measurement applicable to all systems.
5. Collective learning is ethically neutral. It produces vaccines, farming, and science, but also slavery, imperialism, ecological destruction, and nuclear weapons.
6. Human progress is unevenly distributed. Population, state power, and total wealth can increase while many individuals become less healthy or remain poor.
7. The global origin story has its own historical location. Christian acknowledges that it emerged within modern scientific and capitalist societies and may inherit their ecological and cultural blind spots.
8. The future argument is more normative and speculative than the historical chapters. Threshold 9 is a proposed possibility, not an established trend or inevitable outcome.

Overall Conclusion and Significance

Origin Story ultimately establishes that human history cannot be understood in isolation. Every human institution depends on a sequence of earlier transformations: the formation of matter, the birth and death of stars, the production of chemical elements, the creation of Earth, the evolution of cells, the oxygenation of the atmosphere, the appearance of multicellular organisms, and the development of primate brains.

Across these transformations, a recurrent pattern appears. New complexity arises when suitable materials encounter controlled energy flows under rare Goldilocks conditions. The resulting structures possess emergent properties, but they remain temporary and must expend energy to resist entropy.

Life adds a new dimension because organisms use information to preserve themselves, reproduce, and adapt. Humanity adds another because collective learning allows information to accumulate outside genomes. This capacity explains both the astonishing achievements and the escalating dangers of human history.

Agriculture and fossil fuels represent successive enlargements of humanity’s energetic base. Agriculture produced states and civilizations but left most people near subsistence. Fossil fuels created enough surplus to improve life for billions, yet they also gave humans the ability to alter climate, reduce biodiversity, and destroy civilization through warfare.

The book’s most important intellectual contribution is the integration of these processes into one explanatory architecture. Modern society is not portrayed as an inexplicable rupture from nature. It is a new form of complex adaptive organization built from the same matter, energy, and information that shaped earlier thresholds. Human history is continuous with cosmic and biological history without being reducible to them.

Following the argument should change the reader’s sense of scale and dependence. Modern civilization appears powerful when compared with earlier societies but fragile when viewed as a temporary structure resting on planetary feedback systems accumulated over billions of years. Human beings are not outside the biosphere, and economic activity is not separate from energy and material flows.

Christian’s conclusion is nevertheless not simply pessimistic. Collective learning has created the dangers of the Anthropocene, but it also provides the only plausible means of understanding and managing them. A scientifically informed, globally shared origin story may help humanity recognize that national and cultural differences exist within a deeper common history.

The conclusion must remain conditional. Nothing in cosmic history guarantees justice, sustainability, or survival. Entropy will eventually destroy every structure, but this remote fact does not eliminate agency at human scales. The immediate question is whether humanity can convert knowledge into coordination and power into stewardship.

If it succeeds, the modern era may eventually be understood not as the beginning of endless material expansion but as the turbulent formation stage of a more stable planetary civilization. Threshold 9 would occur when collective learning becomes capable of consciously preserving both human flourishing and the biosphere that makes it possible.

Book-Level Keywords

Keywords: big history; modern origin story; increasing complexity; thresholds; emergent properties; Goldilocks conditions; free energy; entropy; complexity tax; information; complex adaptive systems; big bang; stellar nucleosynthesis; biosphere; natural selection; LUCA; photosynthesis; collective learning; noösphere; agriculture; agrarian civilizations; surplus; fossil fuels; Great Acceleration; Anthropocene; Good Anthropocene; Bad Anthropocene; planetary boundaries; mature Anthropocene; threshold 9

Questions for Reflection

1. Does the concept of increasing complexity genuinely explain the book’s diverse historical processes, or does it primarily provide a useful way of organizing them?
2. Where does Christian’s threshold model clarify gradual change, and where might it create artificial breaks between continuous processes?
3. How persuasive is the analogy between the energy requirements of stars, cells, and human societies?
4. Does collective learning adequately explain human distinctiveness, or does it depend on biological and social capacities that deserve equal emphasis?
5. Why did agricultural and fossil-fuel energy bonanzas produce such different effects on average human welfare?
6. To what extent are hierarchy and inequality necessary consequences of surplus and complexity, rather than products of particular institutions?
7. Can the Good Anthropocene realistically be separated from the energy consumption and extraction that created the Bad Anthropocene?
8. Does a shared scientific origin story provide meaningful ethical guidance, or does Christian introduce moral goals that cannot be derived from the history itself?
9. How does the book balance contingency—such as asteroid impacts and climatic disruptions—with recurring mechanisms such as natural selection and collective learning?
10. Is a stable, low-growth mature Anthropocene compatible with modern capitalism, national competition, and existing political time horizons?
11. Which planetary boundaries are most important to Christian’s argument, and how should uncertainty about their precise location affect action?
12. If threshold 9 is never crossed, would this undermine the book’s interpretation of human history as a continuing rise in complexity, or confirm its emphasis on fragility and entropy?

Kindle Highlights

Preface

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Even earlier, but in the same spirit, H. G. Wells wrote a history of humanity as a response to the carnage of World War I. There can be no peace now, we realize, but a common peace in all the world; no prosperity but a general prosperity. But there can be no common peace and prosperity without common historical ideas.… With nothing but narrow, selfish, and conflicting nationalist traditions, races and peoples are bound to drift towards conflict and destruction. 2

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For many, nationalism offered some sense of belonging, but in today’s globally connected world, it is apparent that nationalism divides humanity even as it connects citizens within a particular country.

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Within the creative hurricane of modernity, there is emerging a new, global origin story that is as full of meaning, awe, and mystery as any traditional origin story but is based on modern scientific scholarship across many disciplines.

Introduction

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Utterly impossible as are all these events they are probably as like those which may have taken place as any others which never took person at all are ever likely to be.—JAMES JOYCE, FINNEGANS WAKE We arrive in this universe through no choice of our own, at a time and place not of our choosing. For a few moments, like cosmic fireflies, we will travel with other humans, with our parents, with our sisters and brothers, with our children, with friends and enemies.

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We can all benefit from the maps our ancestors created. The great French sociologist Émile Durkheim insisted that the maps lurking within origin stories and religions were fundamental to our sense of self. Without them, he argued, people could fall into a sense of despair and meaninglessness so profound, it might drive them to suicide. No wonder almost all societies we know of have put origin stories at the heart of education. In Paleolithic societies, students learned origin stories from their elders, just as later scholars learned the core stories of Christianity, Islam, and Buddhism in the universities of Paris, Oxford, Baghdad, and Nalanda.

