Origin Story: A Big History of Everything

by David Christian

So much could have gone wrong. A supernova in a neighboring star system could have blown up. Or we could have collided fatally with another planet. Somehow or other, our Earth avoided these dangers and remained life-friendly for more than three billion years. That was enough time for big life to evolve. And big life really is big. We are to bacteria what Dubai’s 830-meter-tall Burj Khalifa is to an ant crawling past the doorman’s shoes.

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. With the emergence of huge numbers of organisms that got their energy by consuming other organisms, the biosphere became more complex, more diverse, and more hierarchical as energy from sunlight was passed through different trophic levels, from plants to animals and fungi.

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. 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.

Now we know that life had already been around for three and a half billion years; it was just hard to see the evidence. What the Cambrian era marks is not the beginning of life but an exuberant adaptive radiation of multicellular life-forms.

The earliest vertebrates didn’t yet have the concentrated ball of neurons that we call a brain, but they did have nervous systems with hundreds or thousands of networked nerve cells that could process lots of information fed in from sensor cells, then pass on decisions to other organs that could take appropriate action.

Unstable climates may explain the remarkable pace of evolution in the Cambrian period. Oxygen levels began to rise again, supplying some of the energy needed to form multicellular organisms. But carbon dioxide levels rose much faster, reaching levels much higher than today. This was a warm, humid greenhouse world. 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.

Mass extinctions were also hard on the largest organisms, which need more food and reproduce too slowly to keep up with rapid changes. Mass-extinction events reshuffled the genetic deck of cards, created new evolutionary spaces for survivors, and set up new evolutionary experiments. They were always followed by adaptive radiations, periods of rapid experimentation during which new biological products were launched in the mass market of a changing biosphere.

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. It might have been due to rising magmas that broke through the crust in massive volcanic eruptions that sent enough ash into the air to block photosynthesis. We find modern evidence of this in a large volcanic region of Siberia known as the Siberian Traps. The eruptions pumped huge amounts of carbon dioxide into the atmosphere, so when the dust settled, carbon dioxide levels spiked, oxygen levels fell, and the oceans warmed. When Earth burped, the biosphere shuddered. By some estimates, oceans may have been as warm as thirty-eight degrees Celsius, hot enough to kill most marine organisms and stop nearly all photosynthesis in the seas. Warmer oceans could hold less oxygen and support less life, and deep beneath their surface, thawing balls of frozen methane known as clathrates may have released huge bubbles of methane. 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.

Land-dwelling plants had a particularly large impact on the atmosphere, as they inhaled carbon dioxide and exhaled oxygen. Levels of atmospheric oxygen rose fast after the Ordovician period, increasing from about 5 to 10 percent of the atmosphere to levels much higher than they are today, perhaps to 35 percent, before stabilizing. Since about 370 million years ago, oxygen levels have mostly remained between 17 percent and 30 percent of the atmosphere.11 We know this because over this entire period researchers see evidence of spontaneous fires, and fires cannot ignite if oxygen levels fall much below 17 percent. Oxygen levels probably peaked during the Permian period (from 300 to 250 million years ago).

The impact of the first forests was particularly significant because as yet, there were no organisms that could break down the lignin in wood. That’s why forests from the Carboniferous period (from 360 to 300 million years ago) were mostly buried beneath the soil, along with the carbon they had drawn down from the atmosphere. Over time, they fossilized to form the coal seams that later powered the industrial revolution. About 90 percent of today’s coal deposits were buried during the period of high oxygen levels, from around 330 to 260 million years ago.

One long trend was toward bigness. That’s the trend that gave us metazoans in the first place. It also encouraged the evolution of larger and larger metazoans, because being a giant often made good evolutionary sense. After all, larger organisms have fewer predators. Try getting your teeth into a blue whale! Large organisms also need less food for each unit of body weight, and it’s usually easier for them to avoid the catastrophe of desiccation.15 Besides, the high-oxygen atmospheric regime that emerged early in the Phanerozoic eon provided the extra energy needed to power megametazoans. Indeed, it seems likely that very large organisms flourished best when oxygen levels were highest, which usually meant during periods of low carbon dioxide levels and cooler climates. This was true in the oceans as well as on land, because cold water can hold more oxygen than warm water.

