How Human Evolution Prepared Us To Survive Future Disasters

We may be in the early stages of a disaster so profound that it could kick off a mass extinction. Does that mean humanity is doomed? No. Scientific evidence suggests that humans will survive. Find out why, in this excerpt from Scatter, Adapt, and Remember: How Humans Will Survive A Mass Extinction.

We’ve got the preface to the book, plus a chapter on human evolution, which explores two of humanity’s most important survival tools: the ability to explore unknown regions, and the urge to tell stories.


HUMANITY IS AT a crossroads. We have ample evidence that Earth is headed for disaster, and for the first time in history we have the ability to prevent that disaster from wiping us out. Whether the disaster is caused by humans or by nature, it is inevitable. But our doom is not. How can I say that with so much certainty? Because the world has been almost completely destroyed at least half a dozen times already in Earth’s 4.5-billion-year history, and every single time there have been survivors. Earth has been shattered by asteroid impacts, choked by extreme greenhouse gases, locked up in ice, bombarded with cosmic radiation, and ripped open by megavolcanoes so enormous they are almost unimaginable. Each of these disasters caused mass extinctions, during which more than 75 per cent of the species on Earth died out. And yet every single time, living creatures carried on, adapting to survive under the harshest of conditions.

My hope for the future of humanity is therefore not simply a warm feeling I have about how awesome we are. It is based on hard evidence gleaned from the history of survival on Earth. This book is about how life has survived mass extinctions so far. But it is also about the future, and what we need to do to make sure humans don’t perish in the next one.

During the last million years of our evolution as a species, humans narrowly avoided extinction more than once. We lived through harsh conditions while another human group, the Neanderthals, did not. This isn’t just because we are lucky. It’s because as a species, we are extremely cunning when it comes to survival. If we want to survive for another million years, we should look to our history to find strategies that already worked. The title of this book, Scatter, Adapt, and Remember, is a distillation of these strategies. But it’s also a call to implement them in the future, by actively taking on the project of human survival as a social and scientific challenge.

In the near term, we need to improve one of humanity’s greatest inventions, the city, to make urban life healthier and more environmentally sustainable. Essentially, we need to adapt the metropolis to Earth’s current ecosystems so that we can maintain our food supplies and a habitable climate. But even if you’re not worried about climate change, Earth is still a dangerous place. At any time, we could be hit by an asteroid or a gamma-ray burst from space. That’s why we need a long-term plan to get humanity off Earth. We need cities beyond the Blue Marble, oases on other worlds where we can scatter to survive even cosmic disasters.

But none of this will be possible if we don’t remember human history, from our earliest ancestors’ discovery of fire to our grandparents’ development of space programs. Fundamentally, we are a species of builders and explorers. We’ve survived this long by taking control of our destiny. If we want to survive the next mass extinction, we can’t forget how we got here. Now let’s forge ahead into the future that we’ll build forourselves, our planet, and the humans who will exist a million years from now.

Evidence for the Next Mass Extinction

Over the past four years, bee colonies have undergone a disturbing transformation. As helpless beekeepers looked on, the machinelike efficiency of these communal insects devolved into inexplicable disorganisation. Worker bees would fly away, never to return; adolescent bees wandered aimlessly in the hive; and the daily jobs in the colony were left undone until honey production stopped and eggs died of neglect. In reports to agriculture experts, beekeepers sometimes called the results “a dead hive without dead bodies.” The problem became so widespread that scientists gave it a name — Colony Collapse Disorder — and according to the US Department of Agriculture, the syndrome has claimed roughly 30 per cent of bee colonies every winter since 2007. As biologists scramble to understand the causes, suggesting everything from fungal infections to parasites and pollution, farmers worry that the bee population will collapse into total extinction. If bees go extinct, their loss will trigger an extinction domino effect because crops from apples to broccoli rely on these insects for pollination.

At the same time, over a third of the world’s amphibian species are threatened with extinction, too, leading many researchers to call this the era of amphibian crisis. But the crisis isn’t just decimating bees and frogs. The Harvard evolutionary biologist and conservationist E. O. Wilson estimates that 27,000 species of all kinds go extinct per year.

Are we in the first act of a mass extinction that will end in the death of millions of plant and animal species across the planet, including us?

That’s what proponents of the “sixth extinction” theory believe. As the term suggests, our planet has been through five mass extinctions before. The dinosaur extinction was the most recent but hardly the most deadly: 65 million years ago, dinosaurs were among the 76 per cent of all species on Earth that were extinguished after a series of natural disasters. But 185 million years before that, there was a mass extinction so devastating that paleontologists have nicknamed it the Great Dying.

At that time, 95 per cent of all species on the planet were wiped out over a span of roughly 100,000 years — most likely from megavolcanoes that erupted for centuries in Siberia, slowly turning the atmosphere to poison. And three more mass extinctions, some dating back over 400 million years, were caused by ice ages, invasive species, and radiation bombardment from space.

