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

by Rob Dunn

  The villagers were positioned. So far, everything had gone according to plan, but as Corbett and the chief walked, the chief grew tired. He was old. He begged Corbett to let him rest and then, before Corbett had a chance to respond, he took a break. Nothing could be more innocent and natural than his wanting to sit, except that the sinking of his body onto a log, like the lowering of a flag, was taken by the villagers as the signal to begin beating. They began making noise with anger and fury and all of the energy they had pent up in their mourning. They wailed on their makeshift drums. Sure enough, the tiger ran from their bravery straight toward the place where Jim Corbett and the headman were going to shoot it.

  Corbett and the headman were not yet ready. They were still high on the hill and so had to start running down to outpace a tiger running down through the brush. If Corbett and the chief did not make it to the neck of the valley, the tiger would escape and kill again. They ran, but they were too slow, at least when paced against a tiger. Then it happened. The tiger burst from the forest when Corbett was still 300 yards away. The chief was even farther away. Corbett assessed the situation and paused. The chief assessed the situation and acted with all of the fury of the entire village. He fired and missed. So it was that the tiger turned to run toward the villagers.

  There seemed to be only one possible outcome. The tiger would break through the line of villagers, killing one or more in the process, and then run back into the woods, where it could lurk for days or years to come. But none of the villagers knew what had happened. Having heard the chief’s gun, they thought the tiger was dead. They were celebrating. No one was even looking downhill when the tiger began to run their way. They were like lambs. Then dumb luck intervened.

  The tiger heard the prematurely celebrating villagers and fled from the sound back toward Corbett and the headman, who by now were ready. Corbett fired and hit the tiger in the shoulder. The tiger seemed unaffected. It turned toward Corbett, raised its haunches and lowered its shoulders the way a house cat will do before pouncing on a shrew. Corbett was the shrew. He was also out of bullets and yelled to the chief to bring down his gun. The tiger was ready to pounce. What happened next happened so fast as to make all retellings wrong in some detail. Somehow, Corbett grabbed the chief’s gun and squeezed the trigger.

  The shot missed. Fortunately, the damage had already been done. The tiger fell dead from the earlier shot. Meanwhile, the villagers farther up the hill had never stopped celebrating, and so they continued. Corbett had killed the monster of Champawat and was to become the great hunter of man-eaters.

  After Corbett killed the tiger, the villagers would have taken the time for a funeral and sent the body of Bakhul down the river so that her story might make it to the Ganges, to mix with the stories of other women in other villages across India and time who had been eaten. Tigers would kill more people in the years to come, but fewer each year. In the last century, we have escaped our many-million-year burden of being prey. The hundreds of thousands of tigers that once covered India became thousands and then hundreds. More tigers may now live in captivity in Texas than wild in all of Asia. The same is true or nearly so for other predators—cheetahs, leopards, lions, cougars, jaguars, even wolves and bears. The process that began those many thousand years ago, when we first started to hunt in packs like wolves, has nearly reached its completion. Large predators still kill people, a few dozen a so or year, but very few and mostly at the borders of wild lands, where we walk out to pee in the woods and reenact our primitive urge and they, in return, reenact theirs.

  Yet the past’s effects linger in each of our lives and in much of our mental illness, discontent, and perhaps even choices about where and how to live. Because while we killed most of the tigers, most of the wolves, most of the bears, cheetahs, and lions (though not the giant primate-eating eagles, nor the deadly venomous snakes), our bodies’ responses to those predators remain—shaped by the effects of having had to run away for so long.3

  We still have adrenal glands and the amygdala in our brains that translate what we perceive into our body’s response. We still have these structures even though we now stand essentially no chance of being eaten or even chased by a predator. Our fear (and its companion, rage), modulated by these parts of our body, led us to kill most of the animals that triggered that fear. But what then do the systems of bells, whistles, and blood vessels in our bodies that evolved to produce fear have left to do?

