by Rob Dunn
If Isbell was right, and venomous snakes had caused richer vision to evolve in some primates but not others, then she could make a prediction. She had one piece of the puzzle. She knew that in the New World only some primates see the full range of colors visible to the humans, but in the Old World all species do. Could the ancient presence of venomous snakes in the Old World be responsible for these differences? She also knew that in Madagascar, the lemurs, a kind of primitive primate long separate from other primate lineages, not only have poorer color vision but also cannot see in fine detail in the way that we and other Old World monkeys and apes can. Isbell’s theory predicted that there are no venomous snakes in Madagascar.
Among primatologists, Isbell’s idea has no precedent. But often ideas new in one field are accepted truth in another. One field’s radical possibility can be another’s dogma. Predation is not the exclusive fate of primates. Being eaten is a very common way to die, whether one is a primate or a mollusk, especially, I suppose, if one is a mollusk. Perhaps the best precedent for what Isbell was beginning to argue came from just such mollusks. It was work done in the laboratory of Geerat Vermeij. Vermeij works a few buildings over from Isbell in the geology department at the University of California, Davis, and lives in a house a block away from Isbell’s house in town. They are neighbors in work, in life, and, it turns out, in ideas.
On any given Sunday you might find Geerat Vermeij at the beach, on his hands and knees picking through shells. He moves slowly, like some primitive bird, picking over the detritus, trying to find the rare or interesting. Vermeij has spent a life among the shells, whether living or dead, recent or fossil. More than anything, he has specialized in understanding the diverse ways in which animals die. He studies these deaths as though at a crime scene. Instead of blood and bones, he tends to search for holes in shells and the suture lines of old wounds. Instead of weapons, he looks for beaks, raspers, teeth, and the other murderous contrivances of evolution. In this approach, it would be fair to say that he has an uncommon vision of life, except that Vermeij has no vision at all. A case of glaucoma at the age of three rendered him blind. Doctors removed his eyes and left him to find his way around the world with his other senses. Like a snake, he listens, smells, and tastes. It is his sense of touch though from which he has constructed his most detailed understanding of the sea and its history. From the shore, he reaches to the ocean bottom and from there back in time.
One of the mysteries that Vermeij noticed very early in his career was similar to the one upon which Isbell had stumbled, specifically that when new predators—be they crabs, snakes, or modern man—emerge, their prey change. Since his childhood, Vermeij has focused on mollusks (clams and snails). In the shells of those animals, his fingers detect textures, shapes, and nuances of form. Pause for a moment and imagine doing this work as he does it. Walk to the cabinets he opens every day. Move by memory among the echoes of objects. Pull out a drawer filled with shells. Now run your own digits over them. As you do, pay attention to shape and size, but also to the bumps, ridges, and twisted intricacies. Notice too what is missing, though that is the hard part, because to do so, one must know what is supposed to be there in the first place. Notice the gaps, the occasional and seemingly inexplicable hole or divot. Now focus on that divot. Your fingers evolved to pick up fruits and later to grasp stones and spears, but push them to their limits. What could it be? It is perfectly round, this particular hole, as though drilled. But feel inside, push the tip of your pinky down and you might note a rougher texture. All of these subtleties are clues to the history of the shell you hold and, for Vermeij, to the particular stories of the hundreds of thousands, perhaps millions, of shells he has lifted and fondled. Out of those feelings, Vermeij has constructed a world different from the one most of us experience, though no less rich. In his world, some things are obvious that with vision would otherwise be missed.