The term “sixth extinction” was coined in the 1990s by the paleontologist Richard Leakey. At that time, he wrote a book about how this new mass extinction began 15,000 years ago, when the Americas teemed with mammoths, as well as giant elk and sloths. These turbo-vegetarians were hunted by equally large carnivores, including the saber-toothed cat, whose eight-inch fangs emerged from between the big cat’s lips, curving to well beneath its chin. But shortly after humans’ arrival on these continents, the megafauna populations collapsed. Leakey believes human habitat destruction was to blame for the extinctions thousands of years ago, just as it can be blamed today for the amphibian crisis. Leakey’s rallying cry has resulted in sober scientific papers today, where respected biologists detail the evidence of a mass extinction in the making. The New Yorker’s environmental journalist Elizabeth Kolbert has tirelessly reported on scientific evidence gathered over the past two decades corroborating the idea that we might be living through the early days of a new mass extinction.

Though some mass extinctions happen quickly, most take hundreds of thousands of years. So how would we know whether one was happening right now? The simple answer is that we can’t be sure. What we do know, however, is that mass extinctions have decimated our planet on a regular basis throughout its history. The Great Dying involved climate change similar to the one our planet is undergoing right now. Other extinctions may have been caused by radiation bombardment or stray asteroids, but as we’ll see in the first section of this book, these disasters’ most devastating effects involved environmental changes, too.

My point is that regardless of whether humans are responsible for the sixth mass extinction on Earth, it’s going to happen. Assigning blame is less important than figuring out how to prepare for the inevitable and survive it. And when I say “survive it,” I don’t mean as humans alone on a world gone to hell. Survival must include the entire planet, and its myriad ecosystems, because those are what keep us fed and healthy.

There are many ways we can respond to the end of the world as we know it, but our first instincts are usually paralysis and depression. After all, what can you do about a comet hurtling towards us through space, unless you’re Bruce Willis and his crack team of super-astronauts on a mission to blow that sucker up with a bunch of nukes? And what can you do to stop global environmental changes? This kind of “nothing can be done” response is completely understandable, but it rarely leads to pragmatic ideas about how to save ourselves. Instead, we are left imagining what the world will be like without us. We try to persuade ourselves that maybe things really will be better if humans just don’t make it.

I’m not ready to give up like that, and I hope you aren’t either. Let’s assume that humans are just getting started on their long evolutionary trek through time. How do we switch gears into survival mode?

Survivalism vs. Survival

Many of us already have concrete ideas about how we’d survive a disaster. Survivalist groups build shelters stocked with food, preparing for everything from nuclear attack to super-storms. Most of us are survivalists in small ways, too, even if we don’t build elaborate mountain hideaways. I live in San Francisco, where it’s common for people to keep big jugs of water and food supplies in our homes just in case we’re hit with a major earthquake. Our city government recommends that we all stash away enough supplies for a week, including fuel and water-purification tablets. Living here, I’m always aware of the possibility that my city might be in ruins tomorrow. It’s such an ever-present danger that I’ve worked out a quake contingency plan with my family: If a large quake hits and we can’t reach each other by phone, we’re going to meet in the southwest corner of Dolores Park, an open area that’s likely to be relatively safe and undamaged. We picked this location partly because over 100 years ago, people who survived San Francisco’s last great quake met in Dolores Park, too.

One reason I decided to write this book is that I’ve spent so much time thinking about future disasters. I don’t just mean the quake that’s going to wreck my home. For most of my life I’ve been obsessed with stories about the end of the world. The whole thing probably started with the Godzilla movies I watched as a kid with my dad, but by the time I was an adult I’d consumed every story about the apocalypse I could get my hands on, from cheesy movies like Hell Comes to Frogtown to literary novels like Margaret Atwood’s Oryx and Crake. When I was getting my Ph.D. in English, I wrote my dissertation on violent monster stories, exploring why people are drawn to the same tales of disaster over and over again. Eventually I left academia to become a science journalist, which didn’t exactly curb my appetite for destruction. I produced stories about everything from computer hacking to pandemics. While I was at MIT doing a Knight Science Journalism fellowship, I was first exposed to the idea that planetwide mass extinction is a vital part of Earth’s history, and an inevitable part of our future, too. Everything I had read in the fields of fiction and science led me to a single, dark conclusion:

Humans are screwed, and so is our planet.

And so, a few years ago, I set out to write a book about how we are all doomed. I even printed out a brief outline of what I would research, then scribbled at the bottom: “Life is still nasty, brutish and short.” With this idea in mind, I immersed myself in the scientific literature on mass extinction. But soon I discovered something I didn’t expect — a single, bright narrative thread that ran through every story of death. That thread was survival. No matter how horrific things got, in geological and human history, life endured. I began to experience a kind of guarded optimism; perhaps billions of creatures would die in the coming mass extinction, but some would live. I reexamined my assumptions, and started to research what it would take for humans to be part of that bright narrative thread. I interviewed over a hundred people in fields from physics andgeology to history and anthropology; I read about survival strategies in scientific journals, engineering manuals, and science fiction novels; and I traveled all over the world to find evidence of humans’ quest to survive in ancient cities and modern-day labs. I emerged from my research with the belief that humanity has a lot more than a fighting chance at making it for another million years.

