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

by Rob Dunn

  Back in East Africa, no one follows the honeyguide anymore. It has stopped coming to villages. The children who once chased it pursue lollypops instead. We traded partners, so that as much as honeyguides are now more rare than they used to be, sugar beets and sugarcane stems are common, more common even than humans—thousands of stems for every woman and man on Earth. No one chose to ignore the honeyguide; we simply did whatever was necessary to keep our taste buds happy. Because of this tyranny of our taste buds, those few species that could provide us with lots of sugar were favored, and those that provided us with a beehive here and there became more rare.

  In many places in Africa, even where no one collects honey anymore, there are still stories about the honeyguide. They say that if anyone collecting sweets ever failed to reward the honeyguide with a little sugar and wax, the honeyguide would turn on them. It would take the elephant, or maybe the hippo, to beehives, and abandon humans altogether. No elephants have been seen following honeyguides, but moral stories can speak to the nature of a consequence even if the specifics are wrong. We have failed to reward the honeyguide, and we have borne the consequences, albeit from too much rather than too little of all that is sweet.

  Just as we once needed sugar for energy, we long needed salt for reasons of historical contingency. Our circulatory system evolved when we were still fish in the sea, and salt was everywhere. In that context, evolution favored salt and other common compounds for the core switches, levers, pulleys, and other parts of the body. Salt, in particular, was used throughout our bodies. It helped them to regulate blood pressure, which is still one of its main functions in our bodies. Other nutrients might have worked, but in the sea, salt was cheap and easy. Then we left the sea and moved ashore, where salt is scarce. We searched it out, as did other species. Macaws fly to saltlicks, elephants walk to them, and pregnant women can sometimes be found eating fistfuls of salty clay. It was during this time of transition, from oceans to land, that our taste buds for salt became refined and accentuated. The cable that links the taste of salt to our pleasure response was wired deep and strong. It was easy to die without salt. Our brains needed to remind us to search for it.

  In the last several hundred years, our needs for salt changed, just as was the case for sugar. We developed the ability to harvest, store, and even produce salt. Now we are like fish again. We have plenty of salt, but our taste buds are ancient, and so they still beg for the stuff and we give it to them, one chip, bowl of tomato soup, or soda at a time. Our salt taste buds, unlike our sweet or savory taste buds, have limits. We perceive salt concentrations that are too high as bad, but below that concentration we crave more and more. You might blame yourself for your inability to stop eating salty foods, for a lack of self-control perhaps. But the truth is that you are doing what your body evolved to reward you for. Your salty taste buds have no job other than to remind you of the goodness of salt and that you should find more. They beg it of you. Part of the struggle, then, to control the intake of sugar, salt, fat (which the savory taste buds holler for), or anything else, is that while your conscious brain may tell you to avoid it, elsewhere your brain stimulates you to search it out. Ours is a universal struggle not of will power, but between who we are and who we were.4

  Nor is it just salt and sweet. Our other taste buds beg, universally, for fats and proteins, also limited for much of our history. As for bitter and sour flavors, they do the reverse. Bitter flavors signal strong chemicals, and so when we taste them, we have a bad sensation, a sensation that makes us want to spit or vomit. Our bitter and sour responses are actually triggered by many different compounds, compounds that share little in common except for being toxic. Our tongue is amazingly sophisticated. It assesses complexity in the world and simplifies it for us into two possibilities: find more or spit it out. We vomit up bad foods because our body would have it no other way.5 It has saved us, one nasty berry or leaf at a time. So it is that throughout our long history, in a world of toxic and necessary things, we survived.

  Our taste buds are a good point of departure into our preferences more generally, because they evolved for no other reason than to lead us to what we need. Our taste buds, like other aspects of our preferences, are very different from hunger or thirst. Taste buds do not tell us how much sugar or fat we need, or when. They evolved without shutoff valves (other than those related to satiation more generally), and work on the evolutionary “assumption” that we always need both substances. No matter how much you have eaten, when you touch a piece of cookie to your tongue, your brain will ring out “sweet.” Thirst and hunger are different. They tell us when we need water or food (in response, in part, to sensors that measure how much our stomachs are stretched with food). Once our body has enough, or once it thinks it has enough, it stops asking for more. One can even inflate a balloon in the stomach and it simulates the same effect, the same contraction-like feeling of fullness. Not our taste buds; they tell us what we needed a thousand years ago and we oblige. We could be at the point of dying of high blood pressure—collectively we are—and our taste buds would still say, “Salt is good.” They are irrational in their modern context.6

  But what about those other apparently universal preferences and dislikes that I listed, the ones that relate not to tastes but to sights, sounds, or even smells? Some of our perceptions of odors clearly evolved to point us toward or away from factors that influence our well-being. We universally dislike the smell of excrement. It seems plausible that the odor we experience when we smell excrement evolved to keep us away from the stuff (it may not seem as though we need a reminder to stay away from piles of poop, but you overestimate your ancestors). To the extent that dung beetles experience an odor in their head when they smell excrement, it is likely a pleasant one, just as for vultures, dead bodies must smell lovely. Dung and carrion do not intrinsically produce bad odors any more than sugar produces a sweet taste. Such are the vagaries of the senses. Our brains respond differently to different sorts of sounds, and although this effect has been poorly studied, one can imagine contexts in which this would have benefited our ancestors’ fitness. Much of what we experience as universal seems directly related to our survival, if not now, then once upon a time.

