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

by Rob Dunn

  Grasses and cows were not the only species we favored. We also came to choose species that were beautiful to our senses, be they sweet-sounding birds or brightly colored goldfish. That we see such species as beautiful begs the question of whether beauty itself is like sweetness, an adaptive sensation meant to help us survive. No one knows, not yet. In the meantime, tulips and other flowers are shipped around the world at huge expense. Goldfish live in houses in nearly every country. Dogs, which appeal to our social sense of appeasement and connectedness, were brought into our beds. (Cats—well, no one can explain them.) The trains of modernity grind down the tracks, carrying what we call for, one load at a time.

  Beyond the species we have consciously favored (for subconscious reasons) is another group, the group that has perhaps most directly risen to our senses, our pests and guests. They sneak around when we are sleeping or through corners and cracks we ignore. Rats hang close to walls, because it is there that they go unseen. Pigeons and other urban birds nest under ledges, where we will not find them. Nocturnal insects run around our houses. Species such as dust mites and bedbugs, just small enough to avoid being seen, crawl on and off our bodies with impunity. Tiny creatures, bacteria and archaea, flourish. We swipe at them, but they grow on, changed and yet persistent.

  Our senses, coupled with our power, changed the world so quickly and universally that it is easy to forget what the world used to be like. Today, roughly 60 percent of the earth’s surface is managed by humans for production, and most of that land is devoted to one or another kind of grass. Nearly all humans on Earth live by water. (Think of your favorite beach resort, but then also, for example, Manhattan and Los Angeles.) Many of us tend to live by water because we need it, but also because we tend to prefer it. It pulls at us like gravity and makes us feel good. Once upon a time, though, before modern humans, there were more forests and larger animals. Rats were rare, as were mice and roaches. Even grasslands were not nearly so common, and the flowering plants that have arisen around us had not yet called to our senses. In many places, the coastlines along which we now so easily walk were hidden beyond dunes tens of feet high, dunes that while useful to us in protecting our shores, obscured our views. The views won, and so in general the dunes are gone, reduced to a minor row of hills that does little but allow us to see even more of what our eyes and brains demand.

  Not all is left to the fates and taste buds. In small pockets, reason has won out over our urges. We have instituted conservation agencies and plans, public health systems and public sanitation, each of which required us in some way to choose what was reasonable over what was simply appealing. There are real successes, the triumphs not of intuition or our “gut sense” but of the nonintuitive reason that accumulates across peoples and generations. Other times, when we try to figure it out on our own, reason loses out, trapped in a mental world constructed by senses that evolved to detect snakes and fruits, not to puzzle out global crises. Collectively, we have repeatedly tended to make the same decision, regardless of our cultures and differences. We have universal senses and preferences and so we often make universal choices. Aborigines burned down forests in Australia. Amazonians burned Amazonian forests. Others burned in Europe and North America. People burned because they could but also because the result was preferred, everywhere. In some places, this preference for open habitats has been carried to extreme lengths. In the United States, more land is now planted in lawns—perhaps the most perfect manifestation of our minds’ preference for simple, unadulterated openness—than in corn. The sugar remains sweet, the salt tempting, and the open scenes of oceans or grass beautiful in a way that cannot quite be expressed.

  It all might have been otherwise. It would have been were our senses different, if we were, for example, blind like termites. Termites feel their ways along dark tunnels, smelling and groping through the world, as do moles, mole rats, and other denizens of the underground world of holes and chambers. In their world, light is largely irrelevant. In some subterranean species, the eyes of their ancestors became disconnected from their brains until they disappeared altogether. Once the eyes are gone, the colors once meant for them become unnecessary too. To a blind queen, a flashy king was no more attractive than a drab one. Because all colors are costly, termites have lost their hues. They have turned to ghostly versions of their former selves—the color of an onion’s skin. In this alternative world, smells and textures come to rule. Species of beetles, mites, and even fungi sneak into termite nests. They hide there, in plain view. They do not look like termites, but they feel like termites. They smell like them too. As for their foods, the termites use taste and smell to choose those foods that are rotting, which to them must smell some sort of sweet. We are like them in constructing a world catered to our senses, just different in which senses those are.

