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The gods presented her with a box into which each had put something harmful, and forbade her ever to open it. Then they sent her to Epimetheus, who took her gladly although Prometheus had warned him never to accept anything from Zeus. He took her, and afterward when that dangerous thing, a woman, was his, he understood how good his brother’s advice had been. For Pandora, like all women, was possessed of a lively curiosity. She had to know what was in the box. One day she lifted the lid and out flew plagues innumerable, sorrow and mischief for mankind. In terror Pandora clapped the lid down, but too late. One good thing, however, was there—Hope. It was the only good the casket had held among the many evils, and it remains to this day mankind’s sole comfort in misfortune.

(AS RETOLD BY EDITH HAMILTON)

Pandora’s Seed

Image Missing

THE UNFORESEEN COST OF CIVILIZATION

Spencer Wells

ALLEN LANE

an imprint of

PENGUIN BOOKS

Foreword

True, western societies are much better off materially than they were 40 years ago, but why is there so much crime, vandalism and graffiti? Why are divorce rates so high? Why are we seeing declines in civic engagement and trust? Why have obesity and depression reached epidemic proportions, even amongst children? Why do people call this the age of anxiety? Why do studies in most developed countries show that people are becoming unhappier?

As I write this, I am 36,000 feet above the Arabian Sea, sipping a glass of wine and typing on my laptop. I’m returning from Mumbai, India, where I gave a lecture at a science and technology festival organized by the Indian Institute of Technology. I spent twice as long getting there and back as I did on the ground, and with the ten-and-a-half-hour time change I was more than a little disoriented while I was there. Still, India is one of my favorite countries and it was worth the jet-lag whiplash, bouncing halfway around the world and back over a long weekend.

The students at the institute who invited me wanted to hear about my work on genetics and human migration, a subject I have studied for the better part of the past two decades. The work my colleagues and I have carried out has shown unequivocally that all humans have a recent common origin in Africa, within the past 200,000 years, and that we only started to leave Africa to populate the rest of the world in the past 60,000. I spent an hour describing some of the new data from the field, discussing recent unpublished findings, and generally painting a picture of the genetic history of our species. At the end, as I normally do after such lectures, I took questions. These ranged from technical queries about the laboratory methods we use for analyzing human DNA to more general ones. The final question was something that I’ve been asked many times before, and will certainly be asked in the future: What is the broader relevance of your work?

It does seem somewhat esoteric to study the arcane details of distant human history, I suppose, but it has always fascinated me. With the right samples and a smattering of statistics, it is possible to discern the details of how our species populated the globe. “But why is this important?” the student asked. I began my answer as a scientist, describing the importance of basic research in which there is no particular practical application. Governments fund such work in many subjects, I explained, because it is possible that some new finding may end up being very important in fields that are more pragmatic—medicine, for instance. Moreover, what defines us as a species is our complex culture, and scientific inquiry for its own sake is an important part of understanding our role in the world. Imagine encountering intelligent life from another planet, I said; would such an auspicious meeting include explaining mundane details like how the latest video-game console operates or would it focus on who we are as two highly evolved species and what brought us to our present state of being? We need to learn our history to understand who we are, and to speculate on where we might be going. “L’histoire est un grand présent, et pas seulement un passé,” as the French philosopher Alain wrote. History is a grand view of the present, and not simply something in the past.

But there was another reason this information was important, I explained. We live today in a highly globalized world, one where people come into contact with others they may never have encountered only a century before. Africans mix with Europeans, Asians, and Native Americans to create a social mélange that is unprecedented in human history. Couple this exposure with linguistic and cultural differences, and you have a potentially volatile recipe. We are keenly attuned to such differences, and they help define how we see ourselves. Part of what our genetic work shows, though, is how trivial these differences really are—underneath the surface, at the level of our DNA, we are nearly identical. The broader relevance of our work, I explained, is that we should really start to see past the superficial features that divide us, and start to recognize that we are all part of an extended human family. To the extent that we can see ourselves as connected at the genetic level, we might be able to overcome some of our prejudices.

