by Rob Dunn
According to Darwin’s rules, evolution does not overdesign. Natural selection is scrupulous in its editing. No material is wasted and no animal is taller, faster, or stronger than it needs to be in order to do better than its competitors. Were all the animals on Earth tortoises, there would be no advantage to being a hare, just the fastest tortoise. Yet the pronghorn, in their sinuous groups, outrun everything. In the thousands of hours the Byerses, other researchers, hunters, and locals have observed pronghorn, there are few recorded instances of adults being caught by predators. This is true even though many adult pronghorn have been dressed in radio collars and followed out across the plains and even though predation on fawns can be easily observed.10 The fawns are eaten by eagles, coyotes, and other predators. But the fawns do not run in defense. They freeze. The adults are the ones that run and when they do, bears do not come close to catching them, nor do gray wolves or even coyotes. When the Byerses first saw the pronghorn’s speed, it seemed like an affront to natural selection, a kind of gaudy exception flaunted at each opportunity.
John Byers was thinking about this exceptional speed when he started seeing phantoms. He saw animals chasing the pronghorn, sprinting after them. They caught them by their ankles. They took them down, one by one in among the lolling seed heads of the tallest grasses. They were not real, he knew that, but he could see their evidence, the way one might notice wind by seeing what it moves. In a landscape where the biggest predator is a bear, Byers saw the spoor of cheetahs and lions. If he squinted when he watched the pronghorn, he could even see these predators giving chase. He could see them manifest in the pronghorn’s every action. Byers came to believe these ghosts were an answer to the question of the pronghorn’s speed, and to other questions too.
As recently as 10,000 years ago, just as cows were beginning to be domesticated in Asia, the pronghorn lived on the plains with the gray wolf, black bear, grizzly bear, and coyote, but also with other large predators. When humans first arrived in the Americas, they found the pronghorn and alongside them a much greater variety of other herbivores, but, in addition, an even greater variety of predators. The American grasslands were more wild and ferocious than the African plains. The predators that the earliest immigrants to North America found as they colonized the continent fourteen or more thousand years ago were bigger, badder, and faster than anything we know today. There were plundering dogs (Borophagus spp.), short-legged dogs (Protocyon spp.), dire wolves (Canus dirus), giant cheetahs, giant cave lions (Panthera atrox), several kinds of saber-toothed cats, giant short-faced bears (Arctodus simus), and other toothy monsters, many of them fast. The cave lion grew to twelve feet in length. The saber-toothed cat could weigh 1,000 pounds and the giant short-faced bear as much as 2,500 pounds. Most relevant to the story of the pronghorn, though, was the American cheetah (Miracinonyx trumani), a big, long, fast cat built to chase and catch at high speeds.11 Analogies with modern African cheetahs suggest that the American cheetah would have loved to eat pronghorn the way African cheetahs love antelope. And so it was in this context that Byers began to imagine that the pronghorn’s speed and its swarm-running evolved in response to now-extinct predators. The pronghorn once had something to flee. American cheetahs evolved to be faster to pursue faster pronghorn and pronghorn evolved to be even faster in return. Then humans arrived in the Americas and, one way or another, killed off sixty large mammal species, including the cheetah, but also lions, mammoths, mastodons, and even camels. The extinction of these great beasts and, in particular, of the American cheetah left the pronghorn anachronistically and irrelevantly fast.
Once Byers had this insight (which seems correct, in the hindsight provided by more data and analysis), much of the pronghorn’s life seemed to make more sense. All of their biology, but particularly that of females, was built around escaping predators that were no longer present. Females chose fast males so that their children would stand a chance of being fast enough to escape. Even their double-horned uterus and compressed spine seemed a function of their past. They were not an exception, but instead a powerful manifestation of natural selection’s rules. They were, in a way, the rule. What is more, it seems as though the pronghorn’s speed and the traits related to it may be costly. If they are, and if, as time passes, the pronghorn become more abundant or their habitat more rare, the pronghorn may get slower. The individuals that run the fastest may die younger, exhausted from fleeing ghosts but unable to slow down. Given time, each generation of pronghorn might become slower and, in its more ordinary speed, less extraordinary.12
Here was what scientists all search for, a general result derived from the study of something very specific. Because the more Byers talked to other scientists, the more he realized that his case of pronghorn was not unique. His was one well-studied example of the consequences that result when one species misses another with which it had been linked for millennia. Years earlier, in Costa Rica, the tropical biologist and conservationist Dan Janzen had argued that the biggest fruits, those that now sit unmoved beneath their shady mothers, evolved to be dispersed by the now-extinct megafauna, species that disappeared along with the pronghorn’s predators. Janzen’s idea arose from his observations in 1979 of the three-foot-long pods of the Cassia grandis tree. Thirty years later, Janzen seems just as right and those fruits remain just as unmoved. To paraphrase the paleontologist Paul S. Martin, we live in a time of ghosts, their prehistoric presence hinted at by the largest sweet-tasting fruits.13 Many of the fruits that humans have come to favor seem to have evolved to be carried from one place to another in the temporary vehicle of a giant mammal’s guts—papayas make the list, as do avocados, guava, cherimoya, osage oranges, and the foul-smelling but delicious durian.14 Elsewhere, biologists found long flowers without obvious pollinators, flowers that had evolved, they argued, in response to the long tongue of a now-extinct pollinator. With time, more cases like these have been noted, more examples of the consequences of losing partners middance.
