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

by Rob Dunn

  In the biblical story of the Tower of Babel, the people of Babylon come together to erect a great building up to the sky. This tower will be their glory, their great and ambitious accomplishment. Brick by brick they raise it, out of sweat-splashed mud. They raise it using their hands, but their common language also helps them to coordinate efforts—to holler “Over here, we need a brick” and to move the tower up, layer by layer. Their language is as necessary to their endeavor as the pheromones of termites and ants are to their success or the dancing of bees to theirs. Language holds them like a thread, but not all that begins ambitiously ends well. God punishes these people for their arrogance by dividing them. He forces them to speak hundreds of languages, an act that pulls them apart. The moral typically taken from this story is one about the consequences of ambition. But there is a second moral too, implicit in the method chosen to divide these peoples—that the failure to communicate leads to failure. Something analogous has happened in science at an increasing rather than decreasing rate.16 With it, the layers of bricks have grown more difficult to lay. Previous layers exist, of course, on which to stack the mud-baked ideas, but what do they really stand on? More important, where is this tower going? Such answers have grown more difficult to see.

  In looking at science from the outside, one might hope that as our total knowledge increases, we gain a broader and more complete understanding of how the world works. Collectively, we may. Libraries grow. But for individuals, it has become more difficult to have a broad perspective. The scientists of each field have developed more and more specific words and concepts for their quarry. It is now difficult for a neurobiologist to understand a nephrologist and vice versa, but it is even difficult for different neurobiologists to understand each other. The average individual’s ability to understand other scientific realms has become limited. To do so, they must be scientifically multilingual. Biological polyglots are most rare in the study of humans, where territories are divided finely. One might spend a lifetime studying one kind of human heart cell or some attribute of mucous.17 The more divided into tiny parts a field is, the less likely some types of big discoveries become. Mechanistic discoveries still happen as scientists struggle, for example, to understand each tiny part of the ear and how such parts come together to make sound, but very few individuals are standing far enough back from what they are looking at to be able to make big conceptual breakthroughs. Instead, such breakthroughs often come first from scientists studying other, more obscure realms of life, realms in which they are still, at least relatively speaking, their own kings and can stand at a distance between bouts of looking deeply. Ecologists and evolutionary biologists are among those more obscure tribes who still step back a little farther (though less far than they once did). From that slightly greater distance, they can sometimes see what was otherwise missed, lost, as it were, in translation. To really see what is going on, one needs to step back far enough to see parallels, the reverberating similarities between one field or organism and another. I would suggest that an ideal distance is far enough away (figuratively) to see both termite bodies and human bodies, but also the big sweep of the ecological world. From such a distance, it is hard to avoid looking at ants.

  Ants are, like the other ethers, everywhere. Perhaps the classic example of interactions between one species (such as humans) and another species that lives on it (such as our microbes) is that of the relationship between ants and acacia plants. Acacia plants provide ants housing and tiny pearlike fruits in exchange for the ants’ protection of their leaves. Plants with ants grow healthier and faster than those without, because in rewarding the ants, the plants gain a defense against another even more costly group of insects, herbivores. In the story of this relationship is an obvious parallel with the story of our bodies and our microbes. But one can also find closer parallels—one need only look to those ants that farm.

  Farming ants are more like us than any other species. Farming, or as they are better known, leaf-cutter ant colonies, are colossal societies. They are composed of many thousands, sometimes millions, of sterile individuals, all doing their queen’s bidding. Just as in any society, the individuals are imperfect. Some make wrong decisions. Some are eaten. Some carry back toxic leaves. Some persistently walk in the wrong direction. But on average, they get the job done. The job is carrying bits of mandible-cut leaves back to the nest where they are spread as fertilizer on gardens of fungus. The fungus produces sugar-rich bodies—one might call them fruits—which the ants feed to their larvae. For the ants, the fungus serves as an external gut, digesting the leaves in a way the ants on their own cannot. Different leaf-cutter ant species (and there are many) farm different fungi. The ants and the fungi need each other. The tricks the ants have evolved to take advantage of the fungi are many and elaborate. It is not at all easy to farm fungi, and yet the ants seem rarely to fail. Nor is it easy, from the fungus’s perspective, to feed ants. Yet the garden grows. The colony grows. The skin around the queen’s abdomen stretches ever thinner as she fills with the eggs of prosperity.

  Leaf-cutter ant colonies, the culmination of fungus-gardening sophistication, are full of circuslike particulars. Minims, the tiniest leaf-cutters, ride leaves, guarding the ant carrying the leaf they are on from flies bent on laying eggs in their heads. Soldiers, with heads bulbous with muscle at the expense of brain, guard trails. Leaves are cut using a near-perfect, saber-saw-like vibration of the mandibles. And, of course, there is the fat lady, the queen, who, somewhere in a deep chamber, lays thousands of eggs a day, each one as particular and detailed as though it had been produced by Fabergé. Many thousands of tropical biologists have sat with and watched this circuslike civilization proceed down paths. Few have failed to remark at how these cities of ants resemble human cities. It is an almost inevitable comparison, though the ants together also seem to resemble a body. Each individual might be compared to a cell moving around food, shuffling out toxins, doing their unrewarded part to keep the whole alive.