Human beings may be experts at destroying life, including our own, but we are also tremendously talented at preserving it. For all the stories about human selfishness and bloodlust, there are just as many about people putting themselves in mortal danger to rescue strangers from burning houses or oppressive governments. Our urge to live spills over onto everything else around us: We don’t want to live alone. During terrible disasters, we try to save as many other creatures as possible when we save ourselves. The urge to survive, not just as individuals but as a society and an ecosystem, is built into us as deeply as greed and cynicism are. Perhaps even more deeply, since the quest for survival is as old as life itself.

It’s hard to convey in words what it’s like to experience a change of heart based on gathering scientific evidence. I found hope in the historical accounts of human survival that Rebecca Solnit presents in A Paradise Built in Hell: The Extraordinary Communities That Arise in Disaster, and I found a scientific basis for that hope in Joan Roughgarden’s The Genial Gene: Deconstructing Darwinian Selfishness. These thinkers and many more suggest we possess the cultural and evolutionary drive to help each other survive. Put another way, I gained a new appreciation for movies like The Avengers, where our heroes unite to save the world.

All survival strategies, however small, are signs that we harbour hope about the future. The problem is that most of our strategies, like my earthquake plan, are focused on personal survival. I’m only prepared to help myself and a few close companions make it through the coming disaster. Stashing away a week’s worth of canned goods isn’t a plan that scales well for an entire planet and all the human civilisations on it. Though it’s not a bad idea to stock shelters with supplies for our families, we aren’t going to survive a mass extinction that way. Our strategies need to be much bigger.

We have to move from survivalist tactics, aimed at protecting individual lives in a disaster, to survival strategies that could help our entire species make it through a mass extinction.

Learning from the Past

Though this shift in strategy sounds like a daunting task, we can take comfort in knowing that our early ancestors faced near-extinction too. In part one of this book, we’ll plunge into geological deep time, and explore how life has endured through some of the most terrifying mass extinctions that have hit the planet over the past billion years. Then, in part two, we’ll turn to the history of human evolution, and all its perils. Some anthropologists believe Homo sapiens struggled through a population bottleneck that brought our numbers down to thousands of individuals less than 100,000 years ago — possibly due to climate change, or simply from the hardships we faced as we migrated out of Africa. Regardless of what caused the population bottleneck, both the fossil record and genetic analysis suggest that humans were at one time rather sparse upon the Earth. To survive, we adopted strategies similar to other species that lived through centuries of poison skies and gigantic explosions. And one of those basic strategies was adaptability.

“Adaptability” is a term you hear a lot from people who study mass extinction. They talk about it with a weird, gallows-humour kind of optimism. This is evident when you meet Earth scientist Mike Benton, who has spent the past 10 years studying the Great Dying and its survivors. In his line of work, Benton has sifted through the remains of some serious planetwide horrors. Two hundred and 50 million years ago, when the Great Dying happened, megavolcanoes fouled the atmosphere with carbon, and it’s possible that an asteroid hit the planet, too. Despite Benton’s intimate familiarity with mass death, he still maintains hope that our species will survive. He told me that “good survival characteristics for any animal” include being able to eat a lot of different things and live anywhere, just as humans can. Of course, he noted, that doesn’t mean there won’t be a lot of casualties. He continued:

Evidence from mass extinctions of the past is that the initial killing is often quite random, and so nothing in particular can protect you, but then in the following grim times, when Earth conditions may still be ghastly, it’s the adaptable forms that breed fast and live at high population size that have the best chance of fighting through.

We have a fighting chance because our population is large, plus we can adapt to new territories and eat a wide range of things. That’s a good start, but what does it really mean to fight through? In part three of this book, we’ll look at some specific examples of how humans and other creatures have used the three survival strategies of scattering, adapting, and remembering. We’ll also explore how humans survive by planning for the future through storytelling. Fiction about tomorrow can provide a symbolic map that tells us where we want to go.

Stories of the Future

So where, exactly, do we want to go? With parts four and five, we’ll launch ourselves into humanity’s possible future. One of our biggest problems as a species today is that we have become so populous that our mass societies are no longer adaptive. Over half the population lives in cities, but cities can become death traps during disasters, and they are breeding grounds for pandemics. Worse, they are not sustainable; cities’energy and agricultural needs are outpacing availability, which limits their life spans and those of the people in them. Part four is about several ways we’ll want to change cities over the next century to make them healthy, sustainable places that preserve human life as well as the life of the environment.

