A common language connects the members of a community into an information-sharing network with formidable collective powers.
—STEVEN PINKER, THE LANGUAGE INSTINCT
Humanity entire possesses a commonality which historians may hope to understand just as firmly as they can comprehend what unites any lesser group.
—WILLIAM H. MCNEILL, “MYTHISTORY”
The appearance of humans in our origin story is a big deal. We arrived just a few hundred thousand years ago, but today we are beginning to transform the biosphere. In the past, whole groups of organisms, such as the cyanobacteria, have changed the biosphere, but never before has a single species wielded such power. And we’re doing something else that’s utterly new. Because we humans can share individual maps of our surroundings, we have built up a rich collective understanding of space and time that lies behind all our origin stories. This achievement, apparently unique to our species, means that today, one tiny part of the universe is beginning to understand itself.
Our account of human history will barely touch on the things historians usually discuss: the wars and leaders, the states and empires, or the evolution of different artistic, religious, and philosophical traditions. Instead, we will stay with the main themes of our modern origin story. We will watch the appearance of new forms of complexity, created, this time, by a new species that used information in new ways to tap into larger and larger flows of energy. We will see how humans, linked first in local communities but eventually across the world, began to transform the biosphere, slowly at first, then more rapidly, until today we have become a planet-changing species. How we humans will use our power remains unclear. But we already know that humans, and indeed the entire biosphere, stand at a moment of profound and perhaps turbulent change.1
How did we get here? Our modern origin story can help us get our bearings by placing human history within the much larger story of planet Earth and the universe as a whole. The view from the mountaintop can help us see what makes us different.
Culturally, we humans are astonishingly diverse, and that is part of our power. Genetically, though, we are more homogenous than our closest living relatives, the chimps, gorillas, and orangutans. We just haven’t been around long enough to diversify much. Besides, we are extraordinarily sociable, and we love to travel, so human genes have moved pretty freely from group to group.
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.
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.
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. Some species are so disdainful of brains that they treat them as an expendable luxury. 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
However, primate brains do seem to pay their way. They are needed to manage those dexterous hands and feet. And in a very visual species, they are needed to process images (is that a ripe plum three trees away?), because images gobble up processing power in brains just as they do in computers. Even more important, primates are sociable, because living in groups provides protection and support. The pressure to live in large groups increased in open and exposed terrain such as the spreading grasslands and woodlands of a cooling, post-PETM world. To live successfully with other members of your species, you have to keep track of the constantly changing relationships among family, friends, and enemies. Who’s up and who’s down? Who’s friendly and who’s not? Who owes me favors, and who am I in debt to? These are computational tasks whose complexity increases exponentially as groups get larger. If there are just three others, you can probably cope. If there are fifty or a hundred, the calculations are a lot trickier.
To live in groups, you also need some insight into the brains of others. Intuiting the thoughts and feelings of others may have been an important step toward consciousness, the enhanced awareness of what is happening in our own minds.5 Close observation of primate societies shows that if you get these social calculations wrong, you’ll probably eat less well, be less well protected, get beaten up more often, and lower your chances of being healthy and having healthy children.6 So sociability, cooperation, and brainpower seem to have evolved together in the history of primates. Indeed, there seems to be a rough correlation between the size of primate groups and the size of their brains. Apparently, many primate lineages were willing to pay one more entropy tax, the brain tax, if it allowed them to live in larger groups.
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.
Before the 1970s, most paleontologists were convinced from the fossil evidence that humans had diverged from other apes at least twenty million years ago. But in 1968, two geneticists, Vincent Sarich and Allan Wilson, showed that we could estimate when two species diverged by comparing the DNA of species that are alive today. This is because large stretches of DNA, particularly those parts that do not code for genes, change randomly and at a relatively consistent pace. Genetic comparisons using these insights showed that humans, chimps, and gorillas shared a common ancestor until about eight million years ago, at which point the ancestors of modern gorillas decided to go their merry way. 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.