Vermeij noticed many things in his hours among the shells. He noticed what you might see if you took up his occupation—that shells varied from place to place and time to time. He also discovered other things that everyone had missed. Perhaps some features are more obvious to fingers than to eyes. There were patterns in the kinds of shells one found in different places and times. He enjoyed the differences, the way anyone might delight in finding something new, but also wondered about their cause. It is out of specifics that biologists build their universals, and he was beginning to find himself with plenty of specifics. What was most conspicuous to him was that the species of the Pacific had thicker shells than those of the Atlantic, with smaller, more obstructed entrances and longer spines.6 What if, he wondered, those differences were the result of differences in the mollusk-eating predators present in the different oceans? In the Pacific, crabs have bigger claws that are better able to crush an unprotected snail. Yet as he felt his way, there was an even more pronounced pattern at his fingertips, a change not across space but through time. Even as the dinosaurs were going extinct, bigger revolutions were occurring underwater. These seemed, to Vermeij, to have at their root not meteors or some other cataclysm, but instead, once again, the specifics of predators. After crabs and other predators arose, the life on the seafloor responded. It had to. Shells thickened. The openings of shells became narrower and everything became more spiny, armored against fate.7 Mollusks moved from the seafloor down into the sediment. Whole lineages of life disappeared. But there was more. Unlike the fate of dinosaurs, the deaths of mollusks left evidence of the crime, cracks in shells, holes, and other telltale marks that suggested not the two hands of some god, but instead the millions of claws of crabs. In handling the shells, what Vermeij realized was that the evolution of the killing tools of predators had shaped the entire floor of the sea and its inhabitants. Just how the mollusks, his focus, changed was a function of the particulars of what the claws of a given region and time could or could not break open. All of this came together to suggest to Vermeij a kind of law, a rule about life as general as the physical laws are about particles, a rule that applied to mollusks, but also to everything else including snakes, primates, and you.
Vermeij’s law was about a kind of natural gravity, the force predators exert on prey.8 When new predators evolve or when old predators grow more abundant, prey respond. They must, just as clouds must part as they blow over buildings and wet clay must give to the pressure of hands. What Vermeij had uniquely noticed was that the ways in which they responded were predictable and inevitable. To most people before Vermeij (to the extent that anyone had considered the question), it had seemed that prey should respond to those things predators are best at, their most predictable and deadly tools. Vermeij thought the opposite. Imagine a crab that proceeds through four acts in its performance—finding a mollusk, picking it up, breaking in, and then actually killing and eating it. At some tasks, it rarely fails. It finds prey without trouble. It kills with deadly certainty once it has broken through the shell. What it most often fails to do is to get through the shell. Breaking in is hard to do, and so what mollusks have done over time is to change most in those features that prevent the crabs from breaking in. This was Vermeij’s law: prey respond to predators’ weaknesses, the ways they fail rather than the ways they succeed. The main caveat is that the prey must vary genetically in traits related to the predators’ weakness, but in most cases they do. Now that crabs are everywhere, almost all shells in the ocean are thick and hard, but the fleshy bodies of the animals inside are as undefended as the soft bodies of human infants. Crabs rarely fail once they get inside a shell, and so most mollusks do not even try to offer a defense.
Lynne Isbell was beginning to figure out just how snakes fail. It was very different from the ways in which the other predators of primates do so. When lions, leopards, or tigers fail in their attacks on primates, it tends to be in the ambush. These carnivores consistently find primates by scent (we primates are a stinky lot), but they require surprise for a successful attack. When monkeys detect a leopard, the leopard may even turn
away, much as the tiger turned away from Corbett when he turned to look at it, as if to simply admit that the jig is up. Without the element of surprise, a big cat is much less able to kill its prey (though sometimes they will try anyway, hunger being hunger). As Vermeij himself has pointed out, large cats fail to kill their prey as much as half of the time once they have detected it, in large part owing to losing the element of surprise. Because of this, monkey responses to leopards have evolved in such a way as to alert carnivores that they have been detected. Many primate species, including Diana monkeys and Campbell’s monkeys, scream out alarm calls for “large cat.” When doing so, monkeys appear to signal not only to other monkeys but also to the cat itself. So useful is this notice of an ambush that several monkey species are even able to recognize the “large cat” calls of other monkey species and in hearing them know what to do, which is to first look down.9 Alarm calls are at the core of primates’ ability to escape carnivores. They have even been argued to be the precursors to language in humans. My daughter’s first word was “fish” (perhaps she was imagining a very big fish), but our lineage’s first word may very well have been “leopard.”