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Yet, curiously, modern secular education lacks a confident origin story that links all domains of understanding. And that may help explain why the sense of disorientation, division, and directionlessness that Durkheim described is palpable everywhere in today’s world, in Delhi or Lima as much as in Lagos or London.

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The problem is that in a globally connected world, there are so many local origin stories competing for people’s trust and attention that they get in one another’s way.

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The universe of the modern origin story is restless, dynamic, evolving, and huge. The geologist Walter Alvarez reminds us how big it is by asking how many stars it contains. Most galaxies have something like 100 billion stars, and there are at least that many galaxies in the universe. That means that there are (deep breath) 10,000,000,000,000,000,000,000 (1022) stars in the universe. 4 New observations in late 2016 hinted that there may be many more galaxies in the universe, so feel free to add a few more zeros to this number. Our sun is a pretty ordinary member of that huge gang.

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heart of the modern origin story is the idea of increasing complexity.

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The early universe had no stars, no planets, and no living organisms. Then, step by step, entirely new things began to appear. Stars were forged from atoms of hydrogen and helium, new chemical elements were created inside dying stars, planets and moons formed from blobs of ice and dust using these new chemical elements, and the first living cells evolved in the rich chemical environments of rocky planets.

Chapter 1 In the Beginning: Threshold 1

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To make an apple pie from scratch, you must first invent the universe.—CARL SAGAN, COSMOS

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Bootstrapping origin stories is almost as hard as bootstrapping universes. One possible approach is to vanish the problem of beginnings by assuming the universe was always there. No bootstrapping needed.

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Is the idea of an eternal or unchanging universe satisfying? Not really, particularly if you have to smuggle in a creator to kick-start the process, as in “In the beginning there was nothing, then God made…” The logical glitch is obvious, though it has taken some sophisticated minds a long time to see it clearly. At the age of eighteen, Bertrand Russell gave up on the idea of a creator god after reading the following passage in the autobiography of John Stuart Mill: “My father taught me that the question, ‘Who made me?’ cannot be answered, since it immediately suggests the further question, ‘Who made God?’” 2

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We don’t know what Goldilocks conditions allowed a universe to emerge, and we still can’t explain it any better than novelist Terry Pratchett did when he wrote, “The current state of knowledge can be summarized thus: In the beginning, there was nothing, which exploded.” 7

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cosmic microwave background radiation—

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The bare-bones story goes like this: Our universe began as a point smaller than an atom. How small is that? Our species’ minds evolved to deal with things at human scales, so they struggle with things this tiny, but it might help to know that you could squeeze a million atoms into the dot at the end of this sentence. 9 At the moment of the big bang, the entire universe was smaller than an atom. Packed into it was all the energy and matter present in today’s universe. All of it. That is a daunting idea, and at first it might appear plain crazy. But all the evidence we have at present tells us that this strange, tiny, and fantastically hot object really existed about 13.82 billion years ago.

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something really can appear in a vacuum from nothing,

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In modern quantum physics, it is impossible to determine precisely the position and motion of subatomic particles. This means you can never say for sure that a particular region of space is empty, and that means that emptiness is tense with the possibility that something might appear. Like the “neither non-existence nor existence” of the Indian Vedas, this tension seems to have bootstrapped our universe.

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Within a billionth of a billionth of a billionth of a billionth of a second after the big bang, energy itself underwent a phase change. It split into four very different species. Today, we know them as gravity, the electromagnetic force, and the strong and weak nuclear forces.

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Gravity is weak, but it reaches across vast distances and always pulls things together, so its power accumulates. It tends to make the universe more clumpy. Electromagnetic energy comes in negative and positive forms, so it often cancels itself out. Gravity, though puny, shapes the universe on a large scale. But electromagnetism dominates at the level of chemistry and biology, so it’s what holds our bodies together. The third and fourth fundamental forces are known, unexcitingly, as the strong and weak nuclear forces. They reach over tiny distances, so they matter on a subatomic scale. We humans don’t experience them directly, but they shape every aspect of our world because they determine what happens deep inside atoms.

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Today, the mass of this energy may account for as much as 70 percent of the total mass of the universe. But even if it is beginning to dominate our universe, we don’t yet understand what this energy is or how it works, so physicists call it dark energy. The term is a placeholder. Watch this space, because understanding dark energy is one of the great challenges of contemporary science.

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We now know that matter is really a highly compressed form of energy. The young Albert Einstein demonstrated this in a famous paper in 1905. That formula—energy (E) is equal to mass (m) times the speed of light (c) squared, or E = mc2—tells us how much energy is compressed inside a given amount of matter.

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So the modern origin story has to find other ways of explaining the emergence of structures and forms. And that’s not easy, because the second law of thermodynamics tells us that sooner or later, all structures will eventually break down. As the Austrian physicist Erwin Schrödinger wrote: “We now recognize this fundamental law of physics to be just the natural tendency of things to approach the chaotic state (the same tendency that the books of a library or the piles of papers and manuscripts on a writing desk display) unless we obviate it.”

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If there is a bad guy in the modern origin story, it is surely entropy, the apparently universal tendency for structures to dissolve into randomness. Entropy is the loyal servant of the second law of thermodynamics. So, if we think of entropy as a character in our story, we should imagine it as dissolute, lurking, careless of others’ pain and suffering, not interested in looking you in the eye. Entropy is also very, very dangerous, and in the end it will get us all. Entropy stands at the finale of all origin stories. It will dissolve away all structures, all shapes, every star and every galaxy and every living cell. Joseph Campbell described entropy’s role poetically in a book on mythology: “The world, as we know it… yields but one ending: death, disintegration, dismemberment, and the crucifixion of our heart with the passing of the forms that we have loved.”

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fundamental laws of physics. They explain why the frantic and chaotic energies of the primeval atom were not completely directionless—the laws of physics steered change down particular pathways and blocked a nearly infinite range of other possibilities.

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Neutrons, protons, and electrons, the basic constituents of atoms, appeared within seconds of the big bang, as did proton and electron antiparticles (that is, negatively charged protons and positively charged electrons), forming what physicists call matter and antimatter.

