The Wild Life of Our Bodies: Predators, Parasites, and Partners That Shape Who We Are Today

by Rob Dunn

  Maybe, one might think, the Masai did something special to the milk to make it more digestible. Producing cheese out of milk reduces its lactose content, for example. But the Masai and other East African groups were not making cheese. For a while, it seemed possible that there had been migration of individuals (or their genes) from Europe into parts of eastern and western Africa where pastoralism was found. Then, a group of scientists in Europe found some of the genetic variants apparently associated with lactase persistence. When they checked for the presence of those variants in different populations, they found that they were, amazingly, present both in European descendants of dairy people and in the Fulani and Hausa, West African pastoralists. Apparently migration had occurred that allowed the mutant European gene variants for drinking milk as an adult to move across Africa, or at least to the Fulani and Hausa. But this result did not sit totally right with Tishkoff. It seemed like something was missing and so she collected more data on other pastoralists, which is when the problem arose: the Masai and the Dinka and other groups in eastern Africa lacked the gene variants for digesting milk as adults.

  The Masai did not process their milk, nor did they have the European gene for digesting milk. Tishkoff decided to consider a third possibility, that these groups had, in their long history with cows, evolved the ability to digest milk independently of Europeans, that they had relived the history that Leigh Binford imagined was common in the origin of agriculture, but even more than that, that they had done it with exactly the same species. Perhaps individuals with mutant variants of genes that allowed their lactase to persist into adulthood had independently been favored separately in eastern Africa and Europe. It was a long shot, but Tishkoff went ahead.

  The answer Tishkoff found in East Africa was that the ability of human adults to digest lactase evolved more than once. It evolved once in Europeans, around 9,000 to 10,000 years ago, at about the time that archaeological evidence and cow genes point to the domestication of cows in Europe. It then evolved again, at least three times, in Africa, beginning around 7,000 years ago, again just about when evidence suggests cows were domesticated for the second time. At least twice (and probably more like four times) upon a time, aurochsen were domesticated. In each of these cases, Tishkoff has shown that individual humans who had the genes to digest milk as adults had far more children who survived to have children themselves than those who did not, and so on into subsequent generations. Their family tree grew branches and with them the genes that allowed them to drink from the land spread quickly all over Europe and Africa. These were the biggest genetic changes in our recent histories, at least that we know of so far. They were repeated, just as Binford had predicted, with nearly identical fates. In both cases, individuals who could not reap the benefits of the domestication of cows died or simply failed to reproduce. Milk did the body good because these were hungry times, times and places when extra nutrition and extra liquid made the difference between passing one’s genes on and not. With milk, much, but not all, of human population changed.

  In the end, the story of cows and humans is an example of an even bigger, broader story. Wheat would save us, just as the aurochs did, and as would manioc, rice, and our other staples. As our peoples grew denser, we would find ourselves repeatedly at the point of starving and being saved. As the origins of our other crops and animals and the peoples who tamed them are studied in more detail, it seems likely that in many, perhaps most, cases, it will be shown that the genes of those who began to farm changed. It is already known that people who live in regions where grains were domesticated have extra amylase genes, amylase being one of the enzymes that helps to break down starch. Whether these genes too swept quickly among populations has not yet been studied. It seems possible. In fact, it seems possible that in each place that agriculture arose, our bodies changed, independently and differently. Our great human variety reflects, in no small part, the great variety of ways in which we came to depend on individual species, a new less diverse set of species, to make it through the toughest years.

  In the villages of our descent, we turned to these new species and latched on, the way a baby first latches on to its mother. We had been brave and independent, but in those moments, we gave in. We would live, for each day after, where and how those species needed us to live in order to benefit from what they offered. We made an evolutionary contract from which we have never since been separated. It is far easier to divorce your spouse than to divorce agriculture. Of course, one can go off the grid and hunt and gather for food, but it is no longer easy and we can’t do it collectively, not as a species or even as a country. There are not many places to go, and we have forgotten how to live in those wild ways. The same is true for our domestics too. Our dogs can turn feral, but they never go far. They depend only on us, whereas we now collectively depend on many species, but even this is deceptive. By some estimates, 75 percent of all the food consumed in the world comes from just six plants and one animal. If cows went extinct tomorrow, millions of humans would die, just as would happen with wheat or corn and as once did happen with the blight of potatoes. Cows may look at us with mopey-eyed stares, but we are partnered. Whatever our fate is as we move forward, it is largely shared.

  What are not shared are our genes. The genes you or I have today were shaped by those events in recent history when, as a function of our new cultures and ways of surviving, some individuals passed on their genes and others did not. These differences persist. It is hard not to wonder who you are and how your own history, genes, and even (to revisit earlier chapters) microbes influence your modern fate. It is hard, particularly given that despite our differences, we are now converging on similar “modern” diets and lifestyles. The milky, high-fat, high-salt, high-sugar lifestyle we are imposing on our variety of different stories and genes has consequences for our modern health that depend both on who you are and also on who your ancestors were. Even today, it matters whether your people were the ones who crawled under a primitive cow or the few others who looked on from a distance and took the time to point and laugh.