This notion seemed to have special relevance to the members of this audience, many of whom had recently witnessed the brutal terrorist attack in which Islamic militants from Pakistan had bombed and gunned down people in several locations in south Mumbai and taken control of two landmark hotels, the Taj and the Oberoi. Over the course of four days the terrorists killed 164 innocent people. (Nine of the militants died as well.) For India, the social impact was like 9/11 in the United States, though thankfully with fewer deaths. It would have been natural to feel enraged, to want vengeance, to use this attack as an excuse for further violence. However, the Indian reaction, according to my host at the conference, was not to dwell on negative emotions. “It has brought us all together—all India,” he told me as we were weaving through traffic the night I arrived.

The nature of this whole encounter, along with the quote from Richard Tomkins that begins this foreword, highlights the theme of this book. During my career as a geneticist and anthropologist I’ve been lucky to work with many people around the world, ranging from senior politicians and the heads of major corporations to tribal foragers eking out a precarious existence in remote wilderness locations. What has struck me over and over again is the huge amount of change taking place in the world today, regardless of where one lives. Some of this change is good, such as the overall decrease in poverty during the course of my lifetime and the drop in the birthrate in developing countries. Other things, though, like 9/11 and the terrorism in Mumbai, have not been so welcome. Everywhere there is a feeling that the world is in flux, that we are on the brink of a historic transition, and that the world will be fundamentally changed somehow in the next few generations. The pace of technological innovation is accelerating, and we are all swept up in it. Think of all of the things indispensable to your daily life that you have learned to use only in the past decade or so. Email, Google, instant messaging, and mobile phones spring to mind immediately, but there’s also hybrid car technology, curbside recycling, and social-networking sites like Facebook. All have found widespread application only since the mid-1990s, and yet today we can’t imagine living without most of them. Trying to imagine what the world will be like at the close of the twenty-first century is nearly impossible.

With all of these amazing technological advances, though, has come a great deal of ancillary baggage. The unprecedented rise in chronic disease in Westernized societies is perhaps the most obvious example. I say “Westernized” rather than “Western” because of the growing incidence of heart disease, diabetes, and plain old obesity in the developing world, particularly in places such as India and China. As these societies become more like our own, they are taking on many of our worst attributes as well. Psychological disorders such as depression and anxiety are also on the rise, and drugs to treat these disorders are now the most widely prescribed in the United States. This seemingly inexorable march toward Western unhealthiness made me wonder why it happened in the first place. Is there some sort of fatal mismatch between Western culture and our biology that is making us ill? And if there is such a mismatch, how did our present culture come to dominate? Surely we are the masters of our own fate and we created the culture that is best suited to us, rather than our culture driving us?

The answer to this question was a long time in coming, much to the chagrin of my patient editors at Random House and Penguin. It took me on a global quest to discover the similarities between what happened thousands of years ago and what is happening now, as we face what promises to be another apparent turning point in our evolution. During the course of researching my first book, The Journey of Man, I was struck by the effects of the agricultural lifestyle on humans living 10,000 years ago in the Middle East. It turns out, as we’ll see in the first chapter, that early farmers were actually less healthy than the surrounding hunter-gatherer populations. So why did the farmers “win” so resoundingly, to the extent that virtually no one on earth today lives as a hunter-gatherer? What follows is an attempt to excavate today’s “archaeological” record to understand the forces that set in motion the agricultural transition and to understand how that decision created the complex world we now live in. If The Journey of Man was about how humans populated the world, this book is about how we have adapted both psychologically and biologically to live in it during a period of enormous change. They form two bookends to a broad view of human history that takes us from the earliest days of our species to where we might be headed as we hurtle deeper into the twenty-first century.

Stewart Brand, paraphrasing Edmund Leach in the opening sentence of the first Whole Earth Catalog in 1968, said it well: We are as gods; it’s time we got good at it. The biggest revolution of the past 50,000 years of human history was not the advent of the Internet, the growth of the industrial age out of the seeds of the Enlightenment, or the development of modern methods of long-distance navigation. Rather, it was when a few people living in several locations around the world decided to stop gathering from the land, abiding by limits set in place by nature, and started growing their food. This decision has had more far-reaching consequences for our species than any other, and it set in motion the events that we will examine in the following chapters. With the power our species has developed as a result of these changes, we must also learn some humility. In today’s world, where small groups of terrorists can inflict lasting damage on the psyche of entire nations, where apparently simple decisions can affect the biological inheritance of generations far in the future, and where more species are likely to go extinct as a result of our actions than at any point in the past 60 million years, it is time to take stock and realize that with great desires come great consequences.

