Lynne A. Isbell


On her book The Fruit, the Tree, and the Serpent: Why We See So Well

Cover Interview of August 06, 2009

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.