Often, a city-saving idea can start in a lab. Right now, in a cavernous warehouse on the Oregon State University campus, a group of researchers is designing the deadliest tsunami in history. In this cold, windy laboratory, they’ve got a massive water tank, about the size of an Olympic swimming pool, whose currents are controlled by a set of paddles bigger than doors. In the tank, wave after wave buffets a very detailed model city, washing away tiny wooden houses. Whirling in the water are special particles that can be tracked by hundreds of motion detectors, which help scientists understand tsunami behaviour. At the tsunami lab, civil engineers destroy cities to figure out the best places for flood drains and high-ground emergency pathways in coastal cities.

Thousands of kilometers across the country, a revolutionary group of architects is working with biologists to create materials for “living cities” that are environmentally sustainable because they are literally part of the environment. These buildings might have walls made from semi-permeable membranes that allow air in, along with a bit of rainwater for ceiling lights made from luminescent algae. Urbanites would grow fuel in home bioreactors, and tend air-purifying mould that flourishes around their windows. Unlike today’s cities, these living cities will run on biofuels and solar energy. These are the kinds of metropolises where we and our ecoystems could thrive for millennia.

In part five, we’ll look to the far future of humanity and think about our long-term plan to keep our species going for another million years. We know that when early humans were threatened with extinction they fanned out across Africa in search of new homes, eventually leaving the continent entirely. This urge to break away from home and wander has saved us before and could save us in the future. If we colonise other planets, then we will be imitating the survival strategy of our ancestors. Scattering to the stars echoes our journey out of Africa — and it could be our best hope for lasting through the eons.

Engineers at NASA are already preparing more robotic missions to the Moon, nearby asteroids, and Mars, hoping to learn about how the water we’ve discovered on other worlds could sustain a human colony. Every year since 2006, an international group of scientists and entrepreneurs holds a meeting in Washington State to plan for a space elevator that they hope to build in the next few decades. Such a project would allow people to leave Earth’s gravity while using a minimum of energy, thus making travel off-world more economically feasible (and less environmentally damaging) than with rockets. Other groups are figuring out ways to reengineer our entire planet to slow the release of greenhouse gases and grow enough food for our booming population.

These projects, designed to improve cities on Earth while paving the way for life on other worlds, are just a few examples of how humans are getting ready for the inevitable mega disasters that await us. They are also powerful evidence that we want to help each other survive.

Human beings also have one survival skill that we’ve yet to find in creatures around us. We can pass on stories of how to cope with disaster and make it easier for the next group who confronts it. Our tales of survival pass over borders and travel through time from one generation to the next. Humans are creatures of culture as well as nature. Our stories can offer us hope that we’ll make it through unimaginable troubles to come. And they can inspire scientific research about how we’ll do it. Call them tales of pragmatic optimism.

This book is full of such tales — stories about people whose pragmatic optimism could one day save the world. Scientists, philosophers, writers, engineers, doctors, astronauts, and ordinary people are working tirelessly on world-changing projects, assuming that one day our lives can be saved on a massive scale. As their work comes to fruition, our world becomes a very different, more livable place.

If humans are going to make it in the long term, and preserve our planet along with us, we need to accept that change is the status quo. To survive this far, we’ve had to change dramatically over time, and we’ll have to change even more — remoulding our world, our cities, and even our bodies. This book is going to show you how we’ll do it. After all, the only reason we’re here today is because thousands of generations of our ancestors did it already, to make our existence possible.

Chapter 6: The African Bottleneck

MOST OF US are familiar with the basic outlines of the human evolutionary story. Our distant ancestors were a group of apelike creatures who started walking upright millions of years ago in Africa, eventually developing bigger brains and scattering throughout the world to become the humans of today. But there’s another story that has received less attention. Advances in genetics have given us a sharper understanding of what happened between the “walking upright” and the “buying the latest tablet computer” chapters of the tale.

Written into our genomes is the signature left behind by an event when the early human population dwindled to such a small size that our ancient ancestors living in Africa may have come close to extinction. Population geneticists call events like these bottlenecks. They’re periods when the diversity of a species becomes so constrained that evidence of genetic culling is obvious even thousands of generations later. Sometimes the shrinking of a population is the result of mass deaths, and indeed, there is evidence that humans may have been fleeing a natural disaster when we walked out of Africa roughly 70 thousand years ago. But our species probably experienced multiple genetic bottlenecks beginning as far back as 2 million years. And those earlier bottlenecks were caused by a force far more powerful than mass death: the process of evolution itself.

In fact, the African bottlenecks are an example of the paradoxical nature of human survival. They provide evidence that humans nearly died out many times, but also tell a story about how we evolved to survive in places very far away from our evolutionary home in Africa.

The Fundamental Mystery of Human Evolution

Given our enormous, globe-spanning population size, humans have remarkably low genetic diversity — much lower than other mammal species. All 6 billion of us are descended from a group of people who numbered in the mere tens of thousands. When population geneticists describe this peculiar situation, they talk about the difference between humanity’s actual population size and our “effective population size.” An effective population size is a subgroup of the actual population that reasonably represents the genetic diversity of the whole. Put another way, humanity is like a giant dance party full of billions of diverse people. But population geneticists, elite party animals that they are, have managed to find the one ideal VIP area that contains a small group of people who very roughly capture the diversity of the party as a whole. In theory, that room contains the party’s effective population size. If they all started randomly having sex with each other, their children might loosely reproduce the diversity and genetic drift of our actual, billions-strong population.