The Triassic world ended abruptly 200 million years ago in another greenhouse mass-extinction event. Those dinosaur families that survived evolved highly efficient mechanisms for breathing in an oxygen-deprived world. 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. During the Jurassic period (from around 200 to 150 million years ago), oxygen levels rose again, until they approached the levels of today’s world. And dinosaurs got bigger once more.

Mammals, like the other amniotes (reptiles and birds), also appeared after the Permian mass extinction. Mammals would eventually produce some giants, too, but not for almost two hundred million years. Before that, they mostly lived in modest obscurity in the shadows of a world ruled by dinosaurs. Throughout the Triassic, Jurassic, and Cretaceous (from 250 million years ago to 65 million years ago), most mammals were small, burrowing creatures, a bit like modern-day rodents.

Mammals illustrate another powerful evolutionary trend, the tendency toward more elaborate information processing. This is apparent throughout the Phanerozoic but particularly among animals and most strikingly among mammals.

Instead, we all live in a rich virtual reality constructed by our brains. Our brains generate and constantly update maps of the most salient features of our bodies and our surroundings, just as climate scientists model changing environments today.17 Those maps enable us to maintain homeostasis. They help us respond appropriately, most of the time, to the never-ending swirl of changes all around us.

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.

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.

In mammals, the increasing importance of information processing helps explain the evolution and growth of the cortex, the gray, outer layers of the brain. The cortex provides lots of space for calculations and a lot more calculating ability, so it allowed better problem-solving in unfamiliar situations or when other decision-making systems were deadlocked. Eventually, the brainiest mammals would evolve general information-processing and problem-solving systems that were to those of the bacterial world what the Internet is to an abacus.

The evolution of enhanced problem-solving and information-processing systems would eventually lead to the information explosion unleashed by our own remarkable species.

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. Geologists refer to this as the K/T event because it occurred at the border between the Cretaceous period (often abbreviated K, from the German word for “chalk,” Kreide) and the Tertiary period, an older name for the Cenozoic era, which began sixty-five million years ago.

When the asteroid hit, it was moving at thirty kilometers a second (about one hundred thousand kilometers an hour), having taken just seconds to fly through Earth’s atmosphere.

We know exactly where it fell: in the Chicxulub (pronounced “Chikshulub”) crater in the Yucatán Peninsula of modern Mexico. The asteroid evaporated as it punched through the crust, leaving a crater almost two hundred kilometers across. Molten rocks were hurled into the air, where they formed dust clouds that blocked sunlight for many months. Limestone evaporated, spraying carbon dioxide into the atmosphere. An area hundreds of kilometers around the impact point was stripped of life. Hundreds of kilometers beyond that zone, forests lit up in massive firestorms. At sea...

But within weeks, the whole biosphere had changed. Soot blocked sunlight, creating what we might describe today as a nuclear winter. Nitric acid rained from the sky, killing most of the organisms it touched. The surface of Earth would have been in total darkness for a year or two, shutting down photosynthesis, life’s lifeline to the sun. When the dust thinned, and light began to return through the haze, Earth warmed fast, because the atmosphere now contained a lot more carbon dioxide and methane. A few years after the impact, the wretched survivors could start photosynthesizing and breathing again, but they did so in a hot greenhouse world.

There was one more crisis to be survived before mammals could take over the Earth. That was the Paleocene-Eocene thermal maximum (PETM, for lovers of acronyms), a short, sharp shock of greenhouse warming at the border between the Paleocene and Eocene epochs, about fifty-six million years ago. It was damaging enough to drive many species to extinction. The PETM is of interest today because it is the most recent period of rapid greenhouse warming in Earth’s history, so it may help us understand climate change today. The parallels are eerie. The amounts of carbon dioxide released into the atmosphere during the PETM were similar to those being released today by the burning of fossil fuels, and fifty-six million years ago, the result was an increase of between five and nine degrees Celsius in average global temperatures.24

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.