  Leaf-cutter ants are remarkable, as are their fungi. Together, they exemplify the extent to which one species can come to depend on another. But biologists studying the human gut did not know about these ants, at least not any more than you might know about them from seeing them on a Discovery Channel special, in and out of focus or in comparison to some human’s pinky finger. Also, until recently, the story of the leaf-cutter ants was still missing key elements. It was still unclear how the ants’ simple immune systems prevented the fungus, their external gut, from being attacked by disease. (It is a question, you may notice, analogous to that of how our gut prevents itself from being invaded by bacteria that attack our internal guts.) A garden untended tends to be devoured, particularly in the tropics, but these fungi grow relatively pure and, although delicate, untouched. Nor was it clear how the ants themselves, surrounded by fungus, kept from being infected by some pathogen.

  In nature, when things go uneaten, there is a reason. They taste awful, have toxins, or are otherwise defended. But what holds back the demons of the ants’ gardens and, for that matter, on their bodies, bodies that rub daily among microbes? The answer, it was recently suggested, is “good” bacteria. Cameron Currie, a biologist who is now at the University of Wisconsin, found bacteria living in special divots and rough spots on ants’ bodies. The bacteria seem to be more abundant when pathogens are more prevalent in the ants’ colonies. Currie has argued that these bacteria are helping the ants to defend themselves from “bad” microbes on their good fungus. It has long been known that bacteria produce antibiotics (most of our antibiotics such as penicillin come, at least originally, from them). The bacteria on leaf-cutter ants may produce antibiotics that repel the bad fungi (called Escovopsis) that attack the ants’ good fungi. The bacteria, in this theory, are the ants’ defenders and partners, farmed by the ants on their bodies, worn like skin on the bone. The ants appear to sustain the bacteria and even to have evolved traits and maybe rewards that keep them from sliding off. An alternative explanation
for the bacteria is that they actually defend the ants rather than the fungus. Either idea remains possible. In the meantime, the idea that our bodies might farm good microbes, for our defense, came first from the ants. And because ants are easier to study than humans, the intricacies of the ant relationship (though already contentious) are likely to be resolved more quickly than those of our own. Whether or not he is right about just what is going on, Cameron Currie stood far enough back to notice something interesting, something that applies to ants, but also, as it turns out, to you.

  We tend to think of ourselves as complex, or at the very least complicated. In the old telling, we were at the top of the great chain of life. Yet at the same time, we seem to have difficulty imagining that our relationships with other species are as sophisticated as those of, for example, the ants. But our interactions are elaborate too. The leaf-cutter ants were just, until recently, better studied and with more perspective, from a greater distance. We are more like a leaf-cutter ant colony than anyone had imagined, in terms of how we tend our microbial gardens. Our appendices, when they are not bursting, are key to doing just that job. Even as our brains try to tell us that the bacteria in our guts or on our skin are all bad, the appendix mumbles otherwise. In some primitive, wordless way, it knows.

  6

  I Need My Appendix (and So Do My Bacteria)

  On September 11, 1942, Dean Rector of Chautauqua, Kansas, turned nineteen. The celebration was held more than a hundred feet beneath the surface of the ocean. Above Rector were millions of pounds of water and, more ominously, Japanese destroyers searching for American submarines like the one he was in. The submarine was a chamber, meant to hold back both torpedoes and water. In it, Rector was about to have his first birthday at sea.

  Rector’s celebration was short-lived. The next morning he woke up thinking he was about to die. Many dangers surrounded Dean Rector and his shipmates, but on this day it was the internal demons that struck first. As the pain grew worse, he began to whimper. It was a sound more like a dog than a man. One sailor thought what was afflicting Rector was “just the flu.” “Maybe homesickness,” offered another, but as Rector moaned, the reality became inescapable. He had appendicitis.

  Under ordinary circumstances, appendicitis can be dangerous, but for Rector the circumstances were far from ordinary. No trained surgeons were on board, and finding a surgeon so far from home and surrounded by the Japanese was out of the question. They would have to operate, but how? Who would perform the surgery? Officially, Wheeler B. Lipes was the ship’s surgeon, but in title only. His medical experience consisted of having run an electrocardiogram machine. When Lipes was asked by his commanding officer to do the surgery, he declined. So his commanding officer ordered him to do the surgery. Among Lipes’s reasons for hesitation (other than his total lack of experience) were that he did not know how long the ether would last, he did not know where, in a human body, to find the appendix, and he could not imagine, short of kitchen utensils, what to use for surgical tools. Yet Lipes had been ordered and so began his preparations.