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As temperatures fell, matter diversified. Electrons and neutrinos were ruled by electromagnetism and the weak nuclear force. The protons and neutrons that form atomic nuclei were made from triplets of strange particles known as quarks, bound together by the strong nuclear force.

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As nanophysicist Peter Hoffmann writes: “Tempered by physical law, which adds a dash of necessity, chance becomes the creative force, the mover and shaker of our universe. All the beauty we see around us, from galaxies to sunflowers, is the result of this creative collaboration between chaos and necessity.”

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As the universe cooled, the electromagnetic force tugged negatively charged electrons toward positively charged protons until the electrons calmed down enough to fall into orbits around protons. And voilà! We had the first atoms, the basic constituents of all the matter around us.

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Most astronomers since Newton’s time assumed that the universe must be infinite, because if it was not, the laws of gravity should have gathered its contents into a single gluggy mass, like oil in a sump.

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Most astronomers were shocked by the idea of an expanding universe and assumed there was an error in Hubble’s calculations. Hubble himself was not at all sure about it, and Einstein was so convinced the universe was stable that he fiddled with the equations of general relativity so they would predict a stable universe, by adding what he called a cosmological constant.

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Today we know of no astronomical objects older than 13.82 billion years, which is a strong argument in favor of big bang cosmology.

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predicted that the universe would have eventually cooled enough for atoms to form, and when the first atoms formed, there should have been a huge release of energy as photons escaped the charged plasma of the preatomic era and began to flow freely through an electrically neutral universe. Further, they argued that this flash of energy should still be detectable, though its frequency would have fallen to near zero as it was stretched across an expanding universe. If scientists looked carefully enough, they would find radiation at temperatures close to absolute zero coming from all directions. To many this seemed a crazy idea, which was why no one started looking for low-temperature radiation pervading the entire universe.

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The discovery of cosmic microwave background radiation persuaded most astronomers that the big bang was real because no other theory could explain this all-pervading radiation.

Chapter 2 Stars and Galaxies: Thresholds 2 and 3

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For a dazzling few moments, there was enough energy to make and unmake exotic new forms of energy and matter. But as temperatures plummeted, energy and matter froze into a few simple structures. In the kiln of the big bang, forces and particles stabilized like fired pottery.

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Living organisms, including you and me, are held together by delicate and precisely directed flows of energy managed by intricate metabolic processes in our cells. In a post–big bang universe, it takes work to build and maintain new complex structures.

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deep link between form, complexity, and directed or structured flows of energy.

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The first law of thermodynamics tells us that the total amount of energy in the universe never changes. It is conserved. Our universe seems to have arrived with a fixed potential for things to happen. So the first law is really telling us about the primordial ocean of possibilities. The second law of thermodynamics tells us that the things that emerge from the ocean of possibilities can be more or less structured, like the ripples in a stream. But we should expect most of them to be less structured and become even less structured over time. That is because most possible arrangements of matter and energy have little or no structure, and if by chance you do find structure, expect it to decay fast.

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Free energy drove the emergence of the first large structures: galaxies and stars.

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Like a cosmological sheepdog, gravity likes to herd things. And the things it herded were the simple forms of matter created in the big bang. Together, gravity and matter provided the Goldilocks conditions for the emergence of stars and galaxies.

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From the point of view of gravity, the early universe was too smooth. It needed to be clumped up. This tendency of gravity to rearrange the universe explains why we can think of the early universe as having low entropy, a sort of tidiness that entropy could mess up over the next few billion years. Once it got going, gravity took just a few hundred million years to turn the smooth particle mist of the early universe into a messier and lumpier space full of stars and galaxies.

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As gravity forced atoms together, they collided more often and jiggled more frenetically. That drove up temperatures in the clumpier regions, as more heat was concentrated in smaller volumes of space.

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Each star poured energy into the cold spaces around it, generating flows of heat, light, and chemical energy that could be used to build new forms of complexity in nearby regions. Those are the flows of free energy that allow life to flourish here on planet Earth. Gravity had kick-started the transformation of matter into stars by fusing protons despite the barrier created by their positive charges.

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In the story of star formation, gravity provided the activation energy for fusion and star formation and for all that followed.

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more complex phenomena need more dense flows of energy, more energy per gram per second.

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Entropy loves this deal because the energy that props up a star, like the energy of a waterfall, eventually degrades when it is released into space. So, while the star is getting more complex, it’s also helping entropy degrade free energy into heat energy. This is something we will see throughout the modern origin story. Increasing complexity is not a triumph over entropy. Paradoxically, the flows of energy that sustain complex things (including you and me) are helping entropy with its bleak task of slowly breaking down all forms of order and structure.

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Hydrogen and helium were the first elements to be made because they are the simplest. Hydrogen has one proton in its nucleus, so we say it has atomic number 1. Helium has two protons in its nucleus, so its atomic number is 2. When the CMBR was emitted, about 380,000 years after the big bang, there was also a sprinkling of lithium (atomic number 3) and beryllium (atomic number 4).

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stars manufacture new elements in their death throes,

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When stars get very old, eventually they run out of free protons and their cores start filling up with an ash of fused protons—in other words, helium nuclei. Fusing helium nuclei requires much higher temperatures than fusing single protons, so eventually, the furnaces at their core stop working. When that happens, gravity takes over, and the stars collapse under their own mass. But that’s not yet the end of the story. After a star collapses, it heats up again as gravity piles on the pressure. Far from the core, the star’s outer layers expand and cool to keep everything in balance. To us, these cooler outer layers look red, which is why we call stars at this stage red giants. When our sun reaches this stage, it will expand to about two hundred times its present size and vaporize the inner planets, including Earth.