  One place we still see the workings of our fear is in our demand for scary movies and books. Think vampires, Freddy Krueger, and terrible murders on crime shows, each of which terrifies us and as it does, triggers the same chemicals in our bodies once stimulated by the tigers near our villages. We have come to buy the stimuli that trigger our fear response as though to remind ourselves of what it can do, of the way our blood can pump as we flee.

  But the other reality of our modern context is that the same system that once produced fear in us now short-circuits in a world in which many of our stimuli come not from direct threats to our body, but from distant threats. We listen to the news and hear about every murder. We think about our budgets and fear their consequences. These diffuse fears trigger the same responses that the tigers once did, but they do so chronically, a little bit each day. Instead of yielding a resolution, this fear yields anxiety and stress. As many as one in three adults will at some point suffer anxiety disorders brought on by misplaced fear. If prolonged, such misplaced fears can even lead to depression and other stress-related diseases, and to shorter lives. Our chronic and misplaced fears create chronic stress and distress, and make us more rather than less likely to die. We can wake up at night ready to run from our unbalanced checkbooks as often as we like and we will never quite get away. Our anger, in turn, leads to everything from domestic violence to war. We have responded to this chronic stress and rage by taking drugs and buying products that stimulate these ancient parts of our brains.

  Nor are stress and anxiety (and their bestiary of associated disorders) the only manifestation of our ancient urges to flee or freeze in the presence of predators or other dangers. “Phobia,” after all, is simply a word for fear that is directed against something one should not fear. Our modern phobias relate to our ancient system of fear, misplaced in our modern circumstances. Along with these phobias come panic and even post-traumatic stress disorder, both also related to fear cues left to wander, misplaced, among the cells of the mind.

  Some people are more susceptible than others to the modern consequences of tigers, leopards, and our residual fear. Part of the difference from one person to the next is genetic, another part relates, complexly, to experiences, whether in childhood or later. Maybe you are lucky and you sleep easy, afraid of nothing, no rage in your heart (or rather amygdala) and no fear either. Or maybe you live a life in which your fear still makes sense. But if so, your company is sparse. The response of most of our adrenal systems in the context of our modern fear and aggression is no longer adaptive. It is out of context and control and so without recourse we tend to medicate ourselves, whether with prescriptions (which can help with anxiety but not panic or phobias) or street drugs, to quiet the predators that still pace our brains. The medications cost us billions a year. The street drugs cost us far more—economies, livelihoods, and lives. In the future, it has been suggested, we might be able to silence the actions of the genes that make us fearful, phobic, anxious, or angry, to teach ourselves, in other words, genetically, that the tigers we imagine everywhere are gone. Meanwhile, we have nearly extinguished the real tigers from the real world. At zoos, we press our faces up against their cages to be reminded of what they once inspired in us. We laugh when they bat at their bars. Yet we also get the chills, because deep down, under our fat and muscle, our bodies remember. Our bodies remember even as our conscious minds have forgotten. So it will long be. Long after even more of the tigers have been left to go extinct, we will remember them, one sleepless, adrenaline-primed night at a time. Nor is this discontent even the
end of the story. Predators shaped our fears, but that is not the limit of their influence. The legacy of their brutality leaves many marks, the most pervasive of which frame what we hear and smell in the first place, how it is, in other words, that we construct and live in the world. They shaped everything that, through the lenses of our senses, we sought to change.

  11

  Vermeij’s Law of Evolutionary Consequences and How Snakes Made the World

  Imagine it all otherwise. Imagine that you were able to see smaller or more distant things than you do. Imagine that you had a better sense of smell. Each species constructs the world out of the signals received from its senses. Birds and bees see ultraviolet patterns. Ants see the stripes of polarized light in the sky. Vipers see heat, taste chemicals in the air, and feel the vibrations of each banging footstep through their skin. We do not experience their perceptions, except through tools we have invented but not internalized. The world our senses have created in our minds is visual, the other senses only secondary, unnamed characters in the big Hollywood plots of our lives. Look at the chair you are sitting in. Look at the walls around you. They were chosen for their visual appeal and, perhaps to a lesser extent, their texture. They were not chosen for their taste or smells, or by any of the visual cues that would be apparent to other species but not to us.

