What is Big History?
At the risk of sounding insufferably grandiose, it is the study of existence itself. From the very moment of inception—the Big Bang—to this very moment, which is always changing, and onward to the vast array of hypothetical futures that lie well beyond the bounds of perception, let alone attainment. It is an impertinent endeavor, embracing everything and anything within the unbounded realm of the physical Universe.
Consider this. About 13.8 billion years ago, give or take a bit of time, the cosmos was an inconceivably hot, minuscule capsule of radiant energy. How it came to be, what event precipitated its birth, or why it exists in the first place are questions that are, respectively, unknown, unaskable, and perhaps unknowable. Consequently, we won’t jab any of them with a ten-light-year lever. What transpired after this mother of all seminal events is, however, well within our purview.
To appreciate just how extraordinary that history has been, let’s examine what will happen over the next two minutes.
During that brief interval, more than 500 babies will be born. Thousands of airplanes will continue darting around the globe like a hyperactive bee swarm. Earth will be struck by lightning roughly 3,000 times. Nearly 10,000 new blog posts will appear online. More than half a million stars will ignite their core thermonuclear reactions somewhere among the Universe’s roughly 125 billion galaxies. Hundreds of millions of emails will be sent. Billions of miles will be driven by the world’s automobiles. Billions of gigawatts of solar energy will strike Earth’s surface. Deep within the Sun, tens of billions of tons of hydrogen will fuse into helium, releasing the energy that sustains life on our planet.
Meanwhile, inside your own body, your brain will consume thousands of joules of energy. Hundreds of millions of cells will die, billions of neurons will fire, hundreds of millions of red blood cells will be produced, and well over a billion DNA mutations will occur.
This list, far from comprehensive, offers only a fleeting glimpse into what transpires in our bodies, on our planet, and throughout the Universe in the space of two meagre minutes.
All that complexity. That bewildering, almost incomprehensible complexity. The staggeringly intricate interplay of forces required to sustain life, drive civilizations, and shape an ever-evolving Universe. How did the cosmos become transformed from a simple vessel of radiant energy into one capable of producing galaxies, stars, planets, living worlds like Earth, and minds capable of asking where they came from? And where, in all of this, do we humans fit?
Take a moment and look at your feet.
Even with the protruding veins, unclipped nails, dried skin, and every blemish you try to conceal, they’re breathtakingly beautiful. Not because of how they look, but because of what they represent. They are the product of an evolutionary journey stretching back through countless millennia. They were shaped by ancestors who foraged, hunted, migrated, and survived across deserts, tundra, forests, and mountains. Every step they took pressed against the restless Earth, applying Newton’s action-reaction law as they struggled to survive and, ultimately, to flourish. We inherited those feet. We cover them, ignore them, and rarely stop to consider the astonishing story they embody.
In the Big History view of the world, humanity occupies one of the great thresholds of increasing complexity, a concept introduced by the historian David Christian. Ours is not the centre of the Universe, nor are we separate from the rest of nature. Rather, we are one remarkable chapter in a much longer cosmic story that began billions of years before the first human ever gazed at the stars.
Yet we are also unique. No other known species has transformed an entire planet as profoundly as we have. Our influence on Earth’s climate, landscapes, and ecosystems has become so significant that many scientists now describe our time as the Anthropocene. To understand what we have become, how we arrived here, and where we may be heading, we must first understand the story that made us possible.
That story is Big History: the history of the Universe, of Earth, of life, and of ourselves.
We Begin With Two Balloons
Blink once and you will see them both before you, floating above the podium to which they have been firmly tethered. As much as we love to see balloons flying freely, the one thing we do not want is to have two balloons pressed against the ceiling, where they would likely remain forever. As we’ve already infiltrated your imagination, you are free to make them any shape and colour you like. One, however, must contain hydrogen and the other helium, for they will serve an essential role in our defence of Big History.
Perhaps more than any other field, Big History has weathered an unremitting volley of broadside assaults. The principal criticism concerns the sheer breadth of its scope. To create a discipline that embraces subjects as disparate as cosmology, cell biology, and human migration is often regarded as not only audacious but impossible. Ours is an age of increasing specialization, in which even experts within a single discipline often lack a comprehensive view of their own field. The gradual accumulation of knowledge that expanded geometrically through the Victorian era has now exploded exponentially. The old saying that astronomers discover less and less about more and more could just as easily be applied to almost every scientific discipline that emerged from what was once called natural philosophy.
All of this is true. We readily concede that greater breadth inevitably comes at the expense of depth, a complementary principle with which every planetarium astronomer is frustratingly familiar. Such are the limitations of the mortal mind. Yet the aim of Big History has never been to compile every iota of human knowledge into a single work resembling Asimov’s Encyclopedia Galactica.