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).

Why are primate brains so big? This may seem (pardon the pun) a no-brainer. Aren’t brains obviously a good thing? Not necessarily, because they guzzle energy. They need up to twenty times as much energy as the equivalent amount of muscle tissue. In human bodies, the brain uses 16 percent of available energy, though it accounts for just 2 percent of the body’s mass. That’s why, given the choice between brawn and brain, evolution has generally gone for more brawn and less brain. And that’s why there are so few very brainy species.

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.

But in 1924, Raymond Dart, an Australian professor of anatomy based in South Africa, discovered the first important African hominin fossil. It was a skull sitting among a collection of other fossils, the skull of a child from the species now known as Australopithecus africanus, part of a large group of australopithecine species that first appeared about five million years ago. After this discovery, more and more hominin fossils began to turn up in Africa, and most paleoanthropologists now believe that our species evolved somewhere in Africa. From the 1930s, Louis and Mary Leakey began finding hominin fossils and artifacts in Africa’s Rift Valley, where magma pushing up from the mantle has started splitting the tectonic plate on which most of Africa lies.

Walking on two legs required rearrangements of the back, the hips, even the braincase. It also favored narrower hips, which made childbearing more difficult and dangerous and probably means that many hominins, like modern humans, gave birth to infants that were not yet capable of surviving on their own. That would have meant that their babies needed more parenting, which may have encouraged sociability and gotten hominin fathers more involved in child-rearing. There were many indirect effects of bipedalism, but we’re not yet sure exactly why hominins became bipedal. Perhaps bipedalism let our ancestors walk or run farther in the grassy savanna lands that had spread around a cooling world in the past thirty million years. It also freed human hands to specialize in manipulative tasks including, eventually, the making of tools.

The large brains of H. erectus are striking because, as we have seen, brains are costly evolutionary machines. Indeed, the rate of increase of brain size to body weight in hominins was faster than the rates in any other group of species in evolutionary history.9 Perhaps sociability was the driver. The importance of social calculations shows up clearly in the human brain structure, which devotes an exceptional number of neuronal pathways to social calculations. Perhaps more neurons meant more friends, more food, better health, and a better chance of reproducing. Certainly, larger brains allowed hominins to live in larger groups and networks.

Use of fire may have had other important consequences. For example, cooking reduced the digestive work required of the gut, so the gut shrank (and, yes, there is fossil evidence for this), releasing some of the metabolic energy needed to run larger brains. As yet, this interesting hypothesis remains unproven, because good evidence for systematic control of fire appears only from about eight hundred thousand years ago and becomes quite common only after about four hundred thousand years ago.

When you see sudden, rapid changes like this, start looking for tiny changes that have huge consequences. Complexity theory and the related field of chaos theory are full of changes like this. Often, they are described as butterfly effects. The metaphor comes from the meteorologist Edward Lorenz, who pointed out that in weather systems, tiny events (the flapping of a butterfly’s wings, perhaps?) can get amplified by positive feedback cycles, generating a cascade of changes that may unleash tornadoes thousands of miles away. So what tiny changes unleashed the tornado of human history?

Many different features make up the human package, from dexterous hands to large brains and sociability. But what makes us radically different is our collective control of information about our surroundings. We don’t just gather information, like other species. We seem to cultivate and domesticate it, as farmers cultivate crops. We generate and share more and more information and use it to tap larger and larger flows of energy and resources.