But the pronghorn example was different. Giant fruits once benefited from being dispersed by giant fruit-eating mammals, sloths bigger than elephants and their kin. The pronghorn did not benefit from being eaten by a giant cheetah any more than you might benefit from getting eaten by, say, a bear. Yet without the cheetah, the pronghorn’s lifestyle, its leaps and sprints, no longer makes complete sense. The pronghorn suffered from the American cheetahs that were their predators, but in a way the pronghorn may now suffer without their longtime foe present to give chase. They run for no reason. They waste energy, when they might do just as well to stand still. They run from ghosts.
We all do.
3
The Pronghorn Principle and What Our Guts Flee
The Byerses went to the pronghorn to understand the pronghorn. What they found was more general. Let’s call it the pronghorn principle. The pronghorn principle has two elements: First, all species have physical characteristics and genes that relate to the ways in which they interact with other species. Second, when those other species are removed, such features become anachronistic or worse. Plants have evolved toxins to defend their leaves, nectar to entice animals to carry their pollen, and fruits to attract other animals to carry their seeds. Animals, in turn, evolved long tongues to reach nectar or better senses of smell to detect fruit. Carnivores have long, sharp teeth to kill their prey. Intestinal parasites have appendages that mirror, in their contours, the guts of their hosts, to hang on. Pick any organism on Earth and as much of its biology is defined by how it interacts with other species as is influenced by the basics of living, eating, breathing, and mating. Interactions among species (what ecologists call interspecific interactions) are part of the tangled bank to which Darwin referred. What the Byerses newly understood in the context of the pronghorn was the consequences of removing the species our bodies evolved to interact with, be they predators (as in the case of the cheetah), mutualists like the animals that once dispersed the giant American fruits, or even parasites and disease. The loss of other sp
ecies can make key elements of any organism’s body as anachronistic as the giant fruits left sitting in the dirt, waiting for the megafauna that never come to pick them up.
For as much as the pronghorn principle is intuitive to ecologists and evolutionary biologists, a core part of our understanding of the living world, no one, not even the Byerses, had thought about how any of this related to our own bodies. Medical researchers are not usually trained to think about evolutionary history, and even when they are, they tend to learn about humans in isolation, as though the past was one of long, naked walks among the trees picking fruits to eat (yet even the fruit is another species, a species we had to see or smell to find). Until very recently, no research considered what happened when we killed off all of our predators, or, for that matter, removed the tapeworms, hookworms, and their kin from our guts. One wonders which parts of our bodies are, like the pronghorn’s muscles and speed, haunted by ghosts—at least Joel Weinstock would come to wonder. What happens when humans leave behind the species their bodies evolved to interact with, whether they be cheetahs, diseases, honeybees, or giant sucking worms?
Joel Weinstock did not know anything about pronghorn. It is hard to imagine a circumstance in which they would have seemed relevant to him. Like most medical researchers, he had not taken anything resembling a class on ecology or evolution since he was eighteen. He could not have told you who the most recent ancestors of humans were, nor was he particularly a fan of “nature.” He knew about the human immune system and how parasites affect it. These might seem narrow realms of biology to focus on, but in knowing two such fields, he was already broader than most biologists. This modest breadth became useful to him when flying home from his trip to New York. He flipped through a folder of data on the increasing frequency of Crohn’s and other “modern” diseases and wondered why they had become more common. As he did, he remembered that over the same years, many of our parasitic worms had become rare. He connected these observations like dots. Once connected, they were a revelation. The cause of Crohn’s could be, he suddenly thought, the parasitic worms—helminths! The more he looked at the dots he had connected, the more he thought he had the answer, an answer related more to the pronghorn and extinct cheetahs than to standard medical science.
It is joyful to think you have the answer. Your heart pounds. Maybe you run around the lab a little bit and let out a jungle yell. Of course, at some point you have to tell someone else your idea and that, in my experience, is where the raw insight often falls victim to reality. Some overly bright student says something like, “I don’t understand how this would actually work,” which is when you realize that it could not, and you sulk a little. But sometimes the insights are right. Or at least they seem right for a little longer than a single anticlimactic day.
Time would tell for Joel Weinstock. He imagined that the problem with our modern guts was our immune system, and that the problem with our immune system was that it was missing the parasites with which it had evolved. Crohn’s and other inflammatory bowel diseases, he would come to argue, are the consequence of our body still running to escape its ancient assailant. When a pronghorn runs fast to outpace a long-gone predator, it wastes energy. When our bodies run fast to escape nonexistent worms, they trip, he believed, or maybe they never learn to run properly in the first place.
