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 could explain the statistical pattern—a shotgun approach to the study of evolution that was possible only because of the huge leaps in technology that had come about as a result of the Human Genome Project. It also revealed how much biology, and particularly genetics, was becoming a computational field, where much of the study was done in front of monitors and keyboards, rather than at the lab bench. When I’d started graduate school in genetics in the 1980s, the limiting factor in any research project had been simply generating enough data to test a hypothesis. Now data flowed like water from a fire hose, and the hard part was interpreting it and generating the many hypotheses that could explain the statistical patterns.
I asked Pritchard if there were any other interesting types of genes that showed evidence of selection, and he said yes, there were many that were involved in metabolizing food. The gene for alcohol dehydrogenase, which allows your body to break down the alcohol in that glass of wine or beer, showed evidence of strong selection, as did genes involved in metabolizing sugars and fats. He suggested that, as with the lactase gene, these would have been subject to selection as people made the transition to agriculture over the past 10,000 years. Interestingly, several genes in the cytochrome P-450 gene cluster on chromosome 1 had also been subject to strong selection. These genes are expressed in liver tissue and are involved in breaking down foreign compounds in the body, such as drugs. It is possible that new food sources could have introduced new chemicals into our diet that required new versions of these “cleansing” genes to neutralize them.
“The simplest explanation is that the [variant] that was favored could be protective,” he explained, helping to prevent the disease in people with that particular genetic change. It makes sense—if we were in the process of adapting to an increase in dietary calories due to our new agricultural lifestyle, for instance, perhaps a variant that protected against diabetes or heart disease might be selected for. “A more complicated scenario is that things that are diseases now are a consequence of our environment. If your phenotype”—the way your genes have been expressed—“is tuned to a really harsh environment where resources are scarce, and you’re trying to hoard all of your nutrients as efficiently as you can, and then the population switches to an agricultural lifestyle where you’re taking in tons of nutrients, then it may be favorable to not store [nutrients] as carefully.” This was an argument that dated back to the 1960s, when an American geneticist named James Neel had suggested that some noninfectious diseases in modern populations, particularly diabetes, had their roots in the transition from a hunter-gatherer lifestyle to the food-rich environment of agricultural populations, where the genes that had allowed our ancestors to store nutrients very efficiently were no longer advantageous. In other words, the genetic variant that had been good in the old environment had become bad in the new one. Pritchard noted that there are some examples, such as genes known to be involved in diabetes, where the susceptibility variant is ancestral. This is exactly what would be predicted under Neel’s model.
Overall, I was getting a clear sense that the move from hunting and gathering to sedentary farming had had a significant effect on our DNA. Not only had it selected for many potentially positive changes, such as lighter skin in northern latitudes and lactase persistence in milk-drinking populations, but it had also produced some seemingly negative effects. The Big Bang in human populations had been such a rupture with our past that it left a genetic fallout that is still visible. This reminded me of another interesting fact I had learned several years ago, while researching my first book.
WHY DID WE DO IT?
One of the great myths surrounding the development of human culture over the past 10,000 years is that things got progressively better as we moved from our hunter-gatherer existence to the sublimely elevated state in which we live today. Most people assume that the lives of our distant ancestors were, to quote Thomas Hobbes, “solitary, poor, nasty, brutish, and short.” When agriculture and government came along—the inseparable duo we’ll investigate in the next chapter—their obvious superiority was clear, and after that people’s lives improved immeasurably. The explosion in the size of the human population after 10,000 years ago is assumed to be merely the numerical manifestation of the positive impact of growing our own food, the benefits of the new lifestyle writ in the expanding number of happy farmers. In fact, nothing could be further from the truth.
In a classic paper published in 1984, the anthropologist J. Lawrence Angel analyzed the skeletal remains of people living in the eastern Mediterranean before and after the transition to agriculture. He examined several parts of the skeletons of many individuals from each time period, focusing on teeth (which allow an estimate of the person’s age at death), as well as height and something called “pelvic inlet depth index,” both of which are measures of how healthy the person was. When he tabulated the data he saw a surprising pattern, shown in Table 1.