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And that’s the end of the road, because you cannot generate energy by fusing iron nuclei. Eventually, most stars will blast away their outer layers and end up as white dwarfs, which are down in the bottom left corner of the Hertzsprung-Russell diagram. White dwarfs are stellar zombies, with no furnace at their heart. They are extremely dense, often the size of Earth, but with the mass of the sun. If you try to lift a teaspoon of white-dwarf stuff, you’ll fail, because it weighs at least a ton. 4 Though still hot, their corpses will cool over billions of years. But they have done their work by fertilizing their surroundings with new elements. Some white dwarfs die more spectacularly in vast supernova explosions if they get sucked into nearby stars. These explosions are so hot that they can create many of the elements in the periodic table. The spectacular death-by-explosions of white dwarfs generates what are known as type 1a supernovas. These all blow up at about the same temperature, so if you see one, you know how bright it is, and that means you can estimate its true distance. Type 1a supernovas allow astronomers to estimate distances hundreds of times farther away than Cepheid variables. Stars more than about seven times the mass of our sun will also end their lives spectacularly in another type of explosion, known as a core-collapse supernova. When the core has formed a ball of iron larger than our sun, the central furnace will shut down for the last time. Gravity will smash the core together in a fraction of a second and with extreme violence, creating energies and temperatures higher than ever before in the star’s lifetime. The star will explode in a supernova and may briefly emit as much energy as an entire galaxy. In just a few minutes, it manufactures many of the remaining elements of the periodic table and blasts them into space. Perhaps the most famous example of a core-collapse supernova lies at the heart of the Crab Nebula. Betelgeuse could go supernova at any time in the next million years.

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A neutron star just twenty kilometers across would weigh twice as much as our sun, and a teaspoon of neutron-star stuff would weigh a billion tons.

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In these different ways, dying stars enriched and fertilized the young universe. Once forged in dying stars and supernovas, the elements of the periodic table gathered in huge dust clouds between stars; atoms combined to form simple molecules, and, by a sort of fermentation, they brewed up new forms of matter.

Chapter 3 Molecules and Moons: Threshold 4

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Atoms are tiny. You could pack a million carbon atoms into the dot at the end of this sentence. But don’t think of atoms as solid balls of matter. They consist almost entirely of empty space. Each has a tiny nucleus in its center made up of protons (with positive charges) and neutrons (which have no charge) bound together by the strong nuclear force. The rest of the atom is mostly empty. Orbiting the nuclei at huge distances are clouds of electrons, roughly one to each proton in the nucleus. Early in the twentieth century, Ernest Rutherford, one of the pioneers of modern nuclear physics, described the nucleus of an atom as “the fly in the cathedral.”

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Chemists study these flirtations and rivalries and the liaisons and tensions they create as electrons hitch up to neighboring atoms to form molecules linking several atoms, sometimes millions or even billions of them, into structures more complex than the most complex star.

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basic rules of chemistry seem to be universal. We know this because spectroscopes show that many of the simple molecules we find on Earth also exist in interstellar dust clouds. But interstellar chemistry seems to be fairly simple; no interstellar molecules detected so far have more than about a hundred atoms.

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we now know that simple organic molecules are common in the universe, and that increases the likelihood that life exists beyond planet Earth.

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Hydrogen and helium still made up almost 98 percent of those clouds, but it was the remaining 2 percent that made the difference. As in the early universe, gravity liked to make these clouds more clumpy. In our region, gravity may have been helped by a nearby supernova explosion that shook things up and started the contraction of a huge cloud of gas and dust about 4.567 billion years ago.

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Two processes turned a spinning disk of matter into planets, moons, and asteroids. The first was a type of chemical sorting. Violent bursts of charged particles from the young sun, known as the solar wind, blasted lighter elements, such as hydrogen and helium, away from the inner orbits to create two distinct regions. The outer regions of the young solar system, like most of the universe, consisted mainly of the primordial elements, hydrogen and helium. But the inner regions, where the rocky planets—Mercury, Venus, Earth, and Mars—would form, lost so much hydrogen and helium that they had a rare chemical diversity.

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The second process that formed our solar system was accretion. Within different orbits around the young sun, bits of matter slowly gathered together. In the gassier outlying regions, this was probably a fairly gentle process. Gravity collected matter into huge gassy planets, such as Jupiter and Saturn, that consisted mostly of hydrogen and helium with a thin sprinkling of dust and ice. In the inner regions, though, accretion was a more violent and chaotic process, because here, a lot more matter was solid. Particles of dust and ice stuck together to form small blobs of rock and ice, which careened around, sometimes smashing each other into pieces, sometimes sticking together to form larger objects. Eventually, even larger objects appeared, like meteors and asteroids, and within each orbit, these smashed into each other or merged to form objects so large that their gravity could sweep up most of the remaining debris. Eventually, these processes generated the planets we see today, spaced out in distinct orbits around the sun.

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Collisions continued for a long time, even as the solar system stabilized. Indeed, our own moon was probably formed from a collision between the young Earth and a Mars-size protoplanet (Theia) about one hundred million years after the birth of the solar system. That collision sent huge clouds of matter into orbit around Earth, where they probably circled like the rings of Saturn (which might also be the debris from a smashed-up moon) until they accreted to form our moon.

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But all planetary bodies are cooler than stars, and chemically richer and more diverse, and that’s why they provided Goldilocks conditions that allowed the building of new forms of complexity. Eventually, at least one of these objects, and probably many more, generated life.

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In 2014, it was found that the Local Supercluster is part of a vast cosmic empire with perhaps a hundred thousand galaxies, and to cross that would take you four hundred million years traveling at the speed of light. This empire is the Laniakea (Hawaiian for “immeasurable heaven”) Supercluster. At present, this is the largest structured entity we know of in the universe. We assume that Laniakea is built around a scaffolding of dark matter whose gravitational pull holds all these galaxies together as the universe expands.

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The deepest dig so far is about twelve kilometers, which is about 0.2 percent of the distance to Earth’s core.

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Rutherford realized that radioactive breakdown could provide a sort of geological clock if you could measure how much a given sample had decayed. In 1904, he tried to measure the breakdown of a sample of uranium and came up with a figure of about five hundred million years for the age of Earth. The basic idea was right, but his estimate of Earth’s age was controversial because it was much older than the accepted age of less than one hundred million years.

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Earth was about 4.5 billion years old,

Chapter 4 Life: Threshold 5

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What lies at the heart of every living thing is not a fire, not warm breath, not a “spark of life.” It is information, words, instructions.… If you want to understand life, don’t think about vibrant, throbbing gels and oozes, think about information technology.—RICHARD DAWKINS, THE BLIND WATCHMAKER

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The spooky thing about life is that, though the inside of each cell looks like pandemonium—a sort of mud-wrestling contest involving a million molecules—whole cells give the impression of acting with purpose. Something inside each cell seems to drive it, as if it were working its way through a to-do list. The to-do list is simple: (1) stay alive despite entropy and unpredictable surroundings; and (2) make copies of myself that can do the same thing. And so on from cell to cell, and generation to generation. Here, in the seeking out of some outcomes and the avoidance of others, are the origins of desire, caring, purpose, ethics, even love. Perhaps even the beginnings of meaning, if that means the ability to discriminate between the significance of different events and signs. What is the meaning of this great white shark cruising behind me?