Chapter One

Mystery in the Map

CHICAGO, ILLINOIS

My cab wove through the midafternoon traffic, tracing an arc along the frozen shore of Lake Michigan. On my right, the buildings of one of the world’s tallest cities stabbed toward the sky, steel and glass growing out of the Illinois prairie like modern incarnations of the grass and trees that once lined the lake. A thriving metropolis of nearly three million people, Chicago boasts an airport that was once the world’s busiest (it’s now second), with over 190,000 passengers a day passing through its terminals—including, on this particular day, me. This sprawling city prides itself on its dynamic, forward-looking culture—the “tool maker” and “stacker of wheat,” as Carl Sandburg called it. Not the most obvious place to come looking for the past.

The lake took me back in time, though—way back, before it was even there. Lake Michigan is actually a remnant of one of the largest glaciers the earth has ever seen. During the last ice age, the Laurentide ice sheet stretched from northern Canada down along the Missouri River, as far south as Indianapolis, with its eastern flank covering present-day New York and spilling into the Atlantic Ocean. When it melted, around 10,000 years ago, the water coalesced into the Great Lakes, including Michigan. Looking out the window of my cab, at the strong winds ripping across the expanse of ice reaching out from the Chicago shoreline, I felt like history might be rewinding itself. The ice age could have looked a bit like this, I thought.

This wasn’t just idle musing; I’ve spent my life studying the past, effectively trying to rewind history. I became obsessed with it as a child, and devoured anything and everything on ancient Egypt, Greece, and Rome, the great empires of the Middle East, and the European Middle Ages. In high school biology classes I started to think about much more ancient history, its actors playing their parts on a geological stage. I added the history of life to my passion for written history, and when I got to college I decided to study the record written in our own history book—our DNA. The field I became interested in is known as population genetics, which is the study of the genetic composition of populations of living organisms, using their DNA to decipher a record of how they had changed over time. The field originated as an attempt to piece together clues about how our ancestors had moved around, how ancient populations had mixed and split off from each other, and how they had diversified over the eons. In short, really ancient history.

And my quest had brought me here, for the second time. My last visit to the University of Chicago—where I was headed from O’Hare—had been eighteen years earlier, in February 1989, when I was considering going there for graduate school. The lake was frozen then as well, and my early-morning walks to meetings at the university in single-digit temperatures played a small role in my decision to head to school in the somewhat warmer city of Cambridge, Massachusetts. Despite my decision, the University of Chicago was, and is, an outstanding university. Its faculty boasts brilliant researchers and thinkers in many fields, from economics to literature to physics. I had come back to visit one of them.

Jonathan Pritchard had been a graduate student at Stanford when I was a postdoctoral researcher there, and I still clearly remember his early presentations to our group. His mathematician’s mind, coupled with his deep understanding of the processes of genetic change, made him a real asset to the group. We overlapped again briefly when I was at Oxford, but we lost touch over the years, although I followed his work from the papers he published in scientific journals. It was one such publication that led me to get in touch with him to discuss his findings.

This paper, published in the journal PLoS Biology (PLoS stands for Public Library of Science, a prestigious family of scientific journals available on the Web), described a new method his team had developed to look at selection in the human genome. Selection is the Darwinian force that has created exquisite adaptations like the eye and the ear, as well as most of the other really useful traits we humans have. As Darwin taught us, small changes that are advantageous in some way give an organism a greater chance of surviving and reproducing in the perpetual rat race that is life. Since all of these selected characteristics ultimately have their origin in the way our DNA is put together, it is logical to look to our genes to find out about what made us the way we are.

The search for selection at the genetic level has a long history, dating back to way before Watson and Crick deciphered the structure of DNA in the early 1950s. Pioneering scientists such as Theodosius Dobzhansky, a Russian immigrant to America who helped create the modern science of population genetics back in the early twentieth century, were obsessed with looking for genetic changes that could be explained only by invoking Darwin’s seemingly magical force. In the days before DNA sequences could be studied directly, though, researchers observed large-scale changes in the structure of fruit fly chromosomes. (Fruit flies being the geneticist’s favorite model organism, mostly because their huge salivary gland chromosomes made their patterns of genetic variation easy to study in the days before DNA sequencing.) But while they found some evidence for the past action of selection in fruit flies, the ultimate cause of the patterns they observed remained elusive.