  In the end, we have often been like the termite or the ant, at the mercy of our urging senses. But reason can prevail, so long as we do not “trust our bodies.” Our bodies and particularly our senses lie. They are stuck on the front porch rocking back and forth and remembering the old days. And so, when you touch your food to your tongue, savor the pleasure of the good tastes. Culture, of course, can influence how we respond to different tastes, just like different scenes. We can learn to love snakes, just as we have learned to love the stimulus provided by coffee even though it is bitter. Our aversion to snakes and our attraction to sugars, salts, and fats are the murmurings of our history, but such murmurings can be quieted. Our universal fears and ambitions have been our fate, but they need not be. Whatever the right action might be, the honeyguide will not lead us there, nor will our taste buds, which will continue to call out for more, just as our fears cry out for us to fight or flee.

  Part VI

  The Pathogens That Left Us Hairless and Xenophobic

  13

  How Lice and Ticks (and Their Pathogens) Made Us Naked and Gave Us Skin Cancer

  Removing our worms left our immune systems floundering. Changing our mutualist partners left us with too much of the wrong food, and killing our predators left us with ghosts in our brains and nervous systems—ghosts that make us leap with fear and panic with anxiety. Yet it is the changes in our infectious diseases that are potentially most sweeping in their consequences, which is what brings us to the story of ticks and hair.

  Very recently our ancestors were covered in hair (or really fur, hair simply being the word we gave our own fur so as to feel a little special). In the contrast between us and them, our lack of hair is conspicuous. We now recognize the central links between Neanderthals and modern humans. Yet when the Neanderthals in museums are covered in hair, they look nonhuman, more “animal” than cousin.* Our transition, from hairy to smooth bodied and the complicated way we look at our ancestors in the hairy dioramas of our museums, begs the question of just what happened. How is it that we became essentially hairless and that, in the process, many (though not all) cultures came to view hairiness as unattractive? Ninety percent of all American women shave themselves in order to appear more “beautiful.” Nor is this the extent of our eagerness for glabrous bodies. It is one thing to shave a chin, a leg, or an armpit, but we have come to love hairlessness so much that all over the world, people are having their pubic hair ripped off with hot wax.

  It may now seem “normal” that we are hairless. In the context of our ancestry, it is not. We do not actually know whether Neanderthals were hairy. They may not have been, which makes the way in which we stare at them even more interesting. But our ancestors in Africa certainly were, as recently as a million years ago, as were the very first mammals and all of the species in between. Thick hair was part of what made mammals successful. They stayed warm when all else around them had cooled. Early reptiles may have stomped and roared, but when the sun set, so did their body temperatures. Not so mammals, who relied on their fur, along with a more sophisticated heart, to maintain a constant temperature. Fur was an evolutionary breakthrough, a warm hug on a cold day that allowed mammals to
live in conditions too hostile then (and now) for any reptiles other than birds.

  Our hairlessness has become a source of what we think of as beauty, a reality validated in every National Enquirer article about a “wolf boy.” It also has widespread consequences for our health and quality of life. It is the reason for the origin of melanin (the compound that, when present, makes dark skin) in sunny regions. The production of melanin in cells just under the surface of the skin evolved in Africa, along with our loss of hair. All of our ancestors produced melanin and so were dark skinned, but when some of our ancestors moved out of hot climates, melanin blocked too much sun. At least a little sun on the skin is necessary for our bodies to produce vitamin D. Dark-skinned individuals in sunless places suffered rickets. They died, and so, with time, pale-skinned genes were favored, not just once but several times independently, with the northward migrations of humans. In other words, the variety in our skin color would not exist were our skin not exposed in the first place by our lack of hair.

  But why did we lose our covering? Like so many of our modern dilemmas, it may be because of the species we once interacted with, and how their abundance has waxed and waned through time. Blame it on the ectoparasites—lice, ticks, and flies. In the caves of our origins, they climbed through our hair and bit us, and when they did, they transmitted, into our blood, disease.