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This is the “power rating” of a human being: 120 watts, just slightly greater than the power rating of many traditional lightbulbs.

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Life, with its never-ending attempts to push back against entropy, represents a new type and level of complexity. Complexity theorists sometimes describe entities at this level as complex adaptive systems. Unlike the complex physical systems we have seen so far, the components of which behave in ways that can usually be predicted from the universe’s basic operating rules, the components in complex adaptive systems seem to have a will of their own. They appear to follow additional rules that are harder to detect. Indeed, complex adaptive systems, such as bacteria, your dog, or multinational companies, act as if every component is an agent with a will of its own, so each component is constantly adjusting to the behavior of many other components. And that yields extremely complex and unpredictable behaviors. 2

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In its most general form, information consists of rules that affect outcomes by limiting possibilities. One of the most famous definitions of information is “a difference which makes a difference.” 4 Rules determine which changes out of all conceivable options are actually possible at a given time and place, and that makes a difference. Information begins with the laws of physics, the basic operating system of our universe. The laws of physics steer change down particular pathways, like the pathways by which gravity created the first stars. Information in this very general sense limits what is possible, so it reduces randomness. This is why more information seems to mean less entropy, less potential for the disorder that entropy loves. This is universal information: the rules built into every smidgen of matter and energy. No one needed to tell gravity what to do; it just got on with the job.

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The philosopher Daniel Dennett writes: “Animals are not just herbivores or carnivores. They are… informavores.” 5 In fact, all living organisms are informavores. They all consume information, and the mechanisms they use for reading and responding to local information—whether they are eyes and tentacles or muscles and brains—account for much of the complexity of living organisms.

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Entropy, of course, keeps a beady eye on all of this. If more complexity means more information, then when you increase complexity and information, you are reducing entropy and its accompanying uncertainty or disorder. And entropy will notice. Entropy is rubbing its hands at the thought of the energy taxes and fees it can levy as complexity and information increase.

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Over time, as some individuals are selected by “nature” (in fact, by the rules of the local environment), a new species will eventually emerge. Here, as Darwin showed, is the basic mechanism of biological evolution. This is Darwin’s complexity ratchet; this is how life builds more and more complex things, step by step.

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So what are the Goldilocks conditions for chemical experimentation? And why did the young Earth exhibit so many of them? First, our solar system is in the right part of the Milky Way galaxy.

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Second, chemistry works well only at lower temperatures.

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A third Goldilocks condition for rich chemistry is the presence of liquids.

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A fourth Goldilocks condition for rich chemistry is chemical diversity.

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first organisms lived more than three billion years ago,

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bacteria that lived about 3.4 billion years ago.

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In living organisms, energy has a new function that we don’t find in stars: it creates copies of the cell. These copies allow cells to push back against entropy by preserving their complex structures even after individual cells have died. Luca’s descendants evolved the elegant and efficient methods of reproduction that all living things still use today. Those methods are built on a key molecule, DNA, whose structure was first described in 1953 by Francis Crick and James Watson based on earlier research done by Rosalind Franklin. So much of evolution depends on understanding how DNA works that it is worth looking more carefully at this marvelous molecule.

Chapter 5 Little Life and the Biosphere

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The lightest rocks rose to the surface and formed Earth’s crust. Gases and water vapor bubbled up through volcanoes to create Earth’s first atmosphere and oceans. Meteors and asteroids ferried in new cargoes of rocks, minerals, water, gases, and organic molecules.

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Heat from the core still drives a lot of geology and will continue to do so for billions of years. But not until the 1960s did geologists figure out how this huge geological machine worked. Their new understanding of geology was based on one of modern science’s most important paradigms: plate tectonics.

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We now have abundant evidence that Earth’s crust, both oceanic and continental, is broken into distinct plates that jostle for position as they are dragged back and forth by the semimolten magma on which they float.

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This is how the Andes formed, as the Pacific plate burrowed beneath the plate carrying the west coast of South America.

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It generated new types of rocks and minerals. By the time life began to flourish on land, chemical processes within the mantle had already created as many as fifteen hundred distinct types of minerals.

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temperatures at Earth’s surface: heat from the interior and heat from the sun.

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Stars like our sun emit more and more energy as they age, so the amount of heat arriving on Earth has slowly increased. When Earth was young, the sun was emitting 30 percent less energy than today.

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A greenhouse atmosphere was one more important Goldilocks condition for life on the young Earth.

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Where tectonic plates dive back into the mantle at subduction zones, some of this carbon (much of it in the form of limestone) can get buried in the mantle for millions, even billions of years. In this way, the tectonic conveyor belt removes carbon from the atmosphere, and that should eventually reduce carbon dioxide levels and generate colder climates.

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neither Mars nor Venus seems to do plate tectonics, which deprives them of a key component of our planet’s thermostat.

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So we can say with some confidence that humans and chimps shared a common ancestor about seven or eight million years ago, while humans and bananas have followed different genetic paths for about eight hundred million years. Comparing the DNA of different living species allows us to construct family trees that are much more detailed, and probably more precise, than those based just on the fossil record.

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So we can say that humans belong to the species sapiens, the genus Homo, the family Hominidae, the order Primates, the class Mammalia, the phylum Chordata (vertebrates), the kingdom Animalia, and the domain Eukarya.

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The first living organisms probably belonged to the domain of Archaea, though organisms from the second domain, the Eubacteria, also appeared early. Both domains consist entirely of prokaryotes, minute single-celled organisms that have neither a distinct nucleus nor other specialized cellular organelles. Prokaryotes would dominate the biosphere for more than seven-eighths of its history, until about six hundred million years ago. If we meet living organisms elsewhere in our galaxy, we probably won’t be shaking hands with them but peering at them through a microscope.

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This is simple information-processing equipment, but already we have the three key components of all biological information processing: inputs, processing, and outputs.

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By about 3.5 billion years ago, a new evolutionary innovation, photosynthesis, was letting some organisms tap into flows of energy from the sun. This was life’s first energy bonanza, and its impact was the prokaryotic equivalent of a gold strike.