Chimpanzees also hunt monkeys. Whatever squeamishness you might feel at eating an animal that looks back at you with eyes not unlike those of a human child, chimpanzees do not feel it. Some chimpanzees eat many monkeys but in doing so, they fail at something different than do leopards. Once those chimps detect monkeys, they catch and kill them nearly all of the time—chimps hunt actively and pursue. But they are not very good at detecting monkeys. It does not pay for monkeys to evolve defenses against chimps, and it definitely does not pay to sound the alarm. As a consequence, when monkeys spot chimpanzees they respond by running away or huddling close to branches and becoming totally silent, engaged in a life-and-death version of hide-and-seek.
Then there are the snakes. Snakes eat monkeys, but they also kill them in self-defense (because, after all, every so often monkeys kill snakes). Once monkeys spot snakes, alarm calls are useful to tell other monkeys where the snakes are. Several monkey species produce snake-specific calls—“snake, snake, snake”—and may even do so in a way that distinguishes different kinds of snakes (Campbell’s monkeys, for example, sound an alarm when provided with models of Gabon vipers, but not black mambas). There is a difference though between spotting cats and spotting snakes. Monkeys need to see cats, but best at a distance. Spotting snakes at a distance is not necessary. If Vermeij’s law is right (and if, as tends to be agreed, many monkeys die from snake bites), then monkeys should have evolved the ability to notice snakes even as the snakes lie motionless and camouflaged. In other words, Old World monkeys and apes should be better than other animals at detecting snakes. Such a possibility was not considered, not, anyway, until Lynne Isbell stumbled upon it, feeling around in the darkness of ideas the way Vermeij feels around.
If Isbell was right that the particulars of primate vision evolved in response to the presence or absence of venomous snakes, she would expect better vision with greater exposure to venomous snakes. That is just what she found. Venomous snakes evolved in the Old World, and were relatively recent arrivals (10–20 million years ago) in the New. This matched the differences in primate vision. It fit her theory. But what about Madagascar, where prosimian-present primates have relatively poor vision? From the beginning, Isbell had hoped, in a way, that she was wrong. If she was wrong, she could get back to the life she was living before her idea. Maybe she would find that there are venomous snakes in Madagascar, but just as she predicted, there are not. Madagascar has no venomous snakes, and Madagascar’s primates, the lemurs, have the worst vision of all the primates. They are as likely to find their way by taste, smell, or touch as by sight. In this, they are like Vermeij.
Isbell has elaborated her theory in detail in her book The Fruit, the Tree, and the Serpent, and at least two things have emerged as undeniable.10 First, our color vision, and the color vision of African and Asian monkeys and apes more generally, deserves explanation. The only other prominent explanation for patterns in color vision, aside from Isbell’s, is that our color vision evolved to help us discern different kinds of fruit.11 This seems possible, though it is unclear why color vision would be important for fruit in the Old World but not in the New and not at all in Madagascar, where most lemurs eat fruit. Yet, even if the fruit hypothesis were right, we are still left with the certainty that we have good color vision because of our interactions with other species. Second, once the full spectrum of our color vision evolved and our other senses faded, many consequences ensued, both for us and for the rest of the living world.
In concert with the development of our vision, our brains began to expand. That visual and language abilities, both plausibly linked to our evolutionary relationship with snakes, were at the core of this early expansion is beyond doubt. Trichromatic-color vision and antipredator calls seem likely to have been necessary first steps along the brain evolution trajectory that would eventually make us smart enough to be able to type “brain evolution trajectory.” In fact, our vision would become the dominant sense linked to our burgeoning brain. Evidence from the genes of different mammals suggests that just as our vision was becoming ever better, at least some of our other senses grew worse. Genes associated with smell mutated, one after the other, and because smell had become so unimportant relative to sight, the individuals with mutations did no worse. Through time, more and more of our genes for smelling have become broken, unused, and apparently unnecessary, just as has repeatedly happened with vision in cave fish. Whether the same is true for our senses of touch and hearing is unknown, though it seems possible. In other words, for Isbell, snakes are the pea under the pillow of our minds and the ways in which we perceive and have constructed the world.