Modern chimps and humans still share well over 96 percent of their genomes. But with three billion base pairs in each genome, that means that about thirty-five million genetic letters, or base pairs, are different. Lurking among these divergent genetic letters are the clues that can tell us why humans and the chimps have had such radically different histories, particularly in recent millennia. Why are our closest relatives now reduced to remnant populations of a few hundred thousand while there are now more than seven billion humans, and we dominate the biosphere?
All species on the human side of the evolutionary divide between humans and chimps are known as hominins. In the past fifty years, paleontologists have found fossil remains (sometimes just a finger bone or a few teeth) from perhaps thirty or more species of hominins. I say perhaps because deciding what is a distinct species depends on which paleontologist you talk to. Some are splitters; they see many different species of hominins. Others are lumpers; they see fewer species but a lot of variation within each species. Today, we are the only surviving hominin species. That is unusual, because until as recently as twenty or thirty thousand years ago, several different species of hominins cruised the savannas of Africa and Eurasia at the same time. The recent disappearance of other hominin species as we humans took up more and more land and resources is a sign of how dangerous we are.
In the past fifty years, paleontologists have acquired a lot of new forensic toys and tricks that have helped them fill in more details of hominin history. Fossilized teeth are particularly informative. That’s good, because teeth are often the only remains we find. Just as your dentist can tell if you’ve been eating popcorn, chocolate, and ice cream, so, too, a good paleontological “dentist” can tell whether our ancestors were eating meat or plants. The shape of a tooth can tell us whether it was used to cut or grind its owner’s food, and that is very informative. Nuts require grinding teeth, such as molars, while meat requires cutting teeth, such as canines.
Chemical signals found in bones and teeth can also tell us a lot about diets and lifeways. For example, C4 photosynthesizers, such as grasses and sedges, absorb more of the slightly heavier carbon isotope, carbon-13, than they do of the more common carbon-12. Analysis of the teeth of Australopithecus africanus from about 2.5 million years ago shows higher than expected carbon-13 ratios, and, as they surely were not eating grasses (no apes can eat grasses), this suggests that they were eating the meat of animals that were eating grasses. And meat-eating implies they were either scavenging or hunting, and perhaps using stone tools.
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.
The fossil record of hominins is much richer than it used to be. In 1900, anthropologists had fossil remains from only two ancient types of humans: Neanderthals, the first of which was found in Germany in 1848, and Homo erectus, whose remains were first found in 1891 in Java by Dutch paleoanthropologist Eugène Dubois. These finds suggested that humans could have evolved in Europe or Asia. 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. Eventually, a new sea will appear here. Meanwhile, cracks in the African tectonic plate give fossil hunters glimpses into the remote past of our species.
In 1974, in Ethiopia, Donald Johanson discovered 40 percent of the skeleton of another australopithecine species, Australopithecus afarensis. The skeleton was named Lucy and dated to about 3.2 million years ago. Other australopithecine remains have been found that are almost four million years old. Since then, earlier hominin species have been found in other parts of Africa, dating to four and five million years ago (Ardipithecus) and even to six million years ago (Orrorin tugenensis), or perhaps seven (Sahelanthropus tchadensis), which is pretty close to the notional date when the last common ancestor of all hominins lived.
We have so few very early hominin fossils that a single new discovery could change the story radically. It is not even certain that the oldest fossils are really hominins, nor is it always clear whether fossil remains belong to distinct species or not. Should Homo habilis and Homo erectus, species with very different brain sizes, be assigned to different genera, or should H. habilis be regarded as late australopithecines? Our understanding of early hominin history remains sketchy, but parts of the story are getting clearer.
Even the earliest hominin species seem to have walked on two legs, at least some of the time. This is very different from chimps and gorillas, which knuckle-walk. You can tell from bones if a species regularly walks on two legs. In bipedal species, the big toe is no longer used for gripping, so it aligns more closely with the other toes, while the spine enters the skull from below, not from the back (get down on four legs and you’ll understand why). 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.