Once it was known that DNA was the ultimate source of genetic variation, and its structure had been discovered and methods developed to determine the actual sequence of the chemical building blocks that make up the double helix (I’m glossing over about fifty years of pioneering research here), population geneticists began to look at DNA sequences directly. In the early days (only around twenty-five years ago), because of technical limitations, they could examine just a few small regions in the genome (the sum total of the genetic building blocks in an individual), and the search for evidence of natural selection usually proved fruitless. It was only with the completion of the Human Genome Project in the late 1990s, and the massive technological breakthroughs that it spawned, that scientists could finally start to reassess the issue that had obsessed Dobzhansky and his colleagues nearly a century before: Is it possible to find evidence of selection at the DNA level and, perhaps more interestingly, can we figure out why it has taken place?

GENETIC BEADS

I paid the cab driver and got out near the University of Chicago bookstore, taking in the surroundings. Gothic-style edifices, constructed during Chicago’s earlier building boom, toward the end of the nineteenth century, surrounded me on all sides. It had been a conscious attempt on the part of the new university—it was founded in 1890, with funds provided by the oil baron John D. Rockefeller—to connect with an older tradition of learning. I felt as though I were back among the gleaming spires of Oxford, running between undergraduate tutorials. My destination, however, was a much newer structure.

The Cummings Life Science Center was constructed in 1970; as befitted a structure meant to house scientists engaged in the advanced study of biology, then undergoing a revolution as a result of Watson and Crick’s elucidation of the structure of DNA, the building’s brick tower was bracingly modern, even a bit brutal. But I had come to talk to Jonathan Pritchard, who was using the most advanced techniques in genetics to look at the history of our species. The juxtaposition of this building amid a campus of older structures seemed fitting, given what I was here to discuss.

I located his office on one of the upper floors, and we chatted as he made me a cup of tea. An avid distance runner, with the intense, lanky look of a marathoner, he seemed somewhat surprised that I had made the trip just to talk to him. I asked him about his move from Oxford to Chicago, his personal life (one of his son’s drawings hung above his desk), and what it felt like to have been granted tenure at one of the world’s most prestigious universities at the precocious age of thirty-seven. He laughed, confident in his intellectual abilities, like so many of the mathematically gifted people I have known, and explained that his life was going well. We then moved on to the reason for my visit.

I wanted to talk shop. Or, rather, I wanted to get his take on the findings of his important research paper. In their PLoS publication, he and his colleagues had described a new method of detecting selection in the human genome. It made use of something called the HapMap, a collection of data on the so-called haplotype structure of the human genome. And to understand that we’ll need to delve into the science a little.

The long string of DNA that makes up your entire genome is broken into smaller strings called chromosomes—there are twenty-three pairs of them—containing the 23,000 or so genes that direct your body to do what it does. These genes code for things like sugar-digesting enzymes in your gut, or blood-clotting proteins, or the type of earwax you have—all of the physical traits that make you who you are. The chromosomes are linear strings of DNA, composed of four chemical building blocks known as nucleotides: A, C, G, and T. The sequence of these nucleotides—AGCCTAGG, and so on, along the entire length of the chromosome—encodes the information in your genome and determines what each gene will do in your body. The nucleotides are arrayed along the chromosomes like beads on a string, a linear orchestra of musicians, each playing their own part in the symphony that is you. You get one of each of your chromosome pairs from your mother and one from your father.

Something funny happens to these musical beads, though, as they are passed from your parents to you. They shuffle—like a deck of cards—partially mixing up the original linear strings of beads your parents had. That’s right: your parents’ chromosomes literally exchange genetic information along their lengths, breaking and reconnecting their paired strands to produce a completely new version of a chromosome to pass on to you. This is part of the reason why you don’t look identical to other members of your family, but we don’t know exactly why it occurs. The best theory going is that it’s probably a good thing to generate novel chromosomal arrangements of the musical beads in each generation so that your child’s DNA orchestra can play a different tune if times change—think about having to evolve quickly in times of intense climatic upheaval. As it’s pretty much ubiquitous in animals and plants, there’s almost certainly a very good reason it’s there.