It is easy to be skeptical about Isbell’s idea, just as it is easy to be skeptical about many grand theories in primate evolution. The facts are fragmentary, as they long will be, and the ability to experimentally test theories limited, and so the hands of the archetypal anthropologist start waving as though attached by a string to clouds in a storm. Personally, I wondered about the central pillar on which her entire idea rests, that venomous snakes have killed primates often enough so as to have affected primate evolution. Most snakes on Earth, after all, never kill anyone save rodents and insects. They are shy and reluctant to bite, neither tempting nor terrible.
Yet as Isbell points out, there are many records of primates being killed and sometimes eaten by some kinds of snakes. I decided to do my own kind of test—compelled by something stronger than curiosity. I sent an e-mail to friends asking how many of them knew of a biologist who had made a mistake and accidentally grabbed or had been bitten by a venomous snake. I imagined I would get a list of famous (and famously dead) snake biologists who had made one too many a lunge in the field. Instead, I discovered that a striking number of my friends have themselves been bitten by venomous snakes.
Greg Crutsinger, now a faculty member at the University of British Columbia, was working at La Selva Biological Station in Costa Rica when he stepped over a stick that bit him. The stick was a hognose viper. Greg is still a little twitchy around sticks. Piotr Naskrecki was walking down a trail picking up this and that, looking for katydids and more generally new species. Piotr lifted up a rock and a venomous snake bit him. Piotr lived to discover more species. My former adviser Rob Colwell was walking down a trail and talking when he missed a terciopelo (velvet skin) that did not miss him. It emptied all of its venom into his shoulder, as snakes are wont to do only when they are trying to prey on something or, in this case, someone. Maura Maple, whom I met at a field station in Costa Rica, was bitten by a terciopelo at La Selva Biological Station, not far from where Greg was bitten by the hognose viper. The list goes on. Hal Heatwole, whose office is several doors down from mine, was bitten by a sea snake and took the time to take a picture of the bite because he knew that a friend needed such a photo for a book on deadly bites. Vlastimil Zak, an ecolo
gist who lives in Ecuador, has been bitten at least twice by venomous snakes. These friends lived, but not everyone does. Joe Slowinski, a friend of a friend, went to Myanmar with a team to find new snakes. He is one of a wave of biologists who have traveled to faraway places in recent years to find new things. Someone on the trip, a guide, brought him a plastic bag filled with a snake. Someone thought it was venomous. Slowinski thought that someone was wrong. His taxonomy failed him and he died.
Of course, I know many more people who have been stricken by cancer or car accidents than I do who have been bitten by venomous snakes. Yet in all these stories dwells a basic reality. When biologists muck around in the tropics and fail to pay sufficient attention (or their vision does not allow them to pay sufficient attention), they stand a fair chance of being bitten by venomous snakes (less of a chance than being hit by a car, but cars were no threat during our early evolution). What is more, biologists tend to interact with other species more like the ways our ancestors did—hands on—than do most people. To look at the longer history of human relationships with snakes, it is clear that snakebites were once even more common than they are today, and today they are far from rare. They tend to be underreported, but even the cases that are now number 30,000 to 40,000 fatalities a year, not to mention the bite survivors. A study of more than 1,000 rubber tappers in Brazil found one in ten had been bitten by a venomous snake. Half of those who had been bitten had been bitten twice!12 A seven-year study in Benin tallied more than 30,000 bites by venomous snakes, 15 percent of which resulted in death. An older study from Niger estimated that 10,000 people a year are bitten by venomous snakes in that country. There is no reason to believe these studies are unusual. Instead, for the tropical landscape in which the most aggressive venomous snakes dwell (or dwelled), they seem likely to represent our general susceptibility to dying on the end of a snake fang, particularly when in the African tropics of our origins.