The weird part is that compared with our actual population size, the human effective population in that VIP area is very low. In fact, today’s human effective population size is estimated at about 10,000 people. As a point of comparison, the common house mouse is estimated to have an effective population size of 160,000. How could there be so many of us, and so little genetic diversity?

This is one of the fundamental mysteries of human evolution, and is the subject of great debate among scientists. There are a few compelling theories, which we’ll discuss shortly, but there is one point that nearly all evolutionary biologists will agree on. We are descended from a group of proto-humans who were fairly diverse 2 million years ago, but whose diversity crashed and passed through a bottleneck while Homo sapiens evolved. That crash limited our gene pool, creating the small effective population size we have today. Does some kind of terrible disaster lurk in the human past? An event that could have winnowed our population down to a small group of survivors, who became our ancestors? That’s definitely one possibility. Evolutionary biologist Richard Dawkins has popularised the idea that the population crash came in the wake of the Toba catastrophe, a supervolcano that rocked Indonesia 80,000 years ago. It’s possible this enormous blast cooled the African climate for many years, destroying local food sources and starving everybody to death before sending fearful bands of Homo sapiens running out of Africa.

But, as John Hawks, an anthropologist at the University of Wisconsin, Madison, put it to me, a careful examination of the genetic evidence doesn’t reveal anything as dramatic as a single megavolcanic wipeout. Instead of some Hollywood special-effects extravaganza, human history was more like a perilous immigration story. To understand how immigration can turn a vast population into a tiny one, we need to travel back a few million years to the place and time where we evolved.

The Human Diaspora

Humanity’s first great revolution, according to the anthropologist Ian Tattersall of the American Museum of Natural History, was when it learned to walk upright, more than 5 million years ago. At the time, we were part of a hominin group called Australopithecus that shared a very recent common ancestor with apes. Australopithecines hailed from the temperate, lush East African coast. They were short — about the size of an eight-year-old child — and covered in a light layer of fur. They may have started walking on their hind legs because it helped them hunt and find the fruits that dominated their diets. Whatever the reason, walking upright was unique to Australopithecus. Her fellow primates continued to prefer a four-legged gait, as they do today.

Over the next few million years, Australopithecus walked from the tip of what is now South Africa all the way up to where Chad and Sudan are today. Our ancestors also grew larger skulls, anticipating a trend that has continued throughout human evolution. By about 2 million years ago, Australopithecus was evolving into a very human-looking hominin called Homo ergaster (sometimes called Homo erectus). Similar in height to humans today, a couple of H. ergaster individuals could put on jeans and T-shirts and blend in fairly well on a typical city street — as long as they wore hats to hide their slightly prominent brows and sloping foreheads.

Another thing that would make our H. ergasters feel perfectly comfortable loping down Market Street is the way so many in the crowd around them would be clutching small, hand-sized tools. Our tools may contain microchips whose components are the products of advanced chemical processing, but the typical smartphone’s size and heft are comparable to the carefully crafted hand axes that anthropologists have identified as a key component of H. ergaster’s tool kit. H. ergaster wouldn’t need anyone to explain the meat slowly cooking over low flames in kebab stands, either: There’s evidence that their species had mastered fire 1.5 million years ago.

There are many ways to tell the story of what happened to H. ergaster and her children, who eventually built those smartphones and invented the tasty perfection that is a kebab. H. ergaster was one of many bipedal, tool-using hominids roaming southern and eastern Africa who had evolved from Australopithecus. The fossil record from this time is fairly sparse, so we can’t be sure how many groups there were, what kinds of relationships they formed with each other, or even (in some cases) which ones evolved into what. But each group had its own unique collection of genes, some of which still survive today in Homo sapiens. And those are the groups whose paths we’re going to follow.

This path is both a physical and a genetic one. A visitor to the American Museum of Natural History in New York can track its progress in fossils. Glass-enclosed panoramas offer glimpses of what we know about how H. ergaster and her progeny created hand axes by striking one stone against another until enough pieces had flaked off that only a sharp blade was left. Reconstructed early human skeletons stand near sparse fossils and tools, a reminder that our ideas about these people come, literally, from mere fragments of their bodies and cultures. Ian Tattersall has spent most of his career poring over those fragments, trying to reconstruct the tangled root structure of humanity’s evolutionary tree.

One thing we know for sure is that early humans were wanderers. Not only did they spread across Africa, but they actually crossed out of it many times, starting about 2 million years ago. Anthropologists can track the journeys taken by H. ergaster and her progeny by tracing the likely paths between what remains of these peoples’ campsites and villages, often identifying the group who lived there based on the kinds of tools they used.