The average longevity of male Paleolithic hunter-gatherers was 35.4 years, and that of females was 30.0. Women’s shorter life spans were a result of complications due to childbirth, and the pattern of greater male life span has been reversed only in the past century as advances in medicine have led to healthier deliveries. Note, though, that as the population made the transition to agriculture during the Neolithic period, and particularly the Late Neolithic, when the transition was complete, the longevity for both men and women decreased significantly, to 33.1 years for men and 29.2 for women. More strikingly, the measures of health decrease dramatically. Male height drops from nearly five foot ten in the Paleolithic to approximately five-three in the Late Neolithic, and the pelvic index drops by 22 percent. People were not only dying younger, they were dying sicker. Although it is possible that this may be an artifact having to do with the particular populations studied by Angel, similar patterns have been seen in the Americas. Overall, the data shows that the transition to an agricultural lifestyle made people less healthy.
Table 1
Surely for it to have resulted in such a massive expansion in human population, agriculture must have been a huge benefit to humanity. How can we explain the massive increase in human population and the dominance of agriculture, which has pretty much completely replaced hunting and gathering in every inhabited corner of the world, when it actually didn’t improve people’s lives? It is only in the twentieth century that we see a significant increase in longevity, and even then the pelvic index is still lower than that of our Paleolithic ancestors. Looking critically at the numbers in Table 1, an evolutionary biologist would say that hunter-gatherers had an overall 22 percent health advantage over Neolithic agriculturalists, which should have allowed them to win hands down in the game of natural selection. Why did they lose?
As we’ll see in the next chapter, the story of how agriculture won this ancient competition is not a simple one. It involved an extraordinary change in our way of life, not just because it produced more people but because it marked a break from the past of a kind no other organism has ever undertaken. As hunter-gatherers, we were a species that lived in much the same way as any other, relying on the whims of nature to provide us with our food and water. When we developed agriculture we made a conscious decision to modify our environment to suit ourselves. Instead of being along for the ride, we climbed into the driver’s seat. The first person to plant a seed in the Fertile Crescent 10,000 years ago set in motion events that were beyond his or her wildest imagination. Our next destination is to find out how and why those seeds took over the world.
Chapter Two
Growing a New Culture
The first farmer was the first man, and all historic nobility rests on possession and use of land.
—RALPH WALDO EMERSON,
Society and Solitude
The power of population is infinitely greater than the power in the earth to produce subsistence for man.
—THOMAS MALTHUS,
An Essay on the Principle of Population
STAVANGER, NORWAY
It’s worth its weight in gold,” my guide shouted over the roar of the boat’s engines. We were speeding along Jøsenfjord, in western Norway, on our way to see the cutting edge of agriculture. Out past the boat’s wake, water and rock seemed to merge into and out of each other as though in a primeval battle. Here the land was winning, as the sheer granite of a fjord wall rocketed skyward, while elsewhere the sea claimed its supremacy, crashing over a low-lying island. With the gray s
kies spitting rain most of the year, and the dark winter days further blurring the distinction between land and sea, you could be forgiven for thinking that the Norwegians live an amphibious existence. Like some Atlantis in limbo, poised above and below the waterline, western Norway and its way of life are inextricably tied to the ocean. It’s no wonder Viking marauders set sail from here to terrorize much of northern Europe, or that they managed to reach America hundreds of years before Columbus: the sea was in their blood.
Holding the small vial of astaxanthin in his hand, Tor Andre Giskegjerde was explaining to me how the substance is used to create the vivid pink color found in the flesh of farmed salmon. In wild salmon, the color comes from their diet of small krill and microalgae rich in colorful carotenoids, but this culinary staple can’t be easily stored and doled out on fish farms, so an artificial ingredient is used instead. While Giskegjerde’s claim about its market value was a slight exaggeration, it was less of one than you might think. Astaxanthin is worth so much, he told me, because the company that manufactures it still has it under patent—it’s produced from petroleum by-products via a complex chemical process. Atlantic salmon—a species once available only to anglers, accounting for its historically high price in fish markets—is now more common than its wild cousins, thanks to fish farming. The colorful chemical additive fools consumers into thinking they are eating the same fish their grandparents might have landed in a stream in Scotland, and its cost adds nearly 25 percent to the price of farmed salmon feed.
Pandora's Seed Page 3