There are no signs that the earliest hominins were exceptionally brainy by primate standards. Their skulls contained brains much smaller than ours and more like chimpanzee brains, with a volume of about 300 to 450 cubic centimeters. Our brains, in comparison, average about 1,350 cubic centimeters. More significant than absolute size, though not easy to calculate, is the extent to which brain size deviates from the expected brain size for a given body weight within a particular group of organisms. This is the encephalization quotient (EQ). Chimps have an EQ of about 2 (compared to other mammals), and modern humans have an extraordinarily high EQ of about 5.8. The EQs of australopithecines range from 2.4 to 3.1.8 Extreme braininess was not the first distinguishing feature of the hominins. Bipedalism was.
The first fossils that are currently classified within our own genus, Homo, belong to a species known as Homo habilis, which lived in Africa from about 2.5 to 1.5 million years ago. The first evidence of this species, consisting just of a jawbone and some hand bones, was found in 1960 by Mary Leakey and her son Jonathan in Olduvai Gorge in the African rift valley. The close association with stone tools persuaded the Leakeys to classify the new species as a form of Homo, which was a paleontologist’s way of saying “I think these are really humans because they made tools.”
But were they us? Is this when human history began? Today, most researchers are skeptical about a distinct Homo genus that includes both us and habilis. After all, habilis brains were only slightly larger than those of australopithecines, ranging from 500 to 700 cubic centimeters, with an encephalization quotient of just over 3. And their stone tools involved little more than smashing rocks and using the fragments. Given that some australopithecine species probably made stone tools and that chimps, too, can make tools (though not stone tools), it looks as if Homo habilis was similar enough to the australopithecines to be classified with them. Tool use does not make them human, because we now know that toolmaking is not unique to humans.
By two million years ago, at the beginning of the Pleistocene epoch, we find hominin species that were larger, had bigger brains, made more sophisticated stone tools, and exploited a wider range of environments. It is probably no coincidence that they appeared as climates were getting colder and drier. These species are normally classified as Homo erectus or Homo ergaster, but here I will use the label H. erectus for the whole group.
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.10 Most primates, including chimps and baboons, lived in groups of fewer than fifty individuals, and, roughly, the smaller the brain, the smaller the group. But as brain sizes increased in the past two million years, the size of hominin groups increased, too. Homo erectus was probably the first hominin species to live in groups that linked more than fifty individuals.
The first H. erectus remains were found in Java in 1891 by Eugène Dubois. He was looking in Indonesia because of a hunch he had that humans were descended not from African chimpanzees (Darwin’s bet), but from Asian orangutans. He got that wrong. But the remains he found did have brains with volumes of almost 900 cubic centimeters, much closer to the modern human average of about 1,350 cubic centimeters. And they had an EQ of 3 to 4. The fact that the remains were found in Java also showed that H. erectus had the technologies and skills needed to migrate from Africa through much of southern Eurasia. But we shouldn’t be too impressed by this. Many other species, such as lions, tigers, elephants, and even our close relatives the orangutans, had made similar migrations, and that’s because many environments in southern Eurasia are not that different from African environments. Indeed, recent evidence suggests that species closely related to Homo habilis may have traveled as far as Indonesia to become the ancestors of the tiny hominins known as Homo floriensis (or the Hobbits), which lived on the island of Flores as recently as sixty thousand years ago.11
H. erectus were taller than H. habilis, some of them as tall as modern humans. They also made more sophisticated stone tools than H. habilis. These are the beautiful and carefully designed stone tools known as Acheulean axes. Better stone tools may have given H. erectus access to more meat, a crucial source of high-energy food to fuel their expanding brains. They may have also learned to manage, control, and use fire, which would have allowed them to tap into a huge new source of energy. The primatologist Richard Wrangham has argued that H. erectus used fire to cook (in other words, to predigest and detoxify) meat and other foods. This would have increased the range of foods they could eat, because many foods are indigestible or poisonous until cooked. Cooking would also have reduced the time they spent chewing and digesting their food.
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.12 We also know that the stone technologies of H. erectus changed little over a million years, so H. erectus seem to have lacked the technological flair and creativity of our own species.
