by Rob Dunn
Clearly, our ancestors’ diets influence how our bodies respond to our current diet. More genes will be discovered that relate to our distinct histories. Some of the genes associated with how we treat each other, our social behavior, for example, seem to differ dramatically from one group to another. Perhaps, it has been suggested, human lineages that moved into agriculture required different social abilities than those that did not. Perhaps they had to be more docile, less prone to aggression. Perhaps they had to be more like their mopey-eyed cows. Perhaps. A dozen labs are studying this question, so we may know soon. In the meantime, we continue to walk around, each a little different from the other. Some of those differences are pure chance, the wonder and idiosyncrasy of inheritance. Others, like genes for digesting milk or breaking down starch, are adaptations for who and how we once were.
In the end, we are different because “we are each special flowers” as our preschool teacher might have told us. We are special flowers because over the last 10,000 or so years of our evolution, we have lived many different lifestyles in many different ecological contexts. Once we all ate the wild species that were around us. Your malaria is my dairy cow. Your fasting period is my extended history of poor-quality grains. Western medicine does not consider these differences, perhaps in part because of a mistaken assumption about our variety, yet another assumption that Sarah Tishkoff would correct.
In standard evolutionary trees of humans, there is a thin and poorly leafed branch relegated to Africans; other branches support Europe and Asia. From those branches split off the twigs leading to Asia, Australia, and the Americas. What Tishkoff came to realize was that something was missing in this picture. Although modern humans had lived in Africa nearly twice as long as anywhere else, Africans were poorly included in studies of the tree of human life. They were an afterthought, a couple of guys someone had taken a DNA sample from while on a trip. Yet it has long been known that Africa is incredibly diverse culturally, and so it might have been suspected that it would also be diverse genetically. Tishkoff’s work has now shown that it is. In fact, by some measures, the genetic diversity among African groups is as great as that found in all of the rest of the world combined, a finding that reconciles with what is known about the diversity of cultures. Almost a third of the languages are found in Africa and, with them, a third of all ways of living. In other words, in the most common telling, we had the tree of human life precisely backward. The tree of life itself is rooted in Africa, where most of the branches remain. The rest of humans, from Native Americans to aboriginal Australians to Swedes, descend from just a few branches corresponding to one or two migrations out of Africa, and with those migrations came a loss in genetic diversity. A smaller subset of people and genes moved over each further hill.6
There are consequences to the realization that most of human diversity can be found in Africa. For one, it means that our categories of black, white, and brown are useless when thinking about health and disease. The white patients that were long the focus of much of Western medicine come from relatively few, and somewhat anomalous, branches of the human tree. In medicine, where nonwhite races are considered, it tends to be as a contrast. For example, the inability to digest milk as an adult was long thought of as a deficiency. Yet it is the normal condition. If anything, having the ability to drink milk as an adult is unusual, both in our history and in most parts of the world today. Similarly, thousands of studies compare the differences in health and disease between whites and blacks or whites and Asians with a kind of implicit model of trying to understand diversity. Such studies almost invariably find differences that are then attributed to genes, culture, economics, or some complex slurry thereof. But the truth of the matter is that in any such comparison, the artificial category “black” or “Asian” includes much more variation than the category “white.”7 Whiteness, after all, is as unusual as the ability to drink milk as an adult. To the extent that genes influence our modern differences in health and well-being (and they clearly do), this racial split will remain “of limited utility,” or, to mince fewer words, stupid. What we really need is a kind of evolutionary medicine that acknowledges the diversity and contexts of our pasts.
Practicing medicine in a way that acknowledges the differences in our histories is difficult. It depends on knowledge of our many histories, a knowledge that we are losing rather than gaining. Once upon a time, which is to say, 200 years ago, there were, by most estimates, around 20,000 cultures or language groups on Earth. Not all of those cultures were associated with specific genetic adaptations to particular mutualists (or for that matter, worms, pathogens, or climates), but many were. As of today, there are just 6,000 or 7,000 surviving language groups and associated cultures. Perhaps 1,000 or fewer of those are viable in such a way that they are likely to endure another two decades. Each day we lose a few more stories encoded in languages, histories, and spoken and unspoken words.
We have swapped our old partners, wild species of fruits, nuts, and prey, for our new domesticated partners. The relatively new lifestyle associated with these new partners continues to spread, often without its associated genes. In the last 200 years, a few agricultural cultures have grown ever closer to replacing the other remaining cultures on Earth. As this happens, the relationships between our individual histories and our cultures grow muddled. In the future, few individuals are likely to know enough about the livelihoods of their own ancestors to be able to make sense of their genes. Maybe you are already one of them, too disconnected from your past to know whether your ancestors dined daily on seals or tended instead toward the delicacy of wild fruit and ant queens. Soon, few people will be able to say much about where they came from and what they once did. Yet the genes and descendants of these cultures and peoples will persist—rolled into our broader genetic pool and with it our wonderful, tragic, muddy story. When genes are still linked to the cultures in which they evolved, one can understand their origins and significance. But as cultures homogenize, our individual histories blur. Today, Masai individuals can be identified as a cattle people. In that context, their genes for digesting milk make sense. But for the Cavineños, a very small indigenous group in the Bolivian Amazon, the history is less clear. They now farm, somewhat, but for how long have they done so? What did they farm in the past? Some individual Cavineños still know, but most do not, and therefore that knowledge will soon be lost, and they will be culturally like other Bolivians living in the wet tropics. Their genes, though, will be otherwise, a more complicated story that asserts itself without context, a glacial rock in the middle of a field around which one must walk and plow, but which, on its own, never quite makes sense.