How and why our species acquired the linguistic power needed to unleash this powerful new driver of change remain unclear. Was it, as American neuroanthropologist Terrence Deacon has argued, a new ability to compress large amounts of information into symbols (deceptively simple words like symbol that carry a huge informational cargo)? Or was it the evolution of new grammar circuits in the human brain that helped us combine words according to precise rules so as to convey a great variety of different meanings, as the linguist Noam Chomsky has suggested? This is a tempting idea because, as another linguist, Steven Pinker, puts it, the really difficult trick was “to design a code that can extrude a tangled spaghetti of concepts into a linear string of words” and to do this so efficiently that the hearer could quickly re-create the spaghetti of concepts from the linear string.17 Was human language enabled by the increased space for thinking available in an enlarged cortex, which could hold enough complex thoughts in place to form syntactically complex sentences or let an individual memorize the meanings of thousands of words?18 Or do improved forms of language have their roots in the sociability and willingness to collaborate that are particularly well developed in our own species?19 Or was there perhaps a synergy between all these drivers?

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.20

Our own lineage began to spread within Africa starting at least two hundred thousand years ago, which may point to the advantages of collective learning.24 But in a world of small, scattered communities, most of them little larger than extended families, change was slow, erratic, and easily reversed. Whole groups could die out suddenly, along with the technologies, stories, and traditions they had built up over many centuries. The largest catastrophe of this kind occurred about seventy thousand years ago. Genetic evidence shows that the number of humans suddenly fell to just a few tens of thousands, only enough to fill a moderate-size sports stadium. Our species came close to extinction. The catastrophe may have been triggered by a massive volcanic eruption on Mount Toba in Indonesia that pumped clouds of soot into the atmosphere, blocking photosynthesis for months or years and endangering many species of large animals. But then human numbers began to increase again; humans spread more widely, and the machinery of collective learning roared into life once more.

At the Lake Mungo site in Australia, the evidence for religion is compelling. A cremation and burial from about forty thousand years ago and a scattering of other human remains are evidence of rich ritual traditions. Other evidence from the site reminds us that Paleolithic societies, like modern human societies, underwent profound upheavals, many caused by the unpredictable climate changes of the most recent ice age. There were regular periods of aridity from the moment humans first arrived in the Willandra Lakes Region, perhaps fifty thousand years ago. About forty thousand years ago, aridity increased and the lake system began to shrink.

The remains left behind by Paleolithic communities offer grainy snapshots of their societies. But each snapshot represents an entire cultural world, with stories, legends, heroes, and villains, scientific and geographical knowledge, and traditions and rituals that preserved and passed on ancient skills. This accumulation of ideas, traditions, and information was what allowed our Paleolithic ancestors to find the energy and resources they needed to survive and flourish and migrate farther and farther in a harsh, ice-age world.

Whatever the reasons, most Paleolithic communities remained small enough to organize themselves through notions of family or kinship, like most modern foraging societies. That’s why it makes sense to think of Paleolithic communities as families rather than societies. And if modern foraging communities are any guide, they probably had a broad understanding of the term family that extended beyond the world of humans to include other species and even features of the landscape, such as mountains and rivers. Paleolithic societies were embedded in their surroundings ecologically and culturally in ways that modern urban dwellers struggle to understand.

Perhaps they disappeared because climates changed. But they had survived previous ice ages, so it is hard not to think that humans, with their increasingly sophisticated hunting methods, may have tipped them over the edge. The chronology supports this explanation. In Australia, Siberia, and North America, the megafauna vanished not long after the arrival of humans. Perhaps, like the dodo in Mauritius, the megafauna didn’t fear our ancestors enough, unlike African megafauna, which had coevolved with humans and knew how dangerous we could be. In any case, megafauna, like all large animals (including the dinosaurs), are particularly vulnerable to sudden changes.

Removing megafauna changed landscapes. Large herbivores can chomp their way through a lot of plants. Eliminating them increased the frequency of fires, as plant remains were left uneaten. In Australia about forty thousand years ago, the number of fires increased in many regions. A large percentage may have been started by lightning strikes. But we know that here, as in many other parts of the Paleolithic world, humans used fire systematically to fertilize the land. These technologies are known to archaeologists as fire-stick farming, after the fire sticks that indigenous Australians carried to fire the land in historical times.