  Our eyes do not just guide us. They lead to our actions. Children choose among beach shells by their colors and appearance and we have, collectively, chosen among the living things on Earth in similar ways. Wild roses smell lovely, but the roses we have tamed and bred are essentially unscented. We chose visual beauty over aromatic beauty because of the quality of our eyes. We make such choices again and again. We chase the large, visible animals, particularly those perceived to be dangerous, like coyotes, out of our cities, but pay less attention to the smaller and less visible species that cling to night and walls. We kill innocent rat snakes in the garden because they are big and black and visible, but miss most roaches and bedbugs, to say nothing of even smaller lives, as they sneak past us unnoticed, species that would be obvious if our senses were different. We ignored the microbes until someone told us they were everywhere and then we overreacted toward them (though only the species susceptible to our drugs). In other words, all of the ways we have changed the world, changed in particular the species we interact with, are because of our vision. What is more, while our vision has become more dominant, at least some of our other senses have actually atrophied. The genes for smell have been left to wither and break. We can distinguish fewer smells than could our ancestors. But our eyes are marvelous and powerful. The question then is how such eyes and their influence evolved. Right now, as you read, your eyes, eyes that evolved in Africa under the tropical sun, are distinguishing the fine lines of letters, running up over r’s and down and around u’s. The abilities of our eyes and their influence deserve explanation. That explanation, to the extent that we have it, seems to have a great deal to do with a woman named Lynne Isbell and snakes.

  Lynne Isbell is a primatologist at the University of California, Davis. For most of her life she had no intention of thinking about how her own blue eyes evolved, much less those of the monkeys she studied. Then one day she was moving too fast as she ran through the forest chasing monkeys. We tend to think of ourselves as the successful primate and yet as she struggled to keep up, it was hard not to feel as though she was the one who had fallen. Her body was slow, awkward even, in the grasslands of its origin. She high-stepped over logs and branches and listened for the sound of leaping monkeys. Then it happened. Midstride, she realized she was stepping on a thin black snake across the path but she could do nothing to change the outcome. Adrenaline flooded her body. Fortunately, the snake, perhaps a cobra, did not strike. It went on its way, a little worse for the footwear. This would not be her last close encounter. In the coming years, she would come face to scaled-face with a cobra in defensive posture and later a puff adder. What amazed her in these cases was that she saw these snakes at all and then that, in most of the cases, her body, somehow, responded to seeing them before she was aware of them consciously. It was as though while her intellectual self had been looking far ahead, another more self-conscious self had been looking all around. Lynne froze just outside of stepping-on-snake distance before she even knew why she had stopped. That she detected these snakes and responded before she became aware of them was a kind of mystery of vision, the brain, and fate. Her experiences were not quite life or death, but that mystery would eventually change her life.

  Before the snake encounters, and even for a few years after them, what Isbell had planned to do with her career was to carve off a quiet area of intellectual space and spend the next couple of decades studying within it. She was interested in the social behavior of monkeys, including their peregrinations (hence, in part, her pursuit). She wanted to understand why the female monkeys of the Americas—spiders, woollys, squirrels, and all the rest—often move away from home upon reaching adulthood, whereas those of the Old World (Africa and Asia) almost never do. This was not the only curious difference between Old and New World monkeys. Old World monkeys never evolved prehensile tails. They also have color vision that spans the same spectrum as our own from red through orange, yellow, green, blue, indigo, and violet. Many of the New World monkeys of the Americas, on the other hand, have long, grasping tails and, for most species, an inability to see reds and oranges. These are interesting differences, but in the beginning Isbell focused on dispersal of young monkeys away from their mothers. In the broader story of the history and particulars of life on Earth, monkey dispersal is a narrow topic, but it was enough to intrigue her. Then, through her own primate eyes, she literally and figuratively saw the snake. It was a single observation and yet it primed her the way that, when given gas, a pilot light can feed a much larger flame.