Probably a few readers are wondering at this point, “If the chromosomes are paired, then why does shuffling change anything? Surely they are copies of the same beads, so wouldn’t shuffling them just produce the same combinations in each of the two new chromosomes?” The reason for the new combinations is that each member of a pair is actually a slightly flawed version of the other. As the chromosomes get passed down through the generations, they have to be copied by the cellular machinery for each new organism. Although this is done with great care, and there are proofreading mechanisms to make sure the copied beads look like those on the original strand, occasionally a mistake is made. By chance, one color of bead is substituted for another—a red for a green, for instance. It doesn’t happen very often—perhaps a couple of times for each chromosome in every generation—but when it does happen, these changes, which geneticists call mutations, get passed down through the generations. They serve to introduce additional variation into the gene pool. Over time the changes have accumulated to such an extent that, on average, one in every one thousand beads differs between the chromosome pairs. Thus, each chromosome that is passed on is a shuffled version of Mom’s and Dad’s chromosomes, with the shuffling detectable through the patterns of the variable beads. It sounds very complicated in theory, but if you think about it as beads on a string it is a bit easier to grasp.

What the HapMap project did was to assess the way the beads had been shuffled in different human populations. By looking at people from Africa, Europe, and Asia, it deduced that there was an average length to the sections of the string of beads that hadn’t been shuffled. The length was a function of how old the population was, the average size of the population over time, and other factors that helped to determine exactly where on the string recombination could have occurred. The math behind all of this gets pretty tricky, but the take-home message is that there is an average length of these recombined places on the string of beads. Over time, many, many generations of recombination had produced a kind of “signature” for the bead structure of a population—a pattern that served to distinguish one population’s strings of beads from another’s, since people living in the same geographic region tend to share more ancestors than people from different parts of the world.

Pritchard and his colleagues had developed a new statistical method to find regions of the chromosomes that seemed to have too little shuffling. In other words, they found parts of chromosomal bead strings that had long sections that seemed too similar to each other—as if everybody was wearing a uniquely patterned necklace, except that one long section of each person’s necklace was pretty much identical to everyone else’s. For segments like this it was possible to infer that something had happened to produce a long section of beads that seemed to be inherited like a block among many people, as though it had spread through their necklaces like a fashion accessory. One person liked the particular combination of beads they saw in part of someone else’s necklace, and copied it to include in theirs. Fashion tastes served to spread the bead pattern far and wide, and pretty soon lots of people were wearing it.

Of course, chromosome patterns can’t be recognized by looking at someone, and you can’t just take a section of someone’s chromosome and splice it into your own, so the explanation for this genetic “faddishness” had to lie somewhere else. Because the chromosomes carry genes, not beads, the inference was that the particular pattern in one person’s chromosome provided some sort of an evolutionary advantage, allowing it to spread through the population. When this happens in nature, the process is known not as fashion but as selection: Darwin’s force, the one that he got so excited about back in the nineteenth century, that served to create highly adapted organisms over many generations. The opposable thumb, color vision, our amazing brains—all had their origins in small changes that had been selected for in our DNA millions of years ago.

The section of the chromosomal beads that many people shared must have had a particular change in its genetic code that conferred an evolutionary advantage, and because of this the people carrying it were more likely to survive and pass on their DNA—and that popular section of beads. Studying such patterns gives us a way to peer back in time and ask how our genomes have been molded by past episodes of selection. By sifting through clues found in the DNA of people alive today, it is possible to see evidence of events that happened many generations ago, like a forensic detective trying to piece together details about a crime from evidence at the scene. And this is exactly what Pritchard and his colleagues did in their PLoS paper.

I asked Pritchard to explain the method, and he went to the whiteboard in his office and started to draw parallel horizontal lines in different colors, explaining how he and his colleagues had scanned the HapMap data for blocks of similar structure. He explained what they called the integrated haplotype score, or iHS, as a way of correcting for regional variation in the pattern of recombination in the human genome. The iHS accounts for the fact that some parts of the genome will have long regions that appear to be quite similar across all populations, simply due to the physical structure of the DNA in that region (and not necessarily to selection for a popular section of beads), while others will experience more recombination and will vary quite a bit among individuals. In their analysis, Pritchard and his colleagues were looking for chromosomal regions that should have been shuffled and diverse—and thus old—but weren’t. These sections were younger than they should be, suggesting that something had happened relatively recently to cause a change in the pattern of that particular region of the genome—in other words, a block of similar beads had spread throughout the population due to selection.