In the past million years, hominin evolution accelerated. About six hundred thousand years ago, new species appear in the fossil record, with brains and bodies more and more like modern humans’. Not surprisingly, they apparently lived in larger groups, too, groups that linked as many as 150 individuals, which seems to have been the upper limit among our hominin ancestors.13
There are complex debates about how many different species of hominins there were half a million years ago. We know there were many. But more important is the larger trend: Now hominins appear in ice-age Europe and northern Asia, environments that were very different from the African savanna and demanded new skills and technologies. So it is no surprise that their tools were more sophisticated, more varied, and more specialized than those of H. erectus. For the first time, hominins hafted stone points to wooden shafts. In Schöningen, Germany, archaeologists have found four-hundred-thousand-year-old wooden spears made with precision and delicacy. Some anthropologists have even detected evidence of artistic and ritual activity. Among the finds of Eugène Dubois were decorated mussel shells, dated to five hundred thousand years ago, that look suspiciously like simple forms of art.
Still… none of this was revolutionary. The really spectacular changes began only about two or three hundred thousand years ago, after the appearance of our own species, Homo sapiens.
Imagine a team of alien scientists who have been orbiting our planet searching for intelligent life and studying Earth’s life-forms in a longitudinal research project lasting several million years. Two hundred thousand years ago, they wouldn’t have noticed anything unusual about our ancestors. In Africa and parts of Europe and Asia, they might have spotted several species of large, bipedal primates, including the species we call Homo neanderthalensis and Homo heidelbergensis. They might even have seen individuals that a modern human paleontologist would describe as Homo sapiens, because the oldest skull normally assigned to our species is almost two hundred thousand years old. It was found at Omo Valley in Ethiopia in the African rift valley. (In June 2017, human remains from Morocco were dated to three hundred thousand years ago, but their exact relationship to us remains uncertain.) But there was little to distinguish these early humans from many other large or medium-size primate and mammal species. They lived in small, scattered nomadic communities with a total population of, at most, a few hundred thousand individuals. Like all large animals, they gathered or hunted the food and energy they needed from their surroundings.
Today, two or three hundred thousand years later (no time at all for a paleontologist), our orbiting aliens searching for intelligent life would have seen enough changes in the behavior of this particular species to justify a few scholarly high-fives. They would have watched as humans spread around the world. Then, starting from the end of the last ice age, ten thousand years ago, they would have noticed human numbers growing fast. They would also have watched as humans began to change their environments to suit them better by burning down forests, diverting rivers, plowing the land, and building towns and cities. In the past two hundred years, human numbers grew to over seven billion, and our species began to transform the oceans, the land, and the air. Human-built roads, canals, and railways snaked across the continents, linking thousands of human-built cities with populations in the millions. Vast ships navigated the oceans, and planes ferried goods and people through the air and across the continents. Just a hundred years ago, in glowing filaments and patches, Earth started lighting up at night. The aliens’ instruments would also have shown that oceans were getting more acidic, the atmosphere was warming, coral reefs were dying, and polar ice caps were shrinking. Biodiversity was declining so fast that some of the alien biologists might have wondered if this was the start of another mass extinction.
Paleontologically speaking, changes this fast are the equivalent of an explosion. Without planning it, we have become a planet-changing species. We even have the power, if we are foolish enough, to destroy much of the biosphere in just a few hours by launching some of the eighteen hundred nuclear missiles that remain on high alert today. No single species has had such power in the four-billion-year history of the biosphere.
Clearly a new threshold had been crossed. Our alien scientists would surely have been asking themselves, What is it about this strange species?
Historians, anthropologists, philosophers, and scholars in many other fields have wrestled long and hard with the same question. Some feel the question is too complex, too loaded, and too multidimensional to yield a scientific answer. But curiously, when we see human history as part of the larger history of the biosphere and the universe, the distinctive features of our species stand out more clearly. Today, scholars in many different fields seem to be converging on similar answers to the question of what makes us different.
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. New information gave humans improved spears and bows and arrows that allowed them to hunt larger animals more safely. It gave them better boats that gave them access to new fisheries and new lands, and it offered new botanical knowledge that allowed them to leach the poisons from potentially edible plants such as cassava. In more modern times, new information lay behind the technologies that let us tap the energy of fossil fuels and build the electronic networks that link us into a single world system.
