Rorotoko

In a nutshell

The Fruit, the Tree, and the Serpent is the result of my attempt to understand what in the environments of proto-primates might have begun the process of brain modification to expand the visual sense beyond that of other mammals and turn them into what we today call primates.

The currently most widely accepted hypothesis developed to explain why primates evolved linked the traits of good vision and visually guided reaching and grasping via the same selective pressure, that of an insectivorous diet which would have necessitated accurate judgment of distance to insect prey and the ability to catch those insects in the hands.

In The Fruit, the Tree, and the Serpent I provide evidence against this hypothesis. I use recent findings from neuroscience that suggest these traits are not, in fact, inextricably linked. Indeed, it is not even clear anymore that all primates have visually guided reaching and grasping, at least for obtaining their food. Certainly the anthropoid primates (the primate lineage to which we belong) have this ability. But some of the prosimian primates, the lemurs and bushbabies, may not.

I briefly discuss the selective pressures that might have favored visually guided reaching and grasping (forelimb-directed travel in an arboreal environment), but the main emphasis of the book is on explaining other aspects of our enhanced visual sense: our forward-facing eyes, sharp visual acuity, and good color vision, among other qualities. The bottom line: primates have good visual ability not because early primates were predators but because they were prey—notably of constricting snakes, their first predators.

But there’s more to primate evolution than their origin. Although all primates have an expanded visual sense, there is also variation in visual ability within the primate lineage. For example, not all primates see equally clearly and not all primates can distinguish reds from greens. Such variation also needs explaining. The Fruit, the Tree, and the Serpent provides evidence that variation in visual ability among primates is ultimately the result of differential exposure of primates on the landmasses of Madagascar, Central/South America, and Africa/Asia to another deadly threat, that of venomous snakes.

Bringing it back to ourselves, we have superb vision because our own lineage within the primates arose in Africa/Asia, where the longest and most unrelenting evolutionary exposure to venomous snakes occurred.

The wide angle

The idea of Snake Detection actually took me by surprise. I wasn’t looking for it. I was instead pursuing answers to questions that were more consistent with my academic background in primate behavioral ecology. I had been wondering how long ago the various predators of primates appeared on the landmasses of Africa/Asia, Central and South America, and Madagascar. These regions are home to distinct lineages of primates: Old World monkeys and apes (the primate lineage leading to humans), New World monkeys, and lemurs, respectively. Each lineage has distinct behavioral characteristics, including the degree to which females disperse from their natal groups and areas.

What I wanted to find out was whether a longer evolutionary history with predators might be associated with an increase in the tendency of females to stay at home. All I knew about primate vision at the time was that primates on these landmasses differed broadly: Malagasy lemurs have poorer visual acuity than the others (because most lack a fovea for sharp central vision), and most New World monkeys have poorer color vision than Old World monkeys and apes.

As I investigated the question of variation in female dispersal, I came across a reference to a now 30-year-old paper about retroviral transmission from primates to a venomous snake in Asia that apparently occurred quite long ago. Although I knew constricting snakes, the pythons and boas, were predators of primates, I had never really thought about venomous snakes as predators—most of today’s primates are too large to be eaten by them.

Knowing what we know now about retroviral transmission—think of HIV—the inference I drew from the paper was that somewhere in Asia there had been a moment in ancient time when a venomous snake made contact with a primate. This struck me as good indirect evidence that venomous snakes were predators of primates in ancient times; or, at the very least, that venomous snakes were deadly to primates. As we humans know all too well, especially those of us living in the tropics where most primates also live, venomous snakes don’t need to consume us to be deadly.

I followed up on the idea and found that visual ability in primates gets better with longer evolutionary time with venomous snakes.

Venomous snakes first appeared in Africa/Asia about the same time that anthropoid primates appeared. African monkeys and apes (which constitute the lineage leading directly to humans) and Asian monkeys have always had to avoid stepping too near to snakes that could kill them with one bite. Perhaps not coincidentally, they have the best vision of all primates. At the other extreme, venomous snakes have never occurred in Madagascar, and the prosimians there have poorer visual acuity than other primates. Most prosimians also cannot distinguish reds from greens.

Venomous snakes eventually spread from Asia to North America, and finally to South America. The monkeys that live in Central and South America are thought to have originated in Africa, rafting over on floating islands dislodged from the mainland. Their ancestors were exposed to venomous snakes before arriving in the New World. But when these monkeys reached South America, they found themselves in a continent free of venomous snakes. So they began to diversify released from the constraints venomous snakes imposed on their ancestors. Today, not only do New World monkeys have visual abilities intermediate between those of Malagasy prosimians and Old World monkeys and apes, they also have more variability in the details of their visual systems. As the Snake Detection theory predicts, variability would be expected to increase in a less constraining environment.

Reconstructing evolutionary paths that eventually led to what we have in the world today is challenging. We can never really know what exactly happened millions of years ago. All we can do is postulate what might have happened and look at the evidence currently available to decide if what we propose is reasonable. That is what The Fruit, the Tree, and the Serpent does.

primates have good visual ability not because early primates were predators but because they were prey—notably of constricting snakes [...] we have superb vision because our own lineage within the primates arose in Africa/Asia, where the longest and most unrelenting evolutionary exposure to venomous snakes occurred