“We were looking at events that had not gone to fixation,” he explained. This meant cases where the short stretches of fashionable beads were not yet shared by everyone in the population. They did this so that they could estimate the expected level of genetic variation for each region independently, as a kind of internal control. This gave much greater power to their statistical analysis, and made it more likely that the chromosomal regions that returned a positive score really had been subject to selection.

Pritchard discussed the careful work they had done to account for any biases inherent in the analysis. He discussed the limitations of dealing with the three populations included in the HapMap data set, and his plans to look at data from additional populations. The HapMap was the product of an international consortium of biomedical researchers interested in finding genetic variants that could be associated with common diseases, like diabetes or hypertension. It was not planned with the primary intention of telling us more about our evolutionary history, but Pritchard’s small team of researchers had discovered a way to use it to do this.

Their analysis had revealed hundreds of regions of the genome, scattered across all of our twenty-three pairs of chromosomes, that had been strongly selected. There seemed to be smoking guns everywhere, all containing stories about the evolutionary history of our species. But the most incredible single discovery to come out of the study, and the reason I was talking to Pritchard, was how recently these selective events had occurred. All of them had happened in the past 10,000 years.

We usually think of natural selection as a long, slow process. Darwin and other evolutionary biologists typically thought in terms of selection happening over millions of years, as the slight advantage provided by a new trait slowly won out over its less successful rivals. “Survival of the fittest”—the term was actually coined by the nineteenth-century social scientist Herbert Spencer—was about the gradual accumulation of genetic variants that eventually led a species to become better adapted to its environment. In studies that had been done on model experimental organisms such as bacteria or fruit flies, the calculated selective advantage was typically a fraction of a percent, which meant that organisms with the trait took thousands of generations to show evidence of selection—in other words, to show that widespread bead pattern. Pritchard’s finding of hundreds of episodes of selection in the past 10,000 years—only around 350 human generations—implied that our species had been subjected to a very strong selection pressure during this time.

What could have caused this huge change in our genome? Pritchard was quick to point out that his method may have been biased toward these results, although he admitted that his continuing analyses reinforced his finding that there had been stronger selection during this period than there had before. Other researchers have since confirmed Pritchard’s results, suggesting that this time period had indeed produced a significant number of changes in our genome. To understand the timing of these events, we need to look elsewhere, outside our DNA, in the stones and bones of paleoanthropology.

THE IMPORTANCE OF INFLECTION

Our species is a relative newcomer on the biological scene. While horseshoe crabs and sharks are recognizable in the fossil record from over 100 million years ago, the hominid lineage—composed of apes that walk upright like us—doesn’t appear until around 5 million years ago. Our genus, Homo, appears even later, around 2.3 million years ago, with the first large-brained hominids to make stone tools, Homo habilis, and their descendants Homo erectus. Hominids with an even larger brain, looking more like us, appear around 500,000 years ago, but they still don’t belong to our species. In other words, we are rank newcomers on the evolutionary scene.

According to a recent reanalysis of fossils discovered in 1967 by Richard Leakey at the Kibish Formation on the Omo River in southern Ethiopia, our species—Homo sapiens— first appeared as a biologically recognizable entity around 195,000 years ago. These fossils were originally dated to 130,000 years ago, but newer methods have shown them to be 65,000 years older than previously thought. In the highly contentious field of paleoanthropology, where a single discovery can rewrite history, the dates seem pretty certain. Older human remains may be discovered in the future, but as far as we know these were the first humans ever to tread the earth.

The next oldest human fossil finds date to nearly 40,000 years later, at Herto in Ethiopia and Jebel Irhoud in Morocco, but it isn’t until around 120,000 years ago that significant numbers of Homo sapiens start to show up in the fossil record. The best-known finds are at Qafzeh and Skhul Caves in present-day Israel and the Klasies River in South Africa. The dearth of Homo sapiens fossils over a 75,000-year period could be due to low population densities, or perhaps it is simply a result of a poorly studied fossil record, but the implication is that humans were a relatively rare species limited to Africa and the Middle East.