Information management on this scale was not the achievement of individuals. It depended on sharing, on the accumulation of millions of individual insights over many generations. Eventually, community by community, this sharing created what the Russian geologist Vladimir Vernadsky called a noösphere, a single global realm of mind, of culture, of shared thoughts and ideas. “There is,” writes Michael Tomasello, “only one known biological mechanism that could bring about these kinds of changes in behavior and cognition in so short a time.… This biological mechanism is social or cultural transmission, which works on time scales many orders of magnitude faster than those of organic evolution.” This process, which Tomasello calls “cumulative cultural evolution,” is unique to our species.14
The tiny change that allowed humans to share and accumulate so much information was linguistic. Many species have languages; birds and baboons can warn others in their group of the approach of predators. But animal languages can share only the simplest of ideas, almost all of them linked to what is immediately present, a bit like mime (imagine trying to teach biochemistry or wine-making in mime). Several researchers have tried to teach chimps to talk, and chimps can, indeed, acquire and use vocabularies of one or two hundred words; they can even link pairs of words in new patterns. But their vocabularies are small and they don’t use syntax or grammar, the rules that allow us to generate a huge variety of meanings from a small number of verbal tokens. Their linguistic ability seems never to exceed that of a two-or three-year-old human, and that is not enough to create today’s world.
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.15
These linguistic enhancements allowed humans to share information with such precision and clarity that knowledge began to accumulate from generation to generation. Animal languages are too limited and too imprecise to allow this sort of accumulation. If any earlier species did have this ability, it would surely have left traces, including an expanding range and an increasing impact on its environment. In fact, we would see the sort of evidence we find for human history. 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.
How and why our species acquired the linguistic power needed to unleash this powerful new driver of change remains 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 is particularly well developed in our own species?19 Or was there perhaps a synergy between all these drivers?
Whatever happened, our species seems to have been the first to cross the linguistic threshold beyond which information can accumulate within communities and across generations. Like a gold strike, collective learning unleashed a bonanza of information about plants and animals, about soils, fire, and chemicals, and about literature, art, religion, and other humans. Though some information was also lost every generation, in the long run, human stores of information accumulated, and that growing wealth of knowledge would drive human history by giving humans access to increasing flows of energy and increasing power over their surroundings. 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.20
The great world historian W. H. McNeill constructed his classic world history The Rise of the West around the same idea: “The principal factor promoting historically significant social change is contact with strangers possessing new and unfamiliar skills.”21
Human history begins, then, with collective learning. But when did collective learning begin?
Even our alien scientists would hardly have noticed the first flickering of collective learning as they circled Earth two hundred thousand years ago. Some form of collective learning may have been at work even in H. erectus communities, but its consequences were not yet revolutionary. Hints of more rapid technological change begin to appear in the African archaeological record at least three hundred thousand years ago in the form of increasingly delicate stone tools, many of them hafted.22 And it is not just Homo sapiens who show this creativity but also Neanderthals and the hominin species known as Homo heidelbergensis. Perhaps all these species were acquiring improved forms of language that brought them tantalizingly close to threshold 6. Early evidence of ritual or symbolic or artistic activity is particularly significant because it suggests an ability to think symbolically or tell stories about imaginary beings, and that may indicate the arrival of modern forms of language.
Perhaps there was room for only one species to cross the threshold to collective learning. There is an evolutionary mechanism known as competitive exclusion that explains why two species can never share exactly the same niche. One will eventually drive out its rival if it can exploit the same niche slightly more effectively. So we can imagine several species gathering near the evolutionary threshold to collective learning, but then one broke through and began to exploit its environment so efficiently that its numbers multiplied and grew fast enough to lock out its rivals.23 This may help explain why our closest hominin relatives, such as the Neanderthals, have perished, and our closest surviving relatives, the chimps and gorillas, are approaching extinction.
Evidence of technological and cultural change from before a hundred thousand years ago is foggy and difficult to interpret. 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.