  But back to vision. Vision must be different. Whether or not it was shaped by snakes, it is our special sense, our privileged child. Our tongues, noses, and ears respond differently to the various categories of stimuli, but they are marginalized relative to the observed world. Vision responds to intricate scenes in their entirety, whether they are Jackson Pollock paintings or tigers leaping toward us. Our vocabularies to describe touch, taste, and smell are meager. Not so for vision, which we can describe in detail, with shades of color, qualities of light, and hundreds of associated adjectives. It seems implausible that buried in the process of vision are the same kinds of preferences that we find in the way we taste or smell, implausible and yet possible.

  We know certain scenes consistently trigger the same responses in individuals, regardless of their cultures. Snakes trigger us to jump back. They trigger nightmares and fear, except where that fear is counteracted early by culture or education. Scenes with water trigger pleasure, as do open landscapes, grasslands, and forests that look as though they have been tamed, with their understories cleared away as if to make a grove. Could such responses also be the consequence of our evolutionary past? Could it be that some classes of images make us all happy and others all scared, and that these effects are, or at least were, adaptive? Once it was useful to fear snakes. Maybe once it was also useful to move toward grasslands and away from forests’ dark understories. Here we return to Lynne Isbell’s snakes that, like bitter food, once killed us and may have honed our eyes.

  It has been difficult for scientists to study the adaptive aspects of vision. We are wooed by our vision, but we also use our vision as the tool with which we study ourselves, and so just as a dog has trouble biting its own tail, we seem to have difficulty seeing our own eyes. Nonetheless, all of the expl
anations for our sophisticated color vision implicate our ability to detect other living forms, be they fruit or snakes. What if, just as for taste, smell, and maybe even hearing, our visual systems trigger not just our conscious responses, but also subconscious responses to categories of scenes, categories that once saved us and now just influence the world? If our tongue can lead us toward good food, isn’t it possible that our eyes might also lead us toward things that will help us and away from those that will do us harm? We seem to simultaneously imagine that our eyes are the most sophisticated sense, and that they are the least able to recognize specific elements of the world’s goods and bads, stimuli worth running toward or away from. Maybe our eyes trigger preferences too.

  Let’s return to the snakes. It is hard, after all, to turn away from them. Our fear of snakes, or at least our initial wariness around them, seems universal. It is easy to believe that when we all lived outdoors in the tropics of Africa and Asia, where death from venomous snakebites or even constriction by snakes like pythons was relatively common, such wariness would have saved lives. Anyone who was universally unwary of snakes (say, for example, herpetologists) would have been less likely to pass on their genes. What is surprising is that this fear persists, regardless of where and how we now live. We are as predisposed to becoming afraid of snakes in Manhattan as in a rain forest in Cameroon. We are more afraid of snakes, on average, than of cars or guns. Not everyone is afraid of snakes, but surveys tend to suggest that more than 90 percent of us are. This fear develops early and is either innate (we are born with the fear) or easily learned. Monkeys who are shown videos of another monkey responding fearfully to a snake will be afraid of snakes for the rest of their lives. Monkeys shown videos with a rabbit in the place of the snake never become afraid of rabbits. Snakes seem to occupy a unique category in primate brains, unlike other threats, and even unlike other threatening creatures. Monkeys do not seem, for example, to so readily learn fear of cats. Nor is it just the monkeys. Infants too young to talk appear to focus on videos of snakes (instead of a video of another animal) innately (which is to say, without learning) when an adult near them is talking in a fearful voice. But when the adult talks normally, the infants give equal attention to snakes and, say, hippos. It is as though our brains have a rule along the lines of “If you don’t have anything to fear, don’t fear anything, but if you do have something to fear, fear snakes.”7 And so we do.

  One explanation for how some scenes trigger negative responses (in the way that bitter foods also do) relates back to the amygdala, that element of the brain that tugs the strings in our bodies when we are chased and when we fight. Hundreds of biologists spend their lives scaring rats to study their fear, amygdalae, and vision. They can tell you that if you show a rat a scary picture (whether it is of a cat or a biologist), it responds, even if its conscious brain is focused elsewhere. Scary images excite rats’ amygdalae, but not their frontal lobes (the part of the brain greatly exaggerated in you and other humans and associated with our cleverness). More to the point, when the amygdalae is removed from monkeys, they lose their fear of snakes. For a while, it was not clear whether fear was also being conveyed subconsciously like this in humans.