Based on the number of known archaeological sites, along with clues from the human genome, we can estimate the ancient demography of our species since that time. If we plot this on a graph, an interesting pattern emerges (Figure 2). Our species had an unknown but probably fairly stable population size between the time when we originated around 200,000 years ago until around 80,000 years ago. Judging from the sparse fossil record of human remains, the population size was small and the species scattered throughout East and North Africa. Around 120,000 years ago, when we show up in the Middle East and South Africa, there was still no evidence of a significant change in the number of humans. Rather, these small, dispersed groups appear to have wandered into new territory. The Middle East at this time was basically a geographic extension of North Africa, with a similar climate, flora, and fauna, so these early humans did not venture far beyond their African home. There is no evidence of a human presence elsewhere in Asia or Europe at this time.

Between 80,000 and 50,000 years ago, however, something significant happened to the human population. The fossil and archaeological record runs dry, and there is little evidence for humans anywhere, including Africa. The settlements in the Middle East and South Africa were abandoned, as though we were retreating in the face of a catastrophic challenge. Taking the evidence at face value, the inference is that we went through a population crash. And according to recent genetic analyses, this is precisely what happened. By assessing the level of genetic diversity in the present-day human population, which turns out to be remarkably low in comparison with that of our nearest cousins, the great apes, it is possible to calculate that the human population may have averaged no more than a mere two thousand people around 70,000 years ago. Our species was literally on the brink of extinction, at the nadir of our 200,000-year population curve. Then, around 60,000 years ago, something else happened—a change in the direction of the curve, known in mathematics as an inflection point. The human population actually started growing, and this seems to correlate with the first appearance of humans outside of Africa and the Middle East. Within 45,000 years we had spread to every continent (apart from Antarctica), increasing from the couple of thousand who survived the population crash to a few million hunter-gatherers, spread throughout the entire world. What led to this expansion will be discussed in Chapter 4.

FIGURE 2: THE VARIATION IN HUMAN POPULATION SIZE OVER THE PAST 100,000 YEARS. NOTE THE USE OF A LOGARITHMIC SCALE ON THE VERTICAL AXIS (10³ = 1,000, 10⁶ = 1 MILLION, ETC.).

At 10,000 years ago, something really momentous happens: we see another change in the curve, with a massive acceleration in the rate of population growth. Taking us from a few million to over six billion today, this was the true explosion of our species—the Big Bang that led to humans dominating the world stage. What set in motion this sudden growth spurt around 10,000 years ago? If you are an archaeologist, you will know the answer immediately. It was at this time that we settled down and made a conscious decision to change our relationship with nature. We developed agriculture. While hunter-gatherers had relied on finding their food sources, agriculturalists created theirs. This seemingly simple transition in the way we obtained nourishment set in motion a sea change in human history. Instead of being held captive by where we could find enough plants and animals to survive, we gained control of our food supply. The result was that food was no longer the limiting factor in determining how many people could live in one place. We’ll explore how this came about in the next chapter, but for now it is enough to say that by controlling the food supply, we gained the ability to choose how many people could live in a particular location. If the population increased in size, it was relatively easy to grow more food. This radical change in lifestyle set in motion the Big Bang in our population growth curve.

And what’s going on now? Interestingly, as we move into projections of what will happen in the twenty-first century, we see a gradual leveling off in the population curve, leading us to a steady state by the end of the century. There are many reasons for this, ranging from medical to economic, but the result will be another profound shift in our way of life. Our species has been on an accelerating growth curve since around 60,000 years ago, and for the first time since we started to expand from our ancient African homeland we will have to come to terms with life in a stagnant population. According to the United Nations, by 2050 there will be more people alive over the age of sixty than under the age of fifteen, even in much of the developing world. As we’ve read and heard in the news, this population shift will strain our social systems, particularly the twentieth-century concept of “retirement.” It will also provide us with opportunities, as we’ll see later in the book. According to Joel Cohen, a demographer at Rockefeller University, it is at this point that we will have “outgrown our childhood and adolescence as a species.”

Each one of these inflection points marks a change in the fortunes of our species: our comeback from near extinction to populate the world and the period of exponential growth that began 10,000 years ago—each of them has left its mark in our genes and our culture. And in the next century we will be moving from a rapidly expanding population to one that is more stable—or perhaps even in decline. How will this affect us, a species used to expansion and conquest?