Tattersall believes there were at least three major radiations, or population dispersals, out of Africa. Despite the popularity of Dawkins’s Toba volcano theory, Tattersall believes there was “no environmental reason” for these immigrations. Instead, they were all spurred by evolutionary developments that allowed humans to master their environments. “The first radiation seems to have coincided with a change in body structure,” he mused. Members of H. ergaster had a more modern skeletal structure featuring longer legs than their hominid cohorts, which meant they could walk quickly and efficiently over a variety of terrains. Tattersall explained that there were environmental changes in Africa during this time, but not enough to suggest that humans fled environmental destruction to greener pastures. Instead they were simply well suited to explore “unfamiliar environments, ones very unlike their ancestral environments,” he said. H. ergaster ’s rolling gait was an adaptation that allowed the species to continue adapting, by spreading into new lands where other hominids literally could not tread.

As early humans walked into new regions, they separated into different, smaller bands. Each of these bands continued to evolve in ways that suited the environments where they eventually settled. We’re going to focus on four major players in this evolutionary family drama: our early ancestor H. ergaster and three siblings she spawned — Homo erectus, Homo neanderthalensis, and Homo sapiens.

H. erectus was likely the evolutionary product of that first exodus out of Africa that Tattersall described. About 1.8 million years ago, H. erectus crossed out of Africa through what is today Egypt and spread from there all the way across Asia. These hominins soon found themselves in a very different environment from their siblings back in Africa; the winds were cold and snow y, and the steppes were full of completely unfamiliar wild- life. Over the millennia, H. erectus’s skull shape changed and so did her tool sets. We can actually track how our ancestors’ tools changed more easily than how their bodies did because stone preserves better than bone. Scientists have reconstructed the spread of H. erectus by unearthing caches of tools whose shapes are quite distinct from what other groups used. From what we can piece together, it seems that H. erectus founded cultures and communities that lasted for hundreds of thousands of years, and spread throughout China and down into Java.

Over the next million years, other groups of humans followed in H. erectus’s footsteps, walking through Egypt to take their siblings’ route out of Africa. But as the Stanford paleoanthropologist Richard Klein told me, these journeys probably weren’t distinct waves of migration. Walking in small groups, these humans were slowly expanding the boundaries of the hominin neighbourhood.

Fossil remains in Europe suggest that about 500,000 to 600,000 years ago, some of H. ergaster’s progeny, on emerging from Africa, decided to go left instead of right, wandering into the western and central parts of the Eurasian continent. These Europeans evolved into H. neanderthalensis. They often set up homes in generously sized cave systems, and there’s evidence that some groups lived for dozens of generations in the same caves, scattered throughout Italy, Spain, England, Russia, and Slovenia, among other countries. Neanderthals evolved a thicker brow and more barrel-chested body to cope with the colder climate. We’ll talk more about them in the next chapter.

Back in Africa, H. ergaster was busy, too, establishing home bases all over the coasts of the continent, reaching from southern Africa all the way up to regions that are today Algeria and Morocco. By 200,000 years ago, H. ergaster ’s skeletal shape was indistinguishable from that of modern humans. A species we would recognise as H. sapiens had emerged. And that’s when human beings made their next evolutionary leap — one that perfectly complemented our ability to walk upright into new domains.

How We Evolved to Tell Stories

“When Homo sapiens came along there was something totally radical about it,” Tattersall enthused. “For a hundred thousand years, Homo sapiens behaved in basically the same ways its ancestors had. But suddenly something happened that started a different pattern.” Put simply, humans started to use the giant brains they’d evolved to fit inside their gradually enlarging craniums. What changed? Tattersall said there are no easy answers, but evolution often works in jumps and starts like that. For example, birds evolved feathers millions of years before they started flying, and animals had limbs long before they started walking. “ We had a big brain with symbolic potential before we used it for symbolic thought,” he concluded. In what anthropologists call a cultural explosion over the past 100,000 years, humans developed complex symbolic communication, from language and art to fashion and complex tools. Instead of looking at the world as a place to avoid danger and score food, humans disas- sembled it into mental symbols that allowed us to imagine new worlds, or new versions of the world we lived in.

Humans’ new facility with symbols allowed us to learn about the world around us from other humans rather than starting from scratch with direct observations each time we went to a new place. Like walking, symbolic thought is an adaptation that leads to more adaptations. Modern humans could venture into new territory, discover its resources and perils, then tell other bands of humans about it. They might even pass along designs for tools that helped us gain access to foods specific to a certain area, like crushers for nuts or scoops for tubers. Aided by our new capacity for imagination, those bands of humans could familiarise themselves with alien regions before ever visiting them. For the first time in history, people could figure out how to adapt to a place before arriving there — just by hearing stories from their comrades. Symbolic thought is what allowed us to thrive in environments far from warm, coastal Africa, where we began. It was the perfect evolutionary development for a species whose body propelled us easily into new places. Indeed, one might argue that the farther we wandered, the more we evolved our skills as storytellers.