Part V
How Predators Left Us Scared, Pathos-ridden, and Covered in Goose Bumps
9
We Were Hunted, Which Is Why All of Us Are Afraid Some of the Time and Some of Us Are Afraid All of the Time
Our parasites and mutualists influenced our bodies. It is the predators, though, that messed with our minds. We come from a long line of prey; we have been eaten since we were fish. For most of our history, we were more like the pronghorn than the cheetah, more likely to flee than to chase. So it is that time and natural selection had, until very recently, favored the wary over the brave. You can experience your body’s wariness to the threat of a predator when someone jumps out at you from a dark hiding spot. You can feel this past when you watch a scary movie or even by reading about someone else’s scary experience, for example the day in 1907 when a girl in India named Bakhul and her girlfriends went to gather the leaves of walnut trees to feed to the cows. Bakhul had climbed high into the branches to reach the tenderest shoots, the cows’ favorites.1
On that day, she was the first to finish gathering and to start down the tree. As she did, she felt a tug on one of her small feet. Was it one of her girlfriends? No, it was firmer, less playful. A tiger stood at the base of the tree. It looked up at her with big eyes and pawed again with its claws extended. It pulled her as if she were
a lamb. She screamed and held on, but only for a moment. Her leaves fell around the tree, as did the beads of her small blue necklace. The tiger carried her into the woods. She screamed. She was terrified, but still alive.
When they were told about the tiger, Bakhul’s mother and father were despondent beyond words. One town over, a woman had seen her friend taken by this same tiger and it had left her mute, in shock. Bakhul’s parents too seemed unable to speak. The wife stirred the pot of their food. The husband sat, unhinged. The door of his life had swung open and it could not be put back. Somewhere at the edge of town or in the quiet between houses, the tiger was walking. Bakhul might still be alive, but no one chanced searching for her, not yet. The families shook and waited inside their houses for whatever might come. Lightning strikes the same spot only once, but tigers can strike repeatedly. This tiger had already killed more than 200 people in Nepal before armed guards chased her across the border. Once in India, she had killed another 237. Now it was in this town that she would do whatever she did next. Given her history, she would almost undoubtedly eat someone else. If not Bakhul, then who?
In this story, one wants to yell at Bakhul’s family to look for her. “Search her out!” “Be brave!” No one would have listened. The town was shuttered in fear. Doors stayed closed. Children peed into cans and poured them out the windows. Adults too searched for their own vessels or crouched just outside the door. This society had turned inward and gone rancid with fear and excrement. No one wanted to leave their houses, even as food began to run low and the crops rotted on their stems. Even baboons, stronger, faster, and better defended than humans, hang close to their kin when predators are near. They sit back-to-back looking out and groom each other, gently touching each other’s heads and backs, just as it was in this village, people tending to each other with equal parts of tenderness and alarm.
As the villagers waited, they had time to recount other stories they had heard about this tiger and when those stories were exhausted, about other tigers. One town away, in the village of Champawat, a group of men was walking along a path near the village when they heard screaming. Then they saw a tiger coming toward them, and in its mouth, a naked woman, her long hair dragging as she cried for help. In that story too, the men had been too scared to act and so the tiger, carrying the woman, continued on its way. There were dozens more stories. Most ended fatally, but every so often the person was saved, and so they hoped for Bakhul. They hoped for her to stagger back into town, hoped because they were all, each and every one, too terrified to act.
Bakhul’s story has lived on through the writing of Jim Corbett, the great hunter of man-eating animals. It was Corbett who would eventually come to try to find Bakhul and then to kill the tiger. The longer story of humans and predators, though, is embedded in our bodies, in our genes and their products, in particular in a network of ancient cells in our brains called the amygdala. The amygdala is connected to both the more ancient and the more modern parts of our brains. It, along with the adrenal system, is caught halfway between our deep past and the present. From there, it urges us into action or contemplation, depending on the circumstances. If you feel any eagerness on behalf of Bakhul, any modest restlessness for resolution of her story, an interest that perhaps even sent a chill or two down your arms, it was because of your amygdala and its signals. But more generally, it was because you are descended from a long line of individuals who escaped being eaten, at least long enough to mate, a lineage going back not just to grandma but to lizards and then even further. Your heart pounds harder when you are afraid (or angry, a point to which we will return) because of the pulleys and levers of your adrenal glands and the signals sent from your amygdala to your brain stem, that even more primitive root of our actions and wants. This system, sometimes called the fear module, evolved primarily to help us deal with predators, whether by flight or, less often, at least historically, fight, but it’s a finicky system that can be aroused at the mere idea of a threat. Fear, or at least the urge that precedes it, may even be our default reaction to our surroundings. Some elements of our amygdalae appear to constantly send out signals to our bodies that we are afraid. Most of the time, other parts of the amygdala suppress those signals. But when we see, hear, or experience something that triggers fear, the suppression is released and fear courses through us, instantly, like a bomb in our brain.