  8

  So Who Cares If Your Ancestors Sucked Milk from Aurochsen?

  A billion people on Earth are overweight, their stomachs pushing at their waistbands and their bodies burdened by excess. The situation is at its worst in the United States, but other countries are quickly catching up. Yet even in the United States, not everyone is affected. Sixty-five percent of U.S. adults are overweight, but the others are not.1 It is easy to attribute this variety to differences in diet and exercise, but lifestyle is just part of the story. There is also, embedded in our differences, a kind of mystery. After all, the vast majority of Westerners now eat diets that are derived from relatively few domesticated animals and plants. Perhaps you eat only grapefruit or organic camel’s milk. Perhaps you practice moderation and make thoughtful, conscious decisions. If so, you are an exception. The average American and, increasingly, the average Westerner has a diet in which three-fourths of all calories come from dairy, cereals (grass seeds), simple sugars, vegetable oil, or booze.* None of these foods was consumed before the advent of agriculture. Ten thousand years ago, tens of thousands of plants would have been harvested from the wild by humans. Agriculture, even in its earliest incarnations, decreased both the total number of foods we harvested as a species and the number of kinds of foods that any individual human was exposed to. With time, more crops were domesticated, and we recovered some of the diversity of our foodstuffs. But since then, we have come to focus on those few species that grow best and that most simply suit our taste buds.2 In the process, we have both neglected many crop species (several crops are extinct and nearly a thousand are regarded as endangered) and forgotten how to collect the foods we once gathered. Wild berries now sit unpicked on their stems and a small handful of crops constitutes most of the calories we consume globally. Sure, you can find quinoa in your local Whole Foods, but as a proportion of the calories consumed by humans today, such boutique crops are a tiny drop in
an ocean-sized bucket filled with corn, wheat, rice, and a few pieces of crispy meat.

  How we metabolize our relatively new diets differs, at least in part, because of the differences in how our recent ancestors lived. Imagine that we performed an experiment in which we fed every person the same foods in the same quantities. We could come back and check on their (or really our) status through time. What do you predict would happen? We tend to act as though everyone would begin to look the same, or at least similar, in terms of their weight and health. This is the premise on which nearly every diet plan, exercise book, and weight-loss show is based, be it the grapefruit diet, the all-meat diet, the no-fat diet, or something else. It is the premise on which growth charts for babies are derived. It is the premise on which most of medicine, in one way or another, relies. The truth is that we would still differ even when eating exactly the same food. Those differences are the result of the differences among our pasts, differences that assert themselves from just beneath the surface like some sea monster faintly visible in the dim light of our collective minds.

  Let us return to the story of milk. As I have discussed, we do not all have one of the versions of the gene required for digesting milk as adults. Geographically speaking, the ability to drink milk as an adult remains relatively rare. No Native American populations, whether the Incas, the Maya, or any of the several thousand other groups, were able to drink milk prior to the arrival of Europeans and their genes. Roughly 25 percent of the people on earth are totally unable to digest lactose as adults and another 40 to 50 percent can only partially digest lactose. If these billions of individuals happen to drink milk, they will suffer diarrhea and gain, on average, about 5 percent less weight from the standard American diet than do those who can digest milk. In disease-prone environments, such individuals are also at more risk for dehydration associated with diarrhea. In the context of the villages of our origins, any individual who received 5 percent fewer calories would stand a lower chance of passing on their genes, and so it is that historically the milk-digesting genes won, at least where we domesticated cows. In the context of our modern world in developed countries, in which calories abound, the 5 percent extra are more likely to be chalked up as being to our detriment. So, while the advertisements may say that milk “does the body good,” they fail to mention the caveats “but only if your body can digest it” and “only if you need it.” That our bodies respond differently to the same food as a consequence of our ancestry may seem obvious. Yet we ignore such realities every day. The USDA food pyramid still has as one of its main items “milk,” along with fruits, vegetables, meats, and beans, even though most humans worldwide cannot digest milk. Milk is just the beginning of the unraveling of the idea that any one species of plant or animal food (or processed version thereof) might do us all good.

  It is only in light of our recent evolution that the differences among us in our genes make sense. Take your saliva as another example. Amylase is one of several enzymes present in the saliva of many animals, including you. It aids in the breakdown of starches such as those found in corn, potatoes, rice, yams, and other staples of both early agriculture and modern diets. Some humans have extra amylase genes, and so produce more amylase. Those individuals digest starch more quickly and efficiently. Some of us, perhaps you, have sixteen times more amylase than do others. In historical context, this variety appears to exist for a reason. Our preagricultural ancestors had few copies of the amylase genes,3 and therefore less efficient spit. But it seems that in those peoples that began to farm starchy foods, individuals with more copies of the amylase gene did better, and so passed on their genes. In the parts of the world where humans still regularly starve and eat starchy foods, such as much of the developing world but also poor regions of the developed world, having extra amylase genes may still be beneficial. It may allow some individuals to garner more energy than others from the same bag of rice. In those parts of the world where we eat too much, the same genes are likely to help to make us fat. How our bodies respond to the food we give them depends both on the ways in which our recent ancestors lived and on where we now live. One man’s survival gene is another’s belly roll.