  Of the 4,500 or so mammal species on Earth today, nearly all of them are furry. Just a small handful of all living mammal species are essentially hairless. I say essentially because even our bodies are not totally naked. You and I are both covered in tiny hairs that bristle impotently when we get the chills, but never keep us warm. Dolphins and whales are smooth skinned. Their sparse hair is related to swimming; hairless sea creatures are more aerodynamic (though this is not the only way to a smooth ride—seals and sea lions are able to achieve the same effect by having very dense fur). The tendency for some marine animals to be relatively hairless suggested to a few biologists in the 1960s that the earliest humans were swimming apes. Perhaps somewhere between monkey and man, we were mermaids. Maybe we began to live along riverbanks and seashores and then started hunting for seafood when other hominids were beating us up and we could not find anything else to eat. We might have eaten shellfish and sea urchins and then, on one long day, taken the plunge all the way into our naked future. One imagines the scene like something out of The Blue Lagoon. If we were smooth skinned, perhaps we were able to swim faster and farther to that last urchin, and in doing so, survive.

  This theory, though long prominent, does not enjoy much support. It does, however, highlight the extent to which having no hair is an unusual circumstance. Pause and think of other “naked” species, and you come up with the marine mammals and the naked mole rat. Then what else? Very few other species. Rhinos, elephants, and hippos are low on hair, but make up for it by having—like dolphins and whales—thick, insulating skin. Once hair evolved in the first mammals a 120 million years ago, it was very rarely lost.

  So, why then did our species become one of the very few mammals to lose most of its hair if it did not involve better swimming? Perhaps hairlessness helped us keep cool and hydrated in the savannas as we ran around on two legs chasing down prey (or fleeing predators). This hypothesis seems plausible, except that evidence suggests that hairlessness may actually make us more rather than less susceptible to dehydration. Also, none of the other primates that moved into open savannas (or into the dry tops of canopy trees) has become less hairy. Nor have other predators, such as cheetahs, that chase their prey by running. Maybe hairlessness is like a peacock’s tail or a mandrill’s pink butt, useless and extravagant, but lovely and therefore chosen. One can imagine that men have tended to choose less hairy women (or vice versa) because such hairlessness is a demonstration of their good genes, genes so wonderful that their bearer need not worry about sunburn or the discomfort of sitting bare-bottomed on a log. That is what Darwin thought. His own wife had a very smooth face, though one hesitates to draw general lessons about mate preference from a man who married his cousin. But the truth is that one does not have to be terribly fit to be hairless. Instead, the early stages of hairlessness (a little here, a little there) seem likely to have suggested a bad case of mange more than they evinced good health.

  My own favorite theory has been independently suggested by three waves of scientists across more than a century. Each of them has argued that hairlessness evolved because our ancestors were unusually tick, louse, fly, and, more generally, parasite-ridden. This idea was first suggested in the 1800s by the jack-of-all-trades Thomas Belt in his book The Naturalist in Nicaragua. Belt had done his time in the tropics, where his few hairy parts were invaded by ticks, lice, and other life-forms. His own repeated infestations astounded him. “No one,” he wrote, “who has not lived and moved about amongst the bush of the tropics can appreciate what a torment the various . . . parasitical species are.” But, imagine, he said, how bad it would be were his whole body covered with hair, and consequently, with ticks, mites, and their kin. He reasoned what a century of subsequent biology has confirmed as a rule, that the more habitat was available, the more individuals would be found. The habitat in question was the hair on his body, and at that moment, he wished there were less of it. Discomfort, if not the mother of theory, is a reasonable stand-in. This parasite theory was suggested again in 1999 by Markus Rantala, a biologist at the University of Turku in Finland, who (like me) spends much of his time studying ants. Rantala suggested an idea nearly identical, albeit more formally developed, to that of Belt. The idea was then suggested yet again by Mark Pagel and a colleague in 2004, who had discovered Belt’s work but were unaware of Rantala’s paper.