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Living cells now had the energy to reorganize themselves and their surroundings on an entirely new scale. They spread more widely and the amount of life surely increased by several orders of magnitude.

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Many formed coral-like structures known as stromatolites, which grew into reefs at the edges of continents as billons of organisms accumulated over thickening layers of their dead ancestors. Stromatolites still exist in a few special environments, such as Shark Bay, off the coast of Western Australia. They are rare today, but from the time when they first appeared, more than 3.5 billion years ago, until about 500 million years ago—significantly more than half the history of our planet—they were probably the most visible form of life on Earth. If aliens had come looking for life on this planet, they would have found stromatolites. And perhaps that’s what we’ll find when we first detect life on rocky planets in other star systems.

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Powerful evidence for the evolution of the first cyanobacteria is the disappearance from three billion years ago of sedimentary rocks rich in pyrite (fool’s gold), which, like iron, rusts in the presence of free oxygen. But there was a limit to how much oxygen these mechanisms could absorb, and starting about 2.4 billion years ago, levels of atmospheric oxygen began to rise fast, from less than 0.001 percent of today’s levels to perhaps 1 percent or more.

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Protected by the ozone layer, some algae may have started colonizing the land for the first time. Until then, bathed in solar radiation that would have ripped apart any bacteria brave enough to venture onto land, the continents of planet Earth had been more or less sterile.

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As oxygen levels rose and levels of greenhouse gases fell, early in the Proterozoic eon, Earth froze in the first of several snowball-Earth episodes. Glaciers spread from the poles to the equator, turning the Earth white,

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This was a close shave. Organisms for which oxygen was a poison perished or hid deep in the oceans. But even organisms that could cope with oxygen suffered in a world where glaciers covered both the land and the oceans, blocking the sunlight needed for photosynthesis. Life hung by a thread, as most life-forms retreated beneath the ice and huddled around the warm fires of suboceanic volcanoes.

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Glaciers blocked photosynthesis, which slashed oxygen production. Meanwhile, under the glaciers, oceanic volcanoes kept pumping carbon dioxide and other greenhouse gases back into the oceans. Greenhouse gases began to accumulate beneath the ice until, eventually, they broke through the glaciers, and Earth’s surface warmed again.

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Now biology came to the rescue by evolving new types of organisms that could supplement Earth’s thermostats by sucking oxygen out of the air. These organisms, the first eukaryotic cells, didn’t just help stabilize global temperatures. They also marked a biological revolution that would eventually allow the evolution of large organisms such as you and me.

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Eukarya, matters a lot to us because all large organisms, including ourselves, are built from eukaryotic cells. These were the first cells that could use oxygen systematically, exploiting its fierce chemical energy in a process known as respiration, which is what we do when we breathe.

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By using oxygen cleverly, respiration can extract at least ten times as much energy from organic molecules as earlier non-oxygenic ways of breaking down food molecules. 16 With more energy to power their metabolism, rates of primary production—the production of living organisms—may have increased by anything from ten to a thousand times. 17

Chapter 6 Big Life and the Biosphere

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Animals may be evolution’s icing, but bacteria are the cake.—ANDREW KNOLL, LIFE ON A YOUNG PLANET

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Little life ruled the biosphere for three and a half billion years and still rules much of it. It took three billion years to get from Luca to the first specimens of big life—the first multicellular animals, or metazoans. That tells us that evolving multicellular organisms was much trickier than evolving prokaryotes. And that suggests that if there is a lot of life in the universe, metazoans must be rare. Metazoans represent a new level and type of complexity among living organisms.

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Our unusually large moon helped stabilize Earth’s orbit and tilt. And, as we have seen, plate tectonics, erosion, and then life itself provided thermostats that stopped temperatures from wobbling too much at the Earth’s surface.

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We are to bacteria what Dubai’s 830-meter-tall Burj Khalifa is to an ant crawling past the doorman’s shoes.

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In fully multicellular organisms, each cell is so specialized and so interdependent that it cannot survive alone. Genuine multicellularity is really an extreme form of symbiosis. But collaboration is made easier by the fact that most cells in metazoans are genetically identical. They are family. So each cell works to support the whole organism, sometimes by sacrificing its own life for the good of the rest. Indeed, cells, like kamikaze pilots, often self-destruct if they are no longer working well or no longer needed, a process known to biologists as apoptosis. Today, as many as fifty billion cells in your body will commit suicide by apoptosis.

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Metazoans didn’t flourish before about one billion years ago. The first were probably photosynthesizing algae that formed kelplike structures. But at the end of the Proterozoic eon, about six hundred million years ago, big life took off as millions of metazoan species began to explore the many new niches and lifeways opened up by multicellularity.

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Cryogenian period. This started about 720 million years ago and lasted 85 million years. Kilometer-deep glaciers spread over the land and oceans; surface temperatures may have fallen to negative fifty degrees Celsius and photosynthesis would have largely shut down. Once again, the fate of all living organisms hung in the balance.

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true polar wander events, and they have happened at least thirty times in the past three billion years.

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Whatever the cause, these violent changes would have forced the evolutionary pace of life. Beneath the ice, surviving organisms once again huddled around cracks in the Earth’s crust that leaked hot magma. In these biological refugee camps, evolution could explore odd pathways, because new genes can spread fast in small, isolated populations. Indeed, these strange worlds may have witnessed some of the earliest experiments in multicellularity.

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We get the first good evidence for large numbers of multicellular organisms early in the Ediacaran period, which lasted from around 635 million years ago to around 540 million years ago. For the first time, we see the three familiar groups of large organisms: plants, which depend on photosynthesis so they can usually sit still and suck up sunlight; fungi, which scavenge decomposing organic material; and animals, which have to be alert and mobile because they survive by hunting down and eating other organisms.

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Ecologists talk of a food chain, a sort of queue of energy consumers with plants at the front, followed by herbivores (or creatures that consume plants), then by carnivores, which can consume herbivores, then by fungi, which bring up the rear by feasting on the dead. The whole process delights entropy, which exacts a garbage tax at every step.

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Approximately 90 percent of the energy captured by photosynthesis is lost at each trophic level, so much less energy is available for the later links on the food chain. That’s why you find fewer animals than plants on Earth, and fewer carnivores than herbivores. But the fungi do well either way, as they recycle corpses.