  What struck Isbell was a strange little paper about an even stranger disease. She had been trying to understand the evolutionary history of predators and primates, and so was reaching through publications that related in any possible way to the topic. Libraries are full of such bits and pieces of understanding left there for someone to assemble into a story, to put back together in a way that makes sense. The authors of the paper argued that carnivores and monkeys share a specific kind of virus called an RNA retrovirus (HIV, for context, is also an RNA retrovirus).1 That cats and monkeys would have the same virus suggested one of three possibilities: (1) somebody screwed up in a lab somewhere; (2) a cat had eaten a monkey and in the process contracted the virus; or (3) monkeys have more unusual sexual proclivities than have been acknowledged.

  To Isbell, a cat eating a monkey was the most likely of these scenarios. It was the only scenario that occurred to her at the time. Isbell herself had lost whole groups of primates—sometimes primates she had named—to leopards. She had also written an important paper on the influence of predators on primate behavior and evolution.2 All the shared virus really suggested was that something we know happens now had been happening for a long while. The virus was a vestige of a long ago interaction, a living fossil of the sort found in many animals. Isbell paused with the paper for a moment. She held it up and turned it over the way Indiana Jones might look at a clue to a treasure. Maybe it meant something more. She did not quite know what, not yet.

  That paper led Isbell to another paper, this one even more peculiar. It noted that one kind of RNA virus found in Asian monkeys had its closest relatives in a snake, Russell’s viper (Daboia russellii).3 Russell’s viper has, by some counts, killed more humans in recent history than any other snake. It is both winsome and irritable, as its kind may long have been. The authors of the paper had not bothered to discuss why or how this had happened. Had a venomous snake bitten a monkey and led to the passing of the virus a long time ago? She could not prove it. Yet together these two papers suggested to her a long history of the evolutionary game of venomous snake, cat, and poor besieged monkey. Venomous snake bites monkey. Cat eats monkey. Monkey eats fru
its and nuts. The passing of viruses from primate to predator confirmed for Isbell the story of monkey as prey. It was not yet her bold new theory, but these were the first pieces, pieces that were beginning to seem as though they had something to do with her own experiences with snakes. She had been on that trail to learn about monkeys but she began to wonder whether what she had found was a story about humans and herself.

  Still focused on monkey dispersal, Isbell continued. She decided to study more about snakes and their history and geography, particularly as it related to primates. She called Harry Greene, a Cornell professor and the snake biologist’s snake biologist and asked him about the history of snakes.4 She wondered, as she talked to Greene and read more, whether snakes might somehow explain some of the mysteries of why primates are different in different regions. “What if,” she thought aloud to her husband, “the fact that female monkeys are more likely to leave home in the New World has something to do with the density and history of venomous snakes in the New World?” If monkeys were more likely to meet snakes in the Old World, they might also be less likely to recklessly move long distances. Suddenly, her day job and her wild ideas had begun to intersect. It was a point of excitement. She thought about the idea in the car. She thought about it while walking into her office. She thought about it when talking to students or when sitting at the dinner table with her husband. Like a drug, Isbell’s innocent theory became irresistible.

  What Isbell began to wonder was whether venomous snakes influenced the trajectory of primate evolution, not through temptation but rather through death. She wondered if some of the differences between the monkeys of the New and Old Worlds and around the world more generally were due to differences in the likelihood of being killed by a venomous snake—the persistent effects of the idiosyncratic distribution of life.5 What if not only the sedentary lives of Old World monkeys but also their better vision and even their relatively greater intellect were all responses to the threat posed by snakes? Perhaps the traits one uniquely finds in Old World monkeys and apes were those possessed by individuals, like her, who avoided the waiting cobra, viper, or mamba. Maybe our better vision had evolved to detect snakes in much the way that our immune system’s primary role is to detect pathogens. Perhaps it was to this history that she herself, like all of us the descendant of an African primate, owed her own ability to detect the cobra and other snakes near her. Maybe, just maybe. If so, that was scarcely the end of the implications.