  After some soul- and equipment-searching, Lipes eventually readied himself to remove Rector’s appendix. Rector had been placed on his back on a table in the officer’s wardroom. The table “was just long enough so that the [Rector’s] head and feet reached the two ends without hanging over.” Lipes stood over his patient, still flipping nervously through a medical book (looking, one presumes, for a drawing that would indicate the geography of the offending organ). He was wearing a tea strainer as a surgical mask. The men chosen to assist him had been given kitchen spoons to use as muscle retractors. They stood ready on either side of the patient. Then, as would later be described in a Chicago Daily News article, Lipes leaned in toward his patient and mouthed, “Look, Dean, I never did anything like this before.” Dean’s eyes were wide. He watched as Lipes, following “the ancient hand rule, put his little finger on [the] subsiding umbilicus, his thumb on the point of the hipbone, and, by dropping his index finger straight down, found the point where he intended to cut.”

  The appendix is the most frequently removed body part. Often, as Dean Rector’s situation makes clear, such removals happen under dire circumstances. Go to the office and watch the people moving around you. Few will be missing eyes. None will be missing hearts, but quite a number will lack an appendix. These appendixless many walk by largely unworthy of remark, bearing neither stigma nor obvious consequence. Maybe you are one of them. Whether or not you are, it is fair to wonder: If an appendix causes so much trouble and is, from all appearances, less necessary than a pair of pants (whose absence from a coworker would be noticed), why do we have one in the first place? The answer, it would turn out, has to do with our gut microbes and our evolutionary context. The appendix makes sense only in light of our evolutionary past, though no one aboard the submarine had time to care. They were still looking down at Dean Rector, whose mouth was cracked open in a low moan.

  Lipes, having paused to gather his wits, continued to cut.

  The appendix is a dangly bit of flesh that hangs from the lower intestine. It is the size of a pinky finger, so while diminutive relative to other organs, it is large enough to deserve explanation. Yet the question of what the appendix does has long had no pithy answer. The heart pumps blood. The kidneys clean the blood and help to maintain blood pressure. The lungs distribute oxygen and remove carbon dioxide. The appendix, well, it hangs. Various talents have been attributed to this small organ over the 300 years since one was first removed from a living human, some magical, but most rather ordinary. It might be part of the immune system. It might have some neurological role. Maybe it is related to hormone secretion or muscle function. The dominant view, though, has long been that it does nothing at all. It is a vestige, like men’s nipples or the hind leg bones of whales, a prominent and unnecessary relic of history.1 That answer is wrong, but until very recently no one knew.

  The story of our attempts to understand the appendix begins long before Lipes, but with more speculation than gusto. The primary evidence for the mainstream idea that the appendix is vestigial was that nothing happens when you remove it. That was the sum total of the logic. Surgeons (or, in the case of Lipes, electrocardiogram technicians) have cut out millions of appendices. They have watched the consequences the way you might watch the result of removing an unwanted beam from your house. When the house doesn’t collapse, one gains a sense of calmed relief (betrayed only by a mild nervousness when the wind blows). Yet the wind blew and those individuals who had their appendices removed seemed to do no worse and die no younger. Their houses were sound. Had the appendix done something necessary, the appendixless, at least some of them, would have gotten sick.* But just like the guinea pigs in the germ-free chambers, they did not seem to and so it seemed clear that our appendix was a relic of the way our bodies used to work when we were monkeys or perhaps earlier still when we lived, rat-like, among the dinosaurs. Perhaps in our ancestors the appendix did something vital; now it just hangs in our bodies like a clapper in a bell, and every so often, as in the case of Dean Rector, rings out, loud and clear—“I am here. Take me out. Now!”

  Yet there have always been problems with the hypothesis that the appendix is simply an antiquated, useless, vestigial organ. For one, the appendix kills. Were infected appendices not removed, about half of infected individuals would die, old and young alike. Since one in sixteen or so people suffer acute appendicitis,* were their appendices not removed, then one in thirty-two people would die of appendicitis. If, historically, one in thirty people died of appendicitis, and the presence, size, shape, etc., of the appendix has a genetic basis (which it seems to), then it would not take many generations for genes for big appendices or even the presence of an appendix to disappear.2 All things being equal, those of our genes and traits that tend to kill us, or even just weaken us, do not do well in the gene pool. Fish that make the evolutionary move to cave life very quickly lose their eyes, because having eyes is both useless and costly.3 I
f having an appendix is both useless and costly, then it, like cave fish eyes, should disappear. Cave fish can lose not only their eyes but also the circuitry of their eyes within relatively few generations of going underground. The regions of the brain corresponding to vision dwindle. Not so for the appendix, which—millions of deaths to its credit—remains.

  The big problem with the hypothesis, though, is monkeys. If our appendix really were vestigial and useless, we would be able to look at our relatives to infer what its use once was. What did an appendix do in our ancestors when it still had some function, and what does it still do in our close relatives? If our appendix is a vestige of our history, then we might expect monkeys to have more developed and obviously useful appendices than we do. Chimpanzees should have smaller appendices than do most monkeys, since they are our closer kin and their lifestyles (and the usefulness of the appendix to those lifestyles) are more similar to ours. Cave fish may lose their eyes because they are neither useful nor cheap, but one can look to the relatives of cave fish to see what use their eyes had once served. In the same way, we ought to be able to look at our relatives to see what our appendix once did.