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One of the most successful design tricks of this period was modularity. You join together body modules that are pretty similar to form, say, a wormlike creature. Then the tool-kit genes start modifying each module so that some sprout legs or wings while others turn into a head with a mouth, or antennae, or perhaps a brain. Even you and I are modular, though our modules are now so specialized that it’s hard to see the similarities between them.

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So successful were Cambrian designs that all the major groups (or phyla) of large organisms existing today made their first appearance in the Cambrian period. Most showed up in the astonishing ten-million-year interval starting 530 million years ago. In this period (a split second to a paleontologist) was concentrated perhaps the most rapid stretch of biological innovation in the past six hundred million years.

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Cambrian species included the first Chordata, or vertebrates. This is the large phylum of animals that we belong to. Vertebrates are like tubes. They each have a spinal cord, a front (with a mouth), and a back (with an anus).

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Whatever the exact changes, violent climatic and geological swings would have increased the evolutionary pace, driving many species to extinction and forcing the evolution of many new types of large organisms.

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Instead, the history of big life was an unpredictable and dangerous roller-coaster ride. Asteroid impacts, sudden shifts in Earth’s innards, changes in the planet’s atmosphere, and massive volcanic eruptions sent evolution careering down new and unexpected pathways.

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The first mass extinctions happened back in the Archean eon. The great oxygenation event, 2.5 billion years ago, surely killed off many bacterial organisms for which oxygen was toxic. Indeed, this may have been the greatest mass extinction of all. Many groups of species also perished during the snowball-Earth episodes late in the Proterozoic eon, and we know that many disappeared at the end of the Ediacaran period. Since then, we know of at least five mass-extinction events during which more than half of all existing types of species disappeared.

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The greatest of all mass extinctions came at the end of the Permian period, 248 million years ago. This time, more than 80 percent of all genera vanished, including the last of the trilobites. The precise causes of this mass extinction remain uncertain.

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This was a greenhouse mass extinction; it killed by heating rather than freezing. 10 In an extreme greenhouse world, large organisms survived only in the cooler polar environments in the far north and south of the vast supercontinent of Pangaea.

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Colonizing the land was extremely difficult, a bit like colonizing a new planet. Life had evolved and flourished in water for three billion years. Every cell had evolved in a bath of salty water. Organisms floated in water, extracted from it the gases and chemicals they needed, and fished in it for their food. Away from water, they needed support systems as elaborate as any space suit. They needed tough skins that could hold in water and prevent their bodies from drying out. But those skins also had to be permeable enough to let in carbon dioxide or oxygen. There was a tricky balance here. Leaves handle these opposite demands by way of tiny pores called stomata that allow carbon dioxide in and let water leak out. The size and number of stomata change depending on the surrounding temperature, humidity, and carbon dioxide levels.

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Serious colonization of the land by metazoans began only after the late Ordovician extinction, 450 million years ago.

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The first vascular plants, with tissues that could circulate liquids and nutrients, showed up on land about 430 million years ago. Fungi and animals soon followed them. Simple, scorpionlike arthropods may have flourished on land as early as the first vascular plants. Early amphibians certainly walked the land by 400 million years ago, the date of amphibianlike fossil footprints found in Ireland and Poland. Amphibians evolved from fish that could breathe out of water and walk in the shallows of drying lakes and rivers, like modern lungfish. But all amphibians have to stay near water, where they lay their eggs. The first amphibians were the first large, land-based vertebrates. Some were as large as you and I.

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With plenty of oxygen, forest fires were easily ignited by lightning strikes. So the Carboniferous and early Permian world, though chilly, probably had the acrid smell of forest fires, a smell no one will detect on other planets in our solar system because they lack the high oxygen levels and the woody fuel sources needed for the propagation of fire.

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The first reptiles appeared in the Carboniferous period, which started about 320 million years ago. They were part of a new group of animals, the amniotes, which include reptiles, birds, and mammals. Unlike amphibians, amniotes could reproduce away from the water because their young developed in protected eggs, pouches, or wombs. Reptiles would eventually include some of the largest animals that have ever strolled, waddled, lolled, or galloped across the land.

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These mechanisms may have encouraged bipedalism (think T. rex and modern birds), because in bipedal reptiles, the chest is more open and breathing is not checked by motion the way it is in the waddling walk of four-legged reptiles.

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Mammals are a class of warm-blooded animals related to the other amniotes, the reptiles and birds. But mammals differ from reptiles and birds in crucial ways. The mammal brain has a neocortex, which makes mammals superb calculators. They have fur (yes, even humans have fur, though less than most mammals), and for the most part, mammals take more care of their offspring. It was Carl Linnaeus, the founder of modern taxonomy, who first called animals in our class mammals, after another distinctive feature: all mammals nourish their young with milk from mammary glands. For paleontologists, the most visible distinguishing feature of mammal fossils is their teeth. Even the earliest mammal teeth have cusps so that the upper and lower teeth can mesh together, allowing them to chomp down on new types of food and grind it more efficiently than most reptiles do.

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But as organisms became larger and more complex, they needed more information about their environments. Natural selection equipped large organisms with a desire for more information, because good information was vital to their success.

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The simple on/ off switches of pain sensors control a lot of behavior in even the most complex metazoans.

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The emotions, dominated by the limbic system, also allow rapid decision-making by creating predispositions and preferences that drive many important decisions and get them right most of the time. Charles Darwin understood that the emotions are decision-makers that have evolved through natural selection to help organisms survive. The antelope that wants to hug lions is unlikely to pass on its genes to any offspring. The most basic emotions, those least amenable to conscious control, seem to bubble up inside us. They include fear and anger, surprise and disgust, and also, perhaps, a sense of joy. They predispose us to react in certain ways and send the chemical signals that prepare our bodies to run or focus, to attack or hug. 18 Emotions drive decision-making in all animals with large brains, and some emotions, like fear, are probably present in all vertebrates and maybe in some invertebrates, particularly the most intelligent ones such as the octopi. The preferences emotions create for particular outcomes and behaviors lie behind the human sense of meaning and ethics.

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The world of the dinosaurs vanished in just a few hours when a ten-to fifteen-kilometer-wide asteroid crashed into Earth. 21 The crash caused a major extinction event, during which about half of all genera disappeared.

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In the Hell Creek Formation, in Montana and Wyoming, you can find fossils of fish whose gills are full of glass from the asteroid impact.