To understand its real purpose, we must return to those two balloons.
Consider them for a moment. The gases pressing against their thin membranes are composed of the two simplest elements, hydrogen and helium. About 380,000 years after the Big Bang, when the Universe had cooled enough for matter to emerge from the pervasive cosmic radiation, almost all of the material in existence consisted of these two elements. Little else besides.
Now turn away from the balloons and look around you. Chairs, clothing, people, walls, electronics, keyboards, rugs, cups, books, windows, trees, the Sun, clouds, automobiles, streets, buildings, houses. An astonishing variety of objects, assuming countless forms and performing countless functions. The one object you cannot see, because it rests above your eyes, is your own brain: three pounds of neurons capable of observing, remembering, naming, and understanding everything around you.
Now look back at the balloons. If I were to pierce them with a dagger, their contents would burst free and vanish almost instantly into the surrounding air. Invisible. Insubstantial. Yet when the Universe was young, hydrogen and helium constituted almost the entirety of the material cosmos—and, remarkably enough, they still do.
How did the Universe progress from that extraordinary simplicity to the astonishing complexity that surrounds us today? How did the contents of those two balloons ultimately become brains capable of contemplating their own origins? What happened between those first atoms and this present moment?
That question is the true province of Big History.
The Big Bang alone cannot answer it, for the Big Bang explains only the beginning. Beyond that came the separation of the fundamental forces, cosmic inflation, expansion, cooling, and the formation of the lightest elements. The first galaxies and stars appeared. Within their cores, thermonuclear fusion forged heavier elements from hydrogen and helium. When the most massive of those stars ended their lives as supernovae, they created still heavier elements—silver, gold, mercury, uranium, and dozens of others—and scattered them across interstellar space.
Eventually, the debris of long-dead stars became incorporated into new clouds of gas and dust. One such cloud collapsed under its own gravity to form the Sun and the family of planets that surrounds it.
As the young Earth gradually recovered from its violent youth, conditions became suitable for the emergence of the earliest life. Simple cells eventually gave rise to more complex ones. Multicellular organisms appeared, followed by the Cambrian explosion and the long succession of evolutionary innovations and mass extinctions that reshaped life again and again. Throughout it all, Earth’s restless geology raised mountain ranges, opened oceans, shifted continents, altered climates, and continually transformed the environments to which living organisms adapted.
The extinction of the dinosaurs opened opportunities for the mammals, and among them arose the primates. Eventually one primate species, Homo sapiens, spread across the globe as hunters and gatherers. During only the last few percent of its history, agriculture emerged, followed by permanent settlements, writing, mathematics, art, cities, commerce, science, and civilization itself. Each generation inherited the achievements of those before it and extended them further.
Running alongside these developments was another remarkable story: humanity’s growing effort to understand nature itself. Through observation, experiment, and reason, we gradually replaced an Earth-centred cosmos with a Universe so vast that our planet, our Sun, and even our Galaxy occupy no privileged place within it.
In this way, the contents of two simple balloons were transformed, through nothing more than the operation of natural processes over billions of years, into forests, oceans, civilizations, satellites, symphonies, mathematics, and minds capable of asking how it all came to be.
Albert Einstein once remarked, “There are only two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle.”
Big History, for all its ambitious scope, provides one of the clearest paths into that Universe of miracles so often overlooked.
The Thresholds, Part I
Big History places eight thresholds throughout natural history, beginning with the Big Bang and continuing to today’s modern revolution. (Some include a ninth threshold referring to the future.) These designated thresholds serve as markers: periods when conditions were conducive to an increase in complexity. For that is truly one focus of Big History: how complexity arose in small pockets within a Universe that otherwise bends toward entropy and increasing states of disorder. What transpired to transform energy and simple gases into towns, villages, and megacities inhabited by creatures possessed of unfathomably intricate minds?
What follows is a brief introduction to the first four thresholds.
We begin before the beginning: with Threshold Zero.
THRESHOLD ZERO: BEFORE THE BEGINNING
Threshold Zero is not one of the thresholds recognized by Big History co-founder David Christian, but rather my own addition. It serves simply as an acknowledgement that something may have preceded the genesis event we call the Big Bang.