In short, fire-stick farming increases the productivity of the land. Similar techniques were used in many parts of the world in the late Paleolithic. Though not strictly a type of farming, they were a way of increasing the production of usable plants and animals in a given area of land. They count, in other words, as a form of intensification. Fire-stick farming gives us a preview of the bonanza of food, resources, and energy that would be released by farming.

The coldest period of the most recent ice age, just over twenty thousand years ago, was followed by several thousand years of erratic warming until, starting about twelve thousand years ago, global temperatures settled into the warmer and more stable regime that dominated human history during the Holocene epoch. By the end of the last ice age, our alien scientists would already have been very interested in the strange events afoot on planet Earth. As climates got warmer, the behavior of humans would become even more striking. Quite suddenly (on paleontological scales), humans gained access to much larger flows of energy through farming, and these new flows of energy would allow a quantum leap in the complexity, diversity, size, and intricacy of human societies.

Farming was a mega-innovation, a bit like photosynthesis or multicellularity. It set human history off on new and more dynamic pathways by helping our ancestors tap into larger flows of resources and energy that allowed them to do more things and create new forms of wealth. Like a gold rush, the bonanza of energy would generate a frenzy of change. Eventually it would transform the human relationship to the biosphere because, as farming societies grew, they supported much larger populations and evolved many more moving parts than foraging societies. More energy, resources, and people and more links between communities generated positive feedback cycles that accelerated change. For all these reasons, farming counts as our seventh threshold of increasing complexity.

Farmers found that transforming their environments was hard work. But in return for their chopping, plowing, weeding, draining, and fencing, they got a lot more energy and resources from the land, rivers, and forests that surrounded them, because the species they valued flourished spectacularly. That allowed the first farmers to tap more of the photosynthetic energy flowing through the biosphere. The total flow of photosynthetic energy did not necessarily increase, of course. It may even have declined as farmers removed high-productivity plants such as trees. But for farmers, the important thing was that they could now tap a larger share of the existing flows.

For the most part, though, humans adapted to the symbiotic relationships of farming not with genetic changes but with new behaviors: technological, social, and cultural innovations accumulated through collective learning. They developed new ways of working the land, woodlands, and rivers. And as they did that, they had to learn new ways of collaborating and living together. Cultural change happens much faster than genetic change, and this explains why farming transformed human lifeways within just a few generations.

Major changes took place community by community and spread first through local networks. Over time, ideas moved over larger distances, but, until five hundred years ago, there were some fundamental barriers to the movement of people, ideas, and technologies, including farming. Rising sea levels after the end of the last ice age severed links between Eurasia and the Americas, and there was hardly any communication between Eurasia and Australasia or with the islands of the western Pacific, some of which were settled as early as thirty thousand years ago. In effect, humans now lived in a number of separate world islands or zones. Within these zones, human history played out almost as if the inhabitants were living on different planets.

At the end of the last ice age, two worldwide changes coincided to create a small number of regions in which farming began to look tempting. First, climates began to get warmer and wetter around the globe; second, foragers now occupied so much of the Earth that some regions were beginning to feel overpopulated. Both changes nudged humans toward farming. Because these changes were felt to some degree in different regions in all of the world zones, they help explain the strange fact that farming appeared within just a few thousand years in parts of the world that had no contact with one another.

Climates began to warm, erratically, about twenty thousand years ago, and by thirteen thousand years ago, average global temperatures were similar to today. Then, during the cold snap known as the Younger Dryas period, they fell sharply for at least a thousand years, after which they rose again. For about ten thousand years, climates have been unusually stable. Warmer, wetter climates and exceptional climatic stability made farming more viable than it had been for at least one hundred thousand years, providing the Goldilocks conditions for the entire agrarian era.