For now, though, we’re interested in the Big Bang—the massive increase in human population that accompanied our transition to an agricultural way of life. The date of 10,000 years ago is significant because, according to Jonathan Pritchard’s genetic results, that corresponds to the period in which humans have been subject to very strong selection. We modified the plants and animals that allowed us to develop growing agricultural societies, but judging from the genetic data it seems that they could also have modified us.

SIFTING THROUGH THE FALLOUT

Pritchard’s results don’t just point to sections of our chromosomes that have been subject to selection over the past 10,000 years—they also suggest which genes contained within those were the targets of selection. Typically the section where the changes have increased in frequency contains only one, or perhaps a few, genes, and based on their location it’s possible to infer which one was the actual target of selection. Generally, the more centrally located a gene is within such a section, the more likely that it was the key element under selection within the chromosomal region. By comparing these genes to a list of genes with known functions, another of the spin-offs from the Human Genome Project, it’s possible to guess what function was being selected for—and therefore what force might have been doing the selecting.

“The strongest [functional] pattern that we came up with was for skin pigmentation,” Pritchard told me as we started to discuss the types of genes that had been selected. “There are five different genes involved in skin pigmentation that show signals of selection in Europeans.” This helps to explain why Europeans have lighter skin than Africans; this trait appears to have been selected for relatively recently in the European population, consistent with what anthropologists had long argued: that humans evolved originally in Africa with dark skin. It was only as we moved out of the tropics and into higher latitudes, with their lower levels of ultraviolet light, that we had to lose some of our dark pigmentation in order to allow the deeper layers of our skin to synthesize enough vitamin D—something they only do when exposed to enough UV light. The reason Europeans have pale skin—and part of the reason some of us have fair hair—is that our ancient ancestors needed to make enough vitamin D for their bones to survive the rigors of northern life thousands of years ago. I was impressed that Pritchard’s genome-wide analysis had picked up this pattern without any reference to an anthropological hypothesis—he wasn’t looking specifically for genes that might have been selected for skin color. I then asked him what single gene in the human genome had been most strongly selected—not a functional class of genes, like those involved in pigmentation, but the one location in the human genome that had been whipped into shape most vigorously by the action of natural selection.

“Lactase has the biggest, broadest signal,” he said, turning to his computer monitor and showing me a plot of the selection patterns that’s available to the general public on the Web (http://hg-wen.uchicago.edu/selection/index.html). Lactase is the enzyme that allows humans to metabolize lactose, the sugar in milk. Without it, lactose passes through our guts unmetabolized, resulting in the uncomfortable set of symptoms known as lactose intolerance. Human babies have a functioning version of the lactase gene, allowing them to survive on the milk that makes up the majority of an infant’s diet, but in many human populations the gene is switched off after childhood, rendering adults unable to metabolize lactose.

Between 10,000 and 8,000 years ago, however, people living in the Middle East domesticated the goat and the cow. The animals provided a steady supply of meat on the hoof, of course, but also gave our ancestors copious quantities of milk, a nutritious, sterile (if collected properly) food source. It seems that in these Middle Eastern populations, and in their descendants who brought goats and cattle to Europe, milk was an advantageous addition to the diet. Over time, a mutation that caused the lactase gene to remain active after childhood increased in frequency in milk-drinking populations. Today over 90 percent of Europeans have this genetic variant, while the majority of Africans (apart from some cattle-raising populations) and Asians—who never had milk as a major component of their diets—are lactose intolerant as adults. Strong selection for lactose tolerance in Europeans had been detected independently by other researchers investigating this unusual trait, so it was a validation of Pritchard’s analysis.

FIGURE 3: GRAPH OF THE INTEGRATED HAPLOTYPE SCORE (IHS) AROUND THE LACTASE GENE FOR THREE HAPMAP POPULATIONS. CEU = EUROPEAN, YRI = AFRICAN (YORUBAN), ASN = ASIAN (CHINESE AND JAPANESE). NOTE THAT THE SIGNAL OF STRONG SELECTION IS VISIBLE ONLY IN THE EUROPEAN POPULATION. SOURCE: HTTP://HAPLOTTER.UCHICAGO.EDU/.

The beauty of the study was beginning to reveal itself. Instead of focusing on individual genes with a function that could have been useful—such as producing lactase—and trying to find evidence of past selection events, Pritchard’s new technique took a hypothesis-free approach. It simply asked where in our genome there was evidence of selection and then tried to find genes that