The study of origins inevitably confronts us with the problem of first causes. Whether it be the emergence of Gaia, Tartarus, and Eros from the primordial chaos, the marriage of the Māori Rangi and Papa whose union brought the world into existence, or even the Big Bang itself, every account prompts the same nagging question: what preceded that which drew creation itself into existence? Although Stephen Hawking once remarked that asking what occurred before the Big Bang was akin to asking what exists one degree north of the North Pole, many cosmologists today regard the possibility of existence preceding the Big Bang quite seriously. One such idea is the multiverse: a vast collection of distinct universes arising, bubble-like, from an underlying “space-time foam.” The term is only an analogy, of course; no one suggests that space-time literally possesses the consistency of foam. Rather, it is an attempt to describe an underlying physical reality that may lie beyond our present understanding.
According to this idea, the event we call the Big Bang coincided with the emergence of our Universe from some unknown—or, more likely, unknowable—field. Each universe would possess its own distinctive character and be governed by its own physical laws. Only a few, perhaps, would contain conditions conducive to life’s development. Ours is one such Universe. The fundamental physical forces—gravitation, electromagnetism, and the strong and weak nuclear forces—appear to possess precisely the relative strengths required for stars, planets, and ultimately life to exist. Were gravitation even slightly stronger, the Universe would have collapsed back upon itself soon after its birth. Had it been slightly weaker, matter would never have condensed into stars and planets. We are here discussing this Universe precisely because it permitted us to evolve to this point: the first Goldilocks condition.
Yet the mystery immediately returns. If our Universe arose from such a “foam,” from what did that foam arise? And what produced that which produced the foam? Such questions quickly lead us into an infinite regress with no obvious starting point: a column consisting of an indeterminate number of tortoises.
However, much like the deist God who creates and then withdraws, this threshold, if it exists at all, lies well beyond our present ability to investigate. Hence its designation as Zero.
So we give a passing nod to Threshold Zero and continue on our way.
THRESHOLD ONE: THE BIG BANG
The very beginning, at least of our Universe. Current estimates place the birth of the cosmos at approximately 13.8 billion years ago. Although present physical models do not enable us to describe the precise first instant, cosmologists have developed a remarkably detailed timeline beginning with the earliest knowable moment—0.00000000000000000000000000000000000000001 second after the Big Bang—and continuing through the successive stages of the Universe’s earliest development. The Big Bang and the events immediately following it define Threshold One.
THRESHOLD TWO: THE FIRST STARS AND GALAXIES
Approximately 100 million years elapsed between the Big Bang and the formation of the first stars, known as Population III stars. In this metal-poor infant Universe, they consisted almost entirely of hydrogen and helium, the two simplest elements. Clusters of these stars eventually coalesced into the vast stellar systems we call galaxies. Though once described as “island universes,” galaxies frequently collided and merged to form still larger systems. Threshold Two represents the first time large, distinct, and independent structures emerged from the diffuse material scattered throughout the young Universe.
THRESHOLD THREE: CHEMICAL ELEMENTS
The first stars performed a vital role in our story because they enriched the Universe with heavier elements. Every active star generates energy through thermonuclear fusion, the process by which lighter elements are transformed into heavier ones. At this very moment, the Sun is fusing hydrogen into helium while converting a small fraction of its mass into energy. Eventually, about five billion years from now, the Sun will exhaust its core hydrogen and proceed to helium burning, producing carbon.
Astronomers consider every element heavier than helium to be a “metal.”
The Sun is not sufficiently massive to continue fusion beyond carbon. More massive stars, however, proceed through carbon, oxygen, silicon, and finally iron. No star, regardless of its mass, can fuse iron into heavier elements because doing so requires more energy than it releases. The delicate balance between the star’s outward pressure and inward gravitational pull collapses, and the star explodes as a Type II supernova. These extraordinarily energetic explosions create all the naturally occurring elements heavier than iron while dispersing them throughout surrounding space.
The Population III stars served as the first element factories of the Universe. Through thermonuclear fusion they forged many of the lighter heavy elements, and in their deaths they created the heaviest ones while enriching nearby regions with the raw materials from which future generations of stars and planets would form.
THRESHOLD FOUR: EARTH AND THE SOLAR SYSTEM
More than five billion years ago, a Population II star exploded as a Type II supernova. Some of its expelled material encountered a cool, dark nebula consisting primarily of hydrogen and helium. The incorporation of these supernova remnants into the dormant cloud initiated a slow but inexorable collapse that, over more than ten million years, produced an open star cluster, one member of which became our Sun.
Around the newly formed Sun—and most likely around those other young stars as well—a disc of remaining material took shape. From this disc the planets, asteroids, and other attendant bodies gradually formed. As the planets accumulated surrounding material and settled into stable orbits, three of them—Venus, Earth, and Mars—came to reside within the habitable zone, where temperatures could permit liquid water to exist. Yet only on Earth did life take hold and eventually evolve into the astonishing diversity and complexity we observe today.