Let’s go back, for a moment, to that first radiation out of Africa, nearly 2 million years ago when H. ergaster, with her small but effective tool kit, crossed into the Sinai Peninsula. At roughly the same time, we find evidence of humanity’s first genetic bottleneck. And yet, as Tattersall and many others have pointed out, there is no evidence of a giant disaster thinning the population, leaving the survivors to flee across the Middle East and Asia. The bottleneck is clearly a sign of a population crash, but what caused it? As I said earlier, the effective population size for H. sapiens is estimated at roughly 10,000 individuals; but the University of Utah geneticist Chad Huff recently argued that soon after H. ergaster left, our effective population size was about 18,500. It’s likely this bottleneck is actually a record of human groups growing smaller as they thinned out across the Eurasian continent, meeting adversity every step of the way. At the same time, according to anthropologist John Hawks, the bottleneck is a mark of evolutionary changes that could only happen to a population that was always on the move.

It started with that first trek out of Africa, which split humanity into several groups. As Hawks explained in a paper he published with colleagues in 2000, one cause for a genetic bottleneck can be speciation, or the process of one species splitting into two or more genetically distinct groups. We’ve already touched on how H. ergaster evolved into at least three sibling groups, but that’s a dramatic oversimplification. For example, H. ergaster likely evolved into a group called Homo heidelbergensis in Africa, which then speciated into H. sapiens and another group that speciated into Neanderthals and their close relatives the Denisovans later on. There are many complexities in the lineage of H. erectus, too, especially once the group reached Asia. Evolution is a messy process, with many by ways and dead ends. By the time H. ergaster reached the Sinai, the group would have undergone at least one speciation event — the one that led to early H. erectus. That means only a subset of H. ergaster genes survived in H. erectus, and a subset of its genes survived in the H. ergaster groups who stayed behind. If these groups remained small, and there’s ample reason to believe that they did, you now have two isolated gene pools that are less diverse than the original one. That’s how speciation creates a genetic bottleneck.

But even without speciation events, humans’ habit of walking all over the place would have caused a bottleneck. The very act of wandering far from home, into many dangers, can shrink both the population and the gene pool over the course of generations. Population geneticists call this process the founder effect. To see how the founder effect works, let’s follow a band of H. erectus passing through lands edging the Mediterranean Sea and finding its way into India. Remember, this isn’t one long trek. Maybe the coast of today’s state of Gujarat appeals to a few members of H. erectus, and so a band decides to settle down for a while in that region. This settlement is called a founder group, and it has less diversity than the group it came from simply because it has fewer members. In the next generation, a new group splits off from the Gujaratis and heads south along the coast. Generally we assume that each time a group left for untouched lands, it left a group behind. So each new group becomes a founder population in its own right, and has less genetic diversity than the group back in Gujarat — even if you factor in some intermarriage between different founder groups. Multiple founder events in a row would have had the odd effect of increasing humanity’s population while decreasing human genetic diversity. Now, consider the fact that our H. erectus explorers in India are a microcosm of the way all humans spread across the Earth. After hundreds of generations of wandering, humans managed to increase their populations gradually while retaining the low diversity caused by genetic bottlenecks.

Back in Africa, early humans were also speciating and wandering, forming new bands, each of whose genetic diversity was lower than the last. But when a small band of hominins called H. sapiens evolved, about

200,000 years ago, something strange happened. Tattersall believes that humans underwent some kind of genetic change that spurred a cultural shift. Suddenly, between 100,000 and 50,000 years ago, the fossil record is full of sculpture, shell jewelry, complex tools made from multiple kinds of material, ochre-and-carbon cave paintings, and elaborate burial sites. Possibly, as Randall White, an anthropologist at New York University, suggests in his book Prehistoric Art, humans were using jewelry and clothing to proclaim allegiance with particular groups. H. sapiens wasn’t just interacting with the world. They were using symbols to mediate their relationship with it. But why the sudden shift from a hominin with the capacity for cultural expression to a hominin who actively created culture?

It could be that one small group of H. sapiens developed a genetic mutation that led to experiments with cultural expression. Then, the capacity to do it spread via mating between groups because storytelling and symbolic thought were invaluable survival skills for a species that regularly encountered unfamiliar environments. Using language and stories, one group could explain to another how to hunt the local animals and which plants were safe to eat. Armed with this information, humans could conquer territory more quickly. Any group that could do this would have a higher chance of surviving relocation time and again. The more those groups survived, the more able they were to pass along any genetic predisposition for symbolic communication.

Perhaps H. sapiens’ knack for symbolic culture was also a result of sexual selection, in which certain genes spread because their bearers are more attractive to the opposite sex. Put simply, these attractive people get laid more often, and therefore have more chances to spread their genes to the next generation. In his book The Mating Mind, evolutionary psychologist Geoffrey Miller argues that among ancient humans, the most attractive people were good with language and tools. The result would be a population in which sexual selection created successively more symbol-oriented people. Two anthropologists, Gregory Cochran and Henry Harpending, amplify this point. They argue that some of the genes that spread like wildfire through the human population over the past 50,000 years are associated with cranial capacity — brain size — and language ability. “Life is a breeding experiment,” Cochran and Harpending write in their book The 10,000 Year Explosion.