  Nor are the descendants of agriculturalists (whether they were dairy or potato farmers) the only ones likely to have unique genes. The descendants of hunter-gatherers may also have specialized genes associated with their former lifestyles. Ten thousand years ago, all of our ancestors were hunter-gatherers, though they gathered different things as a consequence of where and how they lived. Some populations ate mostly meat, others mostly insects, and still others diets rich with bark (I did not say tasty, just rich). By 5,000 years ago, many fewer peoples survived by gathering and hunting alone. By 1,000 years ago, there were fewer still, and they were located mainly in marginal climates, seasonally cold or dry places like deserts and the Arctic, where crops did not grow well enough to supplant gathering. In general, one would expect that at least some of those peoples that persisted in hunter-gatherer groups might have evolved genes associated with the unique challenges of their ways of living. This should be most likely in those groups at the margins of survival, in places so inhospitable to thriving that the agriculturalists did not ever bother to push them out. In those margins, the genes that were useful are likely to have been very different from those that were useful in dense agricultural societies. But how?

  James Neel, an anthropologist at the University of Michigan, suggested the hypothesis that hunter-gatherers who lived in seasonal environments, where food is plentiful sometimes but scarce other times, would have had an advantage if they could store food on their bodies (get fat) more quickly during the good times.4 This idea, often called thrifty genotype hypothesis, has precedent in ecology, though no one seems to have noticed.

  In the early 1800s, the German doctor Karl Georg Lucas Christian Bergmann argued that animals that live in cold places tend to be bigger and fatter than those that live in warm places. His explanation for this pattern was that big, fat animals have less surface area relative to their volume (a snake is all surface, an elephant all innards, for example) and so they are more likely to stay warm enough in the winter to keep from freezing. Over time, Bergmann’s original rule was broadened when it became apparent that there were two reasons to be fat where it was cold: to stay warm (as Bergmann originally suggested) and to keep from starving during the months when food was too scarce and a period of fasting was inevitable. In other words, fat was useful both where it was cold and where food was seasonal. Fat is a meal when there is otherwise none. Societies, though, have other options. They can, like honeybees, store food in the winter. The question becomes whether humans are more like grizzlies or bees.

  Although often discussed, Neel’s hypothesis has been poorly studied. He may be wrong. To test Neel’s theory, one could look for genes that tend to be favored among hunter-gatherers, though they might be different in different hunter-gatherer groups (much as different mutations in Africans and Europeans allow adults to drink milk). Alternatively, one could check to see whether hunter-gatherers, when confronted with modern agricultural diets, tend to suffer disproportionately from diabetes and obesity, as might be expected if they were especially efficient at converting food to fat and simple, easily used sugars. Neel’s theory predicts that hunter-gatherers from seasonal environments should produce more sugar and fat from this modern diet than do individuals either from hunter-gatherer lineages from less seasonal environments (such as tropical wet forests) or from agricultural societies. In other words, they should be more likely to be obese (good fat storage) and diabetic (really good at turning foods into sugars). One would predict that diabetes should be common in the peoples who lived in those same places where ants and bees store honey or bears get fat, which is to say in deserts, subtropical lands, and tundra.

  In 2007, a large study compared the prevalence of diabetes in hunter-gatherers and former gatherers to those of human populations more generally.5 Many hunter-gatherers store food, particularly th
ose in the north where seal and salmon can be dried. In those groups, diabetes is still rare, actually more rare, for example, than in Western societies. In contrast, in desert and subtropical groups, whether in Australia, Africa, Asia, or the Americas, where storing food is more difficult, diabetes is four times more common than in agricultural societies and Western societies. Only two possible explanations exist for this pattern. Societal pressures may push hunter-gatherer populations consistently and nearly inevitably toward lifestyles that are poorer in nutrients and simpler in sugars than the general population. Alternatively, though not mutually exclusively, hunter-gatherers suffer today because of the consequences of their once uniquely useful, but now misplaced, genes.

  No one yet understands how all the genes for metabolism merge to create the different responses to similar diets seen among different peoples. The genes for bodily processes like storing fat are far more complicated (and ancient) than are those for drinking milk as an adult. Whatever the answer, it is unlikely to be simple. It is also unlikely to be the same from one gathering group to another. At the same time, it seems predictable that as humans spread around the world, their metabolisms changed to reflect differences in diet and lifestyle. The more we look, the more we are likely to find our differences. Those differences are due to our many histories and they bear consequences.