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As is typical in such crises, large species were particularly hard hit, because they need more energy, are less numerous, and reproduce more slowly than smaller creatures. This is why the large dinosaurs perished. But modern birds are descendants of smaller dinosaurs, some of which just made it through. Smaller organisms, such as the rodentlike mammals, did slightly better, and some of these would become our ancestors.

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Though the earliest mammal-like creatures were no larger than mice, they probably already nursed their young, like today’s mammals, and had unusually large brains in comparison to their body size. The basic division between marsupials (mammals whose young need special protection and nourishment, often in pouches) and placentals (mammals whose young are fed within the womb through a placenta) goes back at least 170 million years.

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About 150 million years ago, the world of mammals was changed by the evolution of new types of plants to rival the conifers and ferns that had dominated the plant world so far. These were the angiosperms, plants with fruits and flowers, the types of plants that dominate the forests and woodlands, the parks and backyards of today.

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Many mammal species went big. Within half a million years, there were cow-size herbivorous mammals and equally large mammalian carnivores. There were also primates, members of the order of tree-dwelling, fruit-eating mammals from which we are descended. Though the first primates already existed in the world of dinosaurs, they flourished only after the dinosaurs had left the scene.

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Whatever the precise causes, the cooling trend that began about fifty million years ago has continued to the present day. About 2.6 million years ago, at the beginning of the Pleistocene epoch, the world entered the current phase of regular ice ages. The world had not been this cold for 250 million years, since Pangaea itself had split apart at the end of the Permian period. Fifty million years ago, in this post-dinosaur, post-PETM world of chilly and erratic climate changes, our primate ancestors evolved.

Chapter 7 Humans: Threshold 6

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We belong to the mammalian order Primates, which includes lemurs, monkeys, and great apes. And we share a lot with our primate relatives. The earliest primates almost certainly lived in trees, and young humans (I include my young self here) love climbing trees and are good at it. To climb trees, you need hands and fingers or feet and toes that can grip. If you’re going to leap from branch to branch, it’s a good idea to have stereoscopic vision so you can judge distances. That means having two eyes at the front of your face, with overlapping lines of sight. (Don’t try jumping from branch to branch with one eye closed.) So all primates have hands and feet that can grip and flattish faces with eyes at the front.

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Primates are exceptionally brainy. Their brains are unusually large relative to their bodies, and the top front layer of the brain, the neocortex, is gigantic. In most mammal species, the cortex accounts for between 10 percent and 40 percent of brain size. In primates, it accounts for more than 50 percent, and in humans for as much as 80 percent. 2 Humans are exceptional for the sheer number of their cortical neurons. They have about fifteen billion, or more than twice as many as chimpanzees (with about six billion). 3 Whales and elephants, the next in line after humans on the most-cortical-neurons list, have about ten billion cortical neurons, but they have smaller brains than chimps relative to body size. Large brains mean that primates are wizards at acquiring, storing, and using information about their surroundings.

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In human bodies, the brain uses 16 percent of available energy, though it accounts for just 2 percent of the body’s mass.

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There are species of sea slugs that have mini-brains when they are young. They use them as they voyage through the seas looking for a perch from which they can sieve food. But once they’ve found their perch, they no longer need such an expensive piece of equipment so… they eat their brains. (Some have joked, cruelly, that this is a bit like tenured academics. 4)

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The first primates probably evolved before the dinosaurs were wiped out, but the earliest surviving primate fossils date from several million years after the Chicxulub landing. We belong to the group of large tailless primates known as apes. Apes evolved about thirty million years ago and flourished and diversified in Africa and Eurasia twenty million years ago. The great apes (or hominids) include, today, the orangutans, gorillas, and chimpanzees, as well as humans. Their ancestors evolved in a post-PETM world of falling carbon dioxide levels and chillier and less predictable climates. Climatic instability pressed hard on the evolutionary accelerator, forcing many different species to adapt fast and often. From about ten million years ago, climates became drier and chillier over much of the range of great apes, and the ape lineage was culled, perhaps quite severely, as their forest homes were replaced by grasslands. Our ancestors were survivors of this evolutionary forced march.

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Humans and chimps shared a common ancestor up to about six or seven million years ago. In other words, somewhere in Africa six or seven million years ago, there existed a creature from which modern humans and chimpanzees are both descended. We do not yet have fossil remains of this creature, but modern genetics tells us it was really there.

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Chemical analysis of strontium isotopes in bones can even tell us how widely individuals roamed. 7 Studies of the bones of a group of early hominins known as australopithecines have shown that females traveled more than males, which suggests that females joined groups of males rather than the other way around. In other words, their communities were patrilocal, like those of modern chimps, and that tells us a lot about their social world. These are powerful sleuthing tools. But unfortunately, they often yield more questions than answers, reminding us how complex the story of human evolution really is.

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And here’s where the butterfly flapped its wings. Human language crossed a subtle linguistic threshold that allowed utterly new types of communication. Above all, human languages let us share information about abstract entities or about things or possibilities that are not immediately present and may not even exist outside of our imagination. And they let us do this fast and efficiently. With the partial exception of honeybees, whose dances can tell other bees where to find honey, we know of no animals that can transmit precise information about what is not right in front of them. No animal can swap stories about the future or the past, or warn about the lion pride ten miles to the north, or tell you about gods or demons. They may be able to think about such things, but they cannot talk about them. And that may be why it is hard to find any evidence for teaching within any other species, even among our closest relatives, the monkeys and apes.

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Human language is powerful enough to act like a cultural ratchet, locking in the ideas of one generation and preserving them for the next generation, which can add to them in its turn. 16 I call this mechanism collective learning. Collective learning is a new driver of change, and it can drive change as powerfully as natural selection. But because it allows instantaneous exchanges of information, it works much faster.

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Here is how this mechanism is described by a pioneer of the study of memory, the Nobel Prize winner Eric Kandel: Although the size and structure of the human brain have not changed since Homo sapiens first appeared in East Africa… the learning capability of individual human beings and their historical memory have grown over the centuries through shared learning—that is, through the transmission of culture. Cultural evolution, a nonbiological mode of adaptation, acts in parallel with biological evolution as the means of transmitting knowledge of the past and adaptive behaviour across generations. All human accomplishments, from antiquity to modern times, are products of a shared memory accumulated over centuries.

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Then, as climates cooled, our ancestors seem to have left the Middle East to the Neanderthals, whose bodies were better adapted to colder climates.