In the past one hundred thousand years, we get some glimpses of how our ancestors lived and find clearer evidence for collective learning. Like all large animals, our ancestors collected or hunted resources and game from their surroundings. But there was a crucial difference between those animals and early humans. While other species hunted and gathered using a repertoire of skills and information that barely changed over the generations, humans did so with increasing understanding of their environments, as they shared and accumulated information about plants, animals, seasons, and landscapes. Collective learning meant that, over the generations, human communities hunted and gathered with growing skill and efficiency.
Some sites give us intimate glimpses of how our ancestors lived. At Blombos Cave, on the Indian Ocean shores of South Africa, archaeologist Christopher Henshilwood and his colleagues have excavated sites dating from ninety thousand to sixty thousand years ago. The inhabitants of Blombos Cave ate shellfish, fish, and marine animals as well as land mammals and reptiles. They cooked in well-tended hearths.25 They made delicate stone blades and bone points that were probably hafted to wooden handles with specially prepared glues. But they were also artists. Archaeologists have found ocher stones with geometrical scratch marks on them that look for all the world like symbols or even writing. They also made different-colored pigments and ostrich-shell beads. It is tempting to see this evidence as a sign that the Blombos communities valued collective learning and the preservation and transmission of information, and that surely means that they preserved and told stories that summed up their community’s knowledge.
It is hard not to see similarities with modern foraging communities. If these similarities are not misleading us, we can imagine many groups like those from Blombos Cave with a great diversity of gathering and hunting techniques built up over many generations. We can imagine them migrating through familiar home territories, held together by family ties and shared languages and traditions. They surely danced and sang, too, and told origin stories, and they almost certainly had what we moderns might want to call religions.
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.
Twenty thousand years later, at the coldest phase of the ice age, there were communities living in tundra-like environments on the steppes of modern Ukraine. At sites like Mezhirich, people built huge marquee-like tents, using skins stretched over a scaffolding of mammoth bones, and warmed them with internal hearths. They hunted mammoths and other large animals and stored meat in refrigerated pits for recovery during the long cold winters. They hunted fur-bearing animals and used needle-like objects with ornamental heads carved from bone to sew warm clothing. As many as thirty people may have lived together at Mezhirich during the long ice-age winters. There are similar sites near Mezhirich. This suggests there were regular contacts between neighboring groups, the sort of networks through which information about new technologies, changing climates, animal movements, and other resources would have been exchanged, as well as stories. People, too, would have moved between neighboring groups.
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.
Evidence from ice cores now lets us track global temperature changes with great precision across hundreds of thousands of years. During the Pleistocene epoch, which encompasses the two million years since the evolution of Homo erectus, there were many ice ages. They normally lasted for one hundred thousand years or more, with briefer warm periods, or interglacials, between them. The period we live in now is a warm interglacial that began ten thousand years ago, at the start of the Holocene epoch. The previous interglacial occurred about a hundred thousand years ago and may have lasted for twenty thousand years or more. After it ended, global climates got steadily colder and drier, though with many temporary reversals and local variations. The coldest period of the last ice age was from about twenty-two thousand to eighteen thousand years ago.
As climates cooled, areas that had been occupied for hundreds or thousands of years had to be abandoned. Sites in northern Europe that had been occupied starting about forty thousand years ago were abandoned for thousands of years. Even in the warmer climates of Australia’s far north, people survived by the skin of their teeth.26 Lawn Hill River in the far northwest of Queensland carved gorges through thick layers of limestone and provided local people with a good living from both the fish and marine animals of the rivers and the surrounding highlands. But during the coldest phases, people abandoned the icy highlands entirely and stayed in the protected environments of the gorges.