Our fear modules have been shaped by many thousands of generations of killings and near escapes, since the very earliest moments in which one animal pursued another. Now we retaliate against these predators, but for most of our long history of interacting with predators, we did not have guns. We did not even have the wherewithal to pick up and wield sticks. We screamed (the scream itself being a near innate element of the fear module) and ran. If we did not, it was just a matter of time before we went “one by one into the capacious stomach of [our] arch-enemy, which never neglects an opportunity of reducing [our] numbers and thus fulfilling its mission in life.”2
When asked around campfires and poker tables to construct stories of our identity, we tend to cast ourselves as the predators, both powerful and in control. In storybooks, Little Red Riding Hood is saved, just in the nick of time. That may be who we are now—Johnny to the rescue with his big, bad gun. But the truth is that for most of our history, we were not able to save the little girl in the den of the wolf. We may have tried, but more often than we succeeded, we failed, at least for the first several hundred million years. By the time Bakhul was attacked, such attacks had become less common and yet still occurred (and occur). The tiger that had found Bakhul would turn out to be a “man-eater,” an individual that, through injury or age, had become unable to attack its ordinary prey that might fight back. For most of human history, though, our ancestors would have been eaten simply as one of a variety of possible kinds of prey. As we grew more common, we may have actually become preferred. The more of us there were, the easier we were to find.
Predators did not avoid our ancestors until they had weapons. Even then, the entire concept of man-eaters suggests our ancestral weakness rather than our strength. Injured and old “man-eaters” eat humans because we are the easiest to catch and kill. We have no horns, sharp teeth, or even hair to impede digestion. We are nearly as well packaged for consumption as a hot dog. The “Man-Eaters of Tsavo”3 are rumored to have killed dozens of people in Kenya, enough to impede the construction of the railway from the shores of Lake Victoria to the port of Mombasa in the late 1800s. The two male lions were eventually shot and shipped back to London, where they sat ignored in a museum for a hundred years. However, a study of the bones and mouths of these lions shows that they had been sick, in one case deformed and missing a number of its teeth. In other words, if you are an old predator that cannot catch anything else, a human is your best alternative. We are among the only animals, aside from a three-legged wildebeest or our simple-minded cows, so defenseless that even an animal with a broken leg or missing teeth can catch us. Today, those animals that attack us tend to be killed (as eventually were the Man-Eaters of Tsavo), but for most of our history they operated with impunity. Our ancestors could barely see in the dark, and so when they heard a sound in a cave they crouched, listened, and hoped that if it was a tiger, bear, or other big carnivore, it would eat somebody else first. Imagine peeing in the woods at night with the stars overhead and the sounds of lions or tigers and other animals in the grass all around. It was perhaps with such thoughts in mind that San Bushmen rendered cave paintings of lions dismembering humans.4 Such scenes have long haunted our dreams.5
In the wild history of humans and large predators, we were long unambiguously the prey, which allowed the fear modules in our brains, modules that developed many millions of years ago, to persist and even to become elaborate as we evolved. To find a predator in our ancestry, we may have to go back to the time when we had four feet, a lizard tail, and scales. Even then, we were probably just as likely to be the eaten as the eater. We have been screaming the barbaric, a
nimal equivalent of “Oh shit, don’t eat me” for 300 million years. Four kinds of data tell us that we were eaten up until very recently. First, large numbers of actual predation events on humans have been recorded. In colonial India, tigers may have eaten more than 15,000 humans a year.6 At least 563 people were killed between 1990 and 2004 by lions in Tanzania alone. Nor is it just tigers and lions. Mountain lions eat people. Giant eagles eat (or at least ate) children. Bears of quite a few varieties eat people. Lions, leopards, alligators, crocodiles, sharks, and even snakes eat people, especially children. Even wolves eat a runner or two. And all of this is just during recent years when predators are both more rare and far less diverse than they were during most of our evolutionary past.
Second, the fossil record of our kind is rife with the bones of our frequently gruesome demises. An individual of Australopithecus africanus was discovered with eagle talon marks in its head. It was discovered in a pile of other bones, under the eagle’s nest. In a study of Pleistocene leopard diets, A. africanus individuals were the most common prey item at one site, which is to say the leopards were actually specializing in our ancestors. At a second site, another leopard bone pile revealed similar results. At both sites, the bones that were left to be found by scientists were those of the head, which leopards do not eat and then, in a pile, the regurgitated pieces of the rest of the body.7 One imagines our ancestors living together in a small group that was, night by night, victim not just of leopards but also of the other predators that walked among shadows, invisible to our crude senses,8 predators like lions, hyenas, and wild dogs, their larger, now-extinct kin.9 Nor are these south African caves an exception. Among the earliest fossils of hominids, many come from bones that appear to have been broken to bits by carnivores. We see our own history, our mammalian history, most clearly through the mouths of big dogs and cats.