Our capacity for symbolism evolved quickly, partly because our mating choices would have been shaped by our needs as creatures who evolved to survive by founding new communities. Over the past million years, humans bred themselves to be the ultimate survivors, capable of both exploring the world and adapting to it by sharing stories about what we found there.

How Can We Possibly Know All This?

A lot of the evidence we have for the routes that humans took out of Africa comes from objects and places you can see with your own eyes. Paleontologists have found our ancestors’ ancient bones, as well as their tools. To figure out the ages of these tools and skeletons, we use the same kinds of dating techniques that geologists use to discover the history of rocks. In fact, when an anthropologist talks about “dating the age of fossils,” he or she isn’t actually talking about the bones themselves — to date old bones, anthropologists carefully excavate them and take samples of the rock surrounding them. Then they pin a date on those rocks, under the assumption that the bones come from roughly the same era as the rocks or sand that covered them up. Basically, we date fossils by associa- tion, which is why you’ll often hear scientists suggesting that a particular fossil might be between 100,000 and 80,000 years old. Though we can’t pin an exact month or year on each fossil discovery, we do have ample evidence that certain humans like H. ergaster came before other humans like H. erectus in evolutionary and geological time.

Over the past decade, however, the study of ancient bones has been revolutionised by new technologies for sequencing genomes, including DNA extracted from the fossils of Neanderthals and other hominins who lived in the past 50,000 years (sadly, we don’t have the ability to sequence DNA from Australopithecus or H. ergaster bones — their DNA is too decayed). At the Max Planck Institute in Leipzig, Germany, an evolutionary geneticist named Svante Pääbo and his team have developed technology to extract nearly intact genomes from Neanderthal bones. First they grind the bones to dust and chemically amplify whatever DNA molecules they can find, then analyse this genetic material using the same kinds of sequencers that decode the DNA of living creatures today. We’ll deal with the Neanderthal genome more in the next chapter, but suffice it to say that we have pretty solid evidence about the genetic relationships between H. sapiens and its sibling species H. neanderthalensis.

A lot of the evidence for humans’ low genetic diversity has been made possible by DNA-reading technologies developed since the first human genome was sequenced, in the early 1990s. Though that first human genome took over a decade to sequence, we now have machines capable of reading the entire set of letters making up one genome in just a few hours. As a result, population geneticists are accumulating a diverse sampling of sequenced human genomes, from people all over the world. Many of these genomes are collected into data sets that scientists can feed into soft- ware that does everything from make very simple comparisons between two genomes (literally analysing the similarities and differences between one long string of letters and another), to extremely complex simulations of how these genomes might have evolved over time.

One of the first pieces of genetic evidence for the serial founder theory emerged when scientists had collected DNA sequences from enough people that we could start to analyse genetic diversity in different regions all over the world. Geneticists discovered a telltale pattern: People born in Africa and India tend to have much greater genetic diversity than people born elsewhere. This is precisely the kind of pattern you’d expect to see in a world population that grew out of founder groups originating in Africa. Remember, each successive founder group has less and less genetic diversity. So people descended from groups that stayed in Africa or India are from early founder groups. People in Europe, Australia, Asia, and the Americas were the result of hundreds of generations of founder effects — so we’d expect them to have less genetic diversity. When you add this genetic evidence to the physical evidence from fossils and tools left behind by people leaving Africa, you wind up with a fairly solid theory that founder effects created our genetic bottleneck.

An Eruption That Launched Humanity

Though it’s likely that the genetic bottlenecks we observe in the human population were caused mostly by founder effects and sexual selection, there is some evidence that the final human radiation out of Africa was precipitated by a catastrophe. Ancient humans had been crossing the Sinai out of Africa and into the rest of the world for over a million years, but roughly 80,000 years ago there was an extremely large migration that changed the world and every human on it. H. sapiens, a human with language, clothing, and sophisticated tools, took over Africa, then migrated beyond its borders. Certainly it’s possible that this wave of human immigrants was spurred by mass deaths in the wake of the Toba eruption. But that’s debatable.

What’s certain is another explosion that nobody denies: the one in human symbolic communication. Our capacity for culture is what allowed us to survive in the perilous lands beyond the warm, fecund West African regions where Australopithecus first stood up. We never stayed in any one place for long. We moved into new places, founding new communities. And when we evolved complex symbolic intelligence, our growing facility with tools and language made these migrations easier. We could take advantage of many kinds of environments, teaching each other about their bounties and dangers in advance.

As H. sapiens poured off the continent of our birth, we discovered lands inhabited by our sibling hominins. We had to adapt to a world that already had humans in it. What came next will take us into one of the most controversial areas of population genetics and human evolutionary history.

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Photo via NASA Earth Observatory