As technological and ecological knowledge accumulated, many communities moved into new environments, pulled or pushed by climate change, by conflicts with their neighbors, or, perhaps, by overpopulation. Over thousands of years, small-scale migrations would eventually take our species, kilometer by kilometer, to every continent other than Antarctica. Today, we can track these migrations by following the spread of archaeological remains around the world and by comparing the genes of different modern populations.27
One hundred thousand years ago, during the last interglacial, almost all humans lived in Africa, though a tiny number had left for the Middle East. At sites such as the caves of Skhul and Qafzeh in modern Israel, they may have encountered and occasionally interbred with Neanderthals. (We know this because today, most humans who live outside Africa have some Neanderthal genes.) Then, as climates cooled, our ancestors seem to have left the Middle East to the Neanderthals, whose bodies were better adapted to colder climates. They didn’t return until about sixty thousand years ago. However, some humans may have traveled east into Central Asia and South Asia. One reason for thinking this is that humans reached Sahul (the ice-age continent that included Australia, Papua New Guinea, and Tasmania) between fifty thousand and sixty thousand years ago. Migrants leaving Africa sixty thousand years ago would have had to move extraordinarily fast to get there, so it seems more likely that the first Australians arrived from communities long established in Asia.28 Settling Australia was a major event in human history. We don’t know what drove the first settlers—probably population pressure or conflicts with other communities in the southern parts of what is now Indonesia. But we do know that the crossing required advanced seafaring skills and the ability to adapt fast to an entirely new suite of plants and animals. No other species made the sea crossing. (Dingoes arrived in recent millennia, almost certainly with human help.)
The earliest migrations into Siberia and northern Europe were probably short-lived exploratory probes during brief warm periods. But sites such as Mezhirich show that by twenty thousand years ago, our ancestors could cope with extremely cold environments. Some may have settled permanently in Siberia as early as forty thousand years ago. Twenty thousand years later, at the coldest phase of the last ice age, some Siberians trekked east across the land bridge of Beringia, which was crossable because so much water was locked up in polar glaciers that ocean levels were lower than today. From Beringia, humans spread into the Americas, either by going through Alaska or by traveling in small boats along the northwestern coast of North America. From there, some migrated into South America, probably reaching as far south as Tierra del Fuego within two or three thousand years. At present, the earliest firm evidence for the presence of humans in North America dates to about fifteen thousand years ago.
In the Paleolithic period, migration was probably the most common reaction to innovations or population pressure. A trickle of emigration meant that each human community could remain about the same size as our species spread around the world, and that meant that communities could preserve many of their traditional social rules. This is why we have little evidence for large Paleolithic settlements, though there is plenty of evidence that the total number of communities increased, as well as the total number of humans. The English anthropologist Robin Dunbar has argued that 150 people represents the largest group size that human brains can normally cope with, so it may be that communities naturally split if they got any larger. Dunbar has argued that even today, most humans are embedded in intimate networks that are no larger than 150, even if they have more fleeting relationships with many other people. Modern communities are huge, but only because of the creation of special new social structures to hold them together.
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.
Though small, Paleolithic communities had the universal human knack of accumulating new ideas, insights, and knowledge, so even if it we cannot track their histories in detail, we know that they showed the same cultural and technological dynamism as later human communities did, if on a smaller scale.
Like modern foragers, our Paleolithic ancestors surely had intimate and precise knowledge of the habits and life patterns of the animals and insects they hunted and the plants they used for their food, clothing, and equipment. The looser networks through which people, stories, rituals, and information were exchanged would have linked communities over large areas. From archaeological and anthropological evidence, we can conclude that family groups lived separately for most of the time but gathered periodically in Paleolithic equivalents of the Olympic Games at sites where there was enough food to support temporary gatherings of hundreds of individuals. In the Snowy River region of Southeast Australia, for example, many groups came together when millions of bogong moths hatched, providing the food needed to support the large gatherings known today as corroborees. At these meetings, stories were swapped, rituals and gifts were exchanged, ties of solidarity were maintained in dances and ceremonies, and marriage partners (or disgruntled individuals) moved from group to group. In the south of France fifteen thousand years ago, there were similar gatherings, as human communities followed and hunted herds of horses, deer, and cattle and engaged in periodic rituals that generated beautiful rock art. The art and sculptures produced at sites such as the Lascaux Caves and the La Madeleine rock shelter in the Dordogne region, and the even older stone carvings found in many parts of Australia, are, to modern eyes, as beautiful and sophisticated as any art ever produced by humans. They help illuminate the rich intellectual and mental world of our Paleolithic ancestors.