  I have never had fleas or lice myself, but I do have a story about genital crabs, a lousy story. Genital crabs (Pthirus pubis) are a kind of louse. Like other lice, they spend their lives on bodies. They do not do well anywhere else, and in fact, their survival depends upon staying on their host. They are larger than body or head lice and look, give or take, like miniature versions of Ganesh, the many-armed Hindu elephant god. Unlike Ganesh, they are thin and delicate, so much so that they dry out and die when away from a host for even a few minutes. Genital crabs cement their eggs onto their host’s hair, feed on their host, and otherwise go nowhere else, except in those moments of intimacy when two hosts are close enough for the lice to jump from one host to another. Our genital lice are most closely related to those of a gorilla’s, suggesting that our ancestors and the ancestors of a gorilla once “interacted.” It may seem that depending on a host to touch another host is an unlikely way to survive, but it turns out that touching each other is one of the things we most predictably do. Genital lice have traveled the world at our expense. Like head lice, they rode on the first peoples to migrate into the New World. Head lice and genital lice have both been recovered from Peruvian mummies. Such is the indelicateness of death.

  The first genital louse I saw was among a series in the insect collection at the University of Connecticut. It was alongside another specimen reported to have been collected from a toilet seat recently used by Dennis Leston, a famous, or perhaps infamous, ant biologist who died only recently. Leston was well-known for studying the ways in which ants in the canopies of trees can help (or hinder) pest control in orchards as a function of their identity. Some ant species eat thousands of pests; others farm those same pests for their honeydew and in the process actually increase their numbers. Leston, though, was also known for other things, so much so that in Ghana, where he long studied the role of ants in coffee plantations, locals wrote a popular song about him, something along the lines of “This white dude is not so cool.” It was this lack of coolness that resulted in Leston’s eventual expulsion from the University of Connecticut and also, one suspects, the louse in question. The point of the story, though, is that in order for Leston to rid himself of the genital lice he had “collected,” he would have needed to shave—and let me use a euphemism here—his “louse hot
el.” In fact, shaving oneself is the most effective treatment for nearly every external parasite that afflicts humans, be it flea, louse, or something more exotic. In one study, doctors found that the rising commonness of “bikini waxes” was associated with a decline in genital lice, even as cases of gonorrhea and chlamydia are on the rise.1

  Ectoparasites (“ecto” means on our bodies, as opposed to endoparasites such as intestinal worms) have an innate fondness for living on, in, and among hair. It is what makes outbreaks of head lice so common among children, and so difficult to stop. The nits cling to hair for life, as do their parents, the actual lice. Lice have special hands that are curled in the shape of the hair around which they grip. The size of their grippers is matched to the width of the hair among which they live. Head lice (and body lice, to which we will return) have narrow grippers, for narrow hair. Genital lice have wider grippers, one of the reasons that one of the few other places you can get genital lice is on your eyelashes, which are also a little thicker than head hair.

  Given the intimate links between our parasites and our hair, an association between the amount of hair or fur that we have and the number of ectoparasites we feed seems reasonable. Yet this parasite theory for our nakedness still needs a bit of explanation. For as much as many of us show disdain for the fuzzy muzzle of our ancestors, the loss of our hair also meant the loss of its many advantages. Hairlessness makes humans more vulnerable to UV radiation. It makes it harder for us to stay warm without using clothing.2 It also makes us look smaller, the way a naked mole rat looks small but a husky dog never does unless it happens to be shaved.

  Genetically, it can require very few changes to cause an animal to have no hair, perhaps no more than a single change in a single gene. In general, losing a trait is easy, which is one of the reasons that we have now produced a variety of “naked” domestic animals, including dogs and cats, but also, even, chickens. That natural selection has produced very few bald mammals (and not a single species of featherless bird) suggests that being covered is nearly always useful. Canopy mammals have fur. Nearly all underground mammals have fur. Even most swimming mammals have fur. Fur is good and wonderful. What is required for its loss is a set of conditions under which fur becomes costly, whether because the hairless mate with greater success, or the hairy, bless their hearts, are far more likely to die.