Weinstock had a hunch, but he did not have direct evidence. Of course, it was true that people in developed countries were more likely than those in developing countries to have Crohn’s and less likely to have parasitic worms. In developing countries, as many as a billion people are infected with two species of hookworm alone (Necator americanus and Ancylostoma duodenale), not to mention tapeworms, whipworms, and other possible beasties one meets, however accidentally. All of these species were ancestrally marine creatures. They were able to come ashore by colonizing the guts of animals, each gut being, in a way, like a tiny, albeit not terribly picturesque, sea.
Such a possibility seemed crazy to the medical community, anathema to the long-held idea that medicine is meant, in part, to remove species from our bodies in order to make us healthy. Antibiotics, antiseptics, antihelminthics, and all of the other “antis” are based on the idea of removing life, regardless of its identity. Yet there was something to Weinstock’s argument, and so people listened when he talked. He was also already, it bears mentioning, an esteemed immunologist when he started talking about “our bodies missing their tapeworms.” Esteemed scientists suffer fewer consequences when they advance crazy ideas. They can shout them into a McDonald’s takeout window. They can even announce them on TV. All that is likely to happen is that someone may say, “Damn, Joel, could you just test some of this stuff before you go on Oprah?”
Experiments are the best tests of new theories, but experiments on humans are not always possible, even when moral. It is hard to imagine an experiment to test the effects of refrigeration on Crohn’s or any other disease. Refrigeration could play a role, but conclusive support is probably hard to ever come by. Even sick patients are unlikely to be willing to give up their refrigerators. The effects of the loss of parasites on Crohn’s disease could, though, be tested experimentally. You would test it the same way you might test the effects of losing the megafauna on the American plains and the pronghorn. Reintroduce them. Restore the slender cheetahs of the gut, with their long tails and microscopic claws.
If losing parasites causes Crohn’s disease, then putting them back might remedy Crohn’s. But perhaps this is an overly simple idea, akin in that regard to rewilding the West to keep the pronghorn fast. If the experiment did not work, it would not tell you much. It might be that the effects of the loss of parasites are felt mostly due to their absence during immune system development, or to the loss of chronic infection. Might be. Could be. May be. Yet if you simply added parasites to a patient with Crohn’s and the patient got better, it would be suggestive. If you added parasites to a bunch of patients and most of them got better, it would be more than suggestive, compelling even.
As I began to read the Crohn’s disease literature, I wondered about such an experiment. Would it be moral? Would anyone approve it? Perhaps the clearest precedent for what Weinstock wanted to do came again from the pronghorn. A handful of scientists, friends of the pronghorn biologist Byers, suggests that we ought to rewild western North America. These scientists propose to “change the underlying premise of conservation biology.” We need to, they say, reintroduce the extant carnivores everywhere they once were (bears and wolves, for example, occupy just one percent of the area they roamed just 200 years ago). But we also need to introduce elephants to replace mammoths and mastodons, African cheetahs to replace American cheetahs, and African lions to replace the extinct American lions. We might even introduce the Bactrian camel to fill the role of the many camel species once found in North America. By introducing these species to the American West, we would make it more like what it once was, what it, in their language, “should be.” We would “stomp out the rats, dandelions and weeds.” Maybe then, when the pronghorn again have something to flee, their speed will make sense.
These folks, with Josh Donlan, a conservation biologist currently based at Cornell University, as a kind of radical statesman leader, are macho mammal-trapping, snake-chasing dudes. They are ready for megafauna, ready for it now and unafraid of the consequences of advocating their new vision. They are the kinds of guys (and they are mostly guys) who, given their choice, would rather die in a tiger’s mouth than of a heart attack. In one article Donlan asks, “Will you settle for an American wilderness emptier than it was just a hundred centuries ago?” Donlan will not. Bring back, he argues, the tigers. Bring back the lions. Donlan and his colleagues want these species back so badly that they are willing to go out into the desert and do it themselves. In fact that is just what they did. Under the dark of night they caught a few wild animals from a preserve in Mexico, transported them in the back of a big truck across the border into Texas, and released them, wild and untethered
onto Ted Turner’s ranch. That the individuals were 100-pound Bolson tortoises (Gopherus flavomarginatus), not lions, and that the preserve was a fenced, albeit enormous, backyard is beside the point. The aim was, just as for the lions, to restore their functional roles. For the lions, of course, they would just need a bigger truck.
When Josh Donlan and others proposed rewilding the West, they got hate mail, or at least the academic version thereof, passive-aggressive papers written in response to their papers. The idea was taboo.1 Then they got hate mail from farmers, whose predecessors and ancestors worked so very hard to get rid of the megafauna. In part, the critics’ sentiment was an old one, summed up by the comments of the British biologist William Hunter some 240 years earlier when he wrote, “Though we may as philosophers regret it, as men we cannot but thank Heaven that its whole generation is probably extinct.”2 In other words, tigers are good in Bangladesh, but not in my backyard. Yet there is a key difference between the rewilding of the western United States, whether with a tortoise or a tiger, and the rewilding of our bodies. It is easier to get permission to do an experimental rewilding of a human body than of Idaho’s wild, rippling miles of grass.