  As they tend to do, brain biologists searched long and hard for individuals, both rat and human, with problems. They wanted to find individuals with “blindsight” who could still see but had no conscious knowledge that they were doing so.8 Blindsighted individuals are not aware of seeing, just as our guts taste our food without our being aware they are doing so. Individuals with blindsight are typically surprised to realize that they are aware of the locations of objects. In a recent study, a man with blindsight walked down a corridor, zigzagging around objects he did not know were there. (Some individuals also have emotional blindsight and can respond to fearful faces by cringing, even if they are completely unaware that they are seeing anything at all.) That blindsight can occur means that we all experience both conscious and subconscious responses to what we see, just like the rats. This in turn leads to questions about what those subconsciously encountered visions are doing. Further questions arise, such as how and whether blindsight might, like our guts’ taste buds, subconsciously register distinct categories of scenes, whether they be specific, such as snakes, or just more general varieties of fear.

  Arne Öhman (a brain biologist who happens to be terrified of snakes despite living in snakeless Sweden) and his colleagues have developed a test that mimics the effects of being blindsighted. To do this, they present participants with a picture of a face. Sometimes the face is accompanied by a loud and distracting sound; in other cases, it is not. When the face is accompanied by a loud enough sound and a quick enough image, Öhman can produce situations in which although the image is seen, no image reaches the participant’s conscious brain. When asked, the participant says that he or she did not see a face. In addition, the part of the brain that would be expected to light up if a face were seen (in MRIs being done simultaneously) does not. What lights up instead is a separate part of the brain, a part of the brain that suggests that at least some signals pass directly to the amygdala. They are signals no one had noticed until Öhman’s work and about which we are not consciously aware, ever.

  The chain of connections that researchers have identified are old wires that are more prominent in rats that in humans, but present in all mammals. It is the ancient wiring of fear, aggression, and urges. Some visual stimuli and scenes directly trigger this old wiring, and the body responds to them without the signal’s ever becoming conscious. When Öhman shows subjects snakes or scary faces, the signals travel to their amygdala and trigger a general bodily response to fear. This occurs even when the participants’ conscious brains were unaware that the participant had seen a snake. They could not reason about the snake, because reason was not even involved.

  Just how the old wiring in our brains works is far from well- known, but that it is there, making us jump, shiver, run, or strike something is unambiguous. It does not seem a stretch to implicate this old wiring in some of our preferences (or fears) of images of some animals over others and so too in our views of what is ugly or beautiful, peaceful or terrifying. In rats, there exist cells associated with the old wiring that help to mark how close an individual is to an object. Separate cells, called place cells, keep track of when an animal passes a landmark or the way the face is pointing. How far-fetched is it to think that these cells might also register the signal recorded when the eyes follow the sinewy shape of a snake, that narrow fellow in the grass? How much of a stretch is it to imagine that these subconscious parts of the brain might record even more nuanced aspects of the world, aspects that produce bad or good feelings, anger or joy?

  What we know for now is that we all seem to be born with an easy ability to learn or have triggered a wariness of snakes, and an ability, once we become afraid, to develop a heightened wariness, a real fear. Most of us are also born preferring relatively open landscapes over forested ones. A scene with a tree that has branches for climbing is judged by most of us as more beautiful than one that is long and straight. These preferences, like fear of snakes, can be modulated by learning, made stronger or weaker by experience and reason, but they seem to begin deep and innate. There are others too, such as universal preferences for water, or shimmering blue colors. Just how all of this works, which scenes we truly prefer, how they are learned and not learned, and how our body responds to them is fascinating, a primitive gem as fundamental to who we are as our taste buds, an undocumented kingdom just beginning to be explored.

  Even as the cogs and gears of our universal preferences remain enigmatic, their consequences are clear. By leading us to our choices, they have shaped the living world and removed us from the nature in which we evolved. It began when we turned from prey to predator, from fear to some more complicated mix of fear and aggression. Just as for the crab, our influences ramified out from our tools and senses. Once we had weapons, the first species that we influenced were those we could
see and catch. We sought them because our eyes and ears detected them and because when we did catch them, their fat rewarded our tongues. Survival in the face of our weapons meant and means hiding from our vision or breeding very fast. We chased the big and obvious animals. They may have tried to escape by taking advantage of our weaknesses (as in Vermeij’s law), but with spears and society, our weaknesses would prove fewer each year. As we looked out across the landscape, we also began to burn. We lit dry grass, dry leaves, trees, and anything else that would ignite. We turned forests to grass over which we could newly see into the distance. With agriculture, we again chose species that grew in the open and planted grasslands of millet, wheat, and corn. The corn was short, the millet too. Where we could not plant crops, we raised cattle that beat down the tall grass, making even more acres of Earth open and, to most of our sensibilities, beautiful. In some places, we made and make subtle distinctions, for example between truly dangerous and mostly or entirely innocuous snakes. In other places, such as Texas, we killed them without such discernment, as we still do each year in rattlesnake roundups. Each of these changes made the world more like our preferred landscape, the one that brings us pleasure whether we are conscious of it or not, whether it is good for us or the future of life or not.