As hunting and gathering techniques became more sophisticated, our ancestors began to shape their environments in new ways. In some parts of the world, they changed the mix of surrounding species. The first humans in Australia found many species of large animals, or megafauna. Some were as big as the rhinoceroses, elephants, and giraffes of South Africa, the one part of the world in which large numbers of megafauna survive today. In Australia, there were giant kangaroos and wombats and huge flightless birds such as Genyornis newtoni. Then, quite suddenly, most of the Australian megafauna disappeared, as they would eventually disappear in Siberia and the Americas.
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. There are many modern examples of megafaunal extinctions, such as the disappearance of the large New Zealand birds known as moas within a few centuries of the arrival of humans. In Siberia and the Americas, we even have direct evidence of kill sites, so we know that humans hunted megafauna such as mammoths.
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. Systematic use of fire, not just to cook or protect yourself but to transform your environment, represents one of the first signs of the growing ecological power of our species. If you had the skills needed to manage fires safely, regular firing of the land provided many advantages. Burn an area of grassland, then wander back in a day or two, and the first thing you will find is plant and animal barbecues. Wait a few weeks and you will find new growth, because the fire has scattered ash as a fertilizer and sped up the decomposition of plant and animal remains. Grasses and other plants will sprout and can be harvested sooner. And new plants will usually attract herbivores and small reptiles, making the hunting easier and more productive. 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.
As people shared information, ideas, and insights, as well as jokes, gossip, and stories, over many generations and among neighboring communities, there slowly accumulated, region by region, a body of information that I am tempted to call scientific. Paleolithic science included knowledge about usable resources, whether hunted or gathered, whether for eating or making clothes or healing; knowledge about techniques, whether for navigation or hunting or digging for root crops; knowledge about astronomy; and social knowledge about how to approach and talk to elders or strangers and how to mark important transitions in the lives of individuals. It was valuable knowledge because it was needed for survival, so tending it and passing it on was a matter of great seriousness. Knowledge was filtered through many minds, tested for its authoritativeness, accuracy, and usefulness, and eventually incorporated in the origin stories that lay at the heart of education. And this slow increase in available information and the control that this accumulated information gave our species over the natural world and over energy flows through the biosphere would turn out to be the primary driver of change in human history. As humans spread, so did knowledge. Though knowledge was still compartmentalized, community by community, we can imagine the slow emergence, for the first time in the planet’s history, of a new sphere of shared knowledge, the noösphere.
During the Paleolithic period, the noösphere expanded through Africa, Eurasia, Australasia, and then to the Americas, as human numbers increased. When human communities spread within Africa, their populations may have risen to a few tens of thousands, or even hundreds of thousands, though there were surely local fluctuations in numbers. And as we have seen, human numbers plummeted to just a few tens of thousands just seventy thousand years ago. The Italian demographer Massimo Livi-Bacci estimates that thirty thousand years ago, there may have been five hundred thousand humans, and by the beginning of the Holocene, just ten thousand years ago, there may have been five or six million.29
If we take just these last two figures, they suggest that human populations increased by about twelve times (or by an average of a quarter of a million every thousand years) in the last twenty thousand years of the Paleolithic period. On the reasonable assumption that each individual was using no less energy than before, that suggests that total human energy consumption also increased by about twelve times. Collective learning, over more than one hundred thousand years, had significantly increased human control over energy and resource flows in many different parts of the world.
Most of these increasing flows of energy supported population growth. Not much energy was spent on increasing complexity at the local level; as we have seen, human communities remained small and intimate. At the species level, though, there is no doubt that the spread of humans around the world represented an increase in complexity, because by ten thousand years ago, humans employed a much greater diversity of technologies and information than any other species on Earth, and they deployed it across much of the planet.
We have no evidence that more energy increased affluence. Some foragers may have lived pretty well. Indeed, the anthropologist Marshall Sahlins argued that in some environments, Paleolithic communities enjoyed varied diets, high levels of health, and large amounts of leisure time, which they could use for storytelling, for sleeping or relaxing, and for the marathon dances that seem to have bound most small communities together.30 But there cannot have been significant differences in wealth, because foragers had no reason to accumulate goods when they could get most of what they needed from their surroundings. Besides, when you’re regularly on the road, you want only the most valuable and portable of goods.
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.