Good Eyesight? Thank Snakes

What miracle of evolution gave humans the ability to see details just inches away?

Snakes, says an anthropologist who has spent years trying to answer that question.

Especially venomous snakes that forced our distant relatives to improve their vision or perish.

Lynne Isbell of the University of California at Davis has brought forth a new hypothesis that could explain why humans are equipped with such extraordinary eyesight. It's the product, she says, of a "biological arms race" tens of millions of years ago between primates and vipers.

The primates survived, but the trauma may help explain all these years later why snakes are still reviled by so many humans.

Isbell's theory, published in a recent issue of the Journal of Human Evolution, flies in the face of conventional wisdom and will require much more research to be verified. The current thinking holds that primates developed near-vision capabilities to help them capture bugs, or at least reach out and grab a piece of fruit.

But Isbell points out that when snakes began to eat primates, it made sense for the monkeys and apes of Africa to develop better eyesight. A snake, after all, is pretty harmless unless it's close by. It's not likely to romp across the lawn to attack a human, or even a smaller mammal.

But if it's close, you better be able to see it.

She didn't set out to disprove an old theory, or to rattle the cages of scientists in disciplines as diverse as neurology and primatology. In fact, she was researching a totally different question when evidence she was collecting from around the world led her in another direction.

She decided to change course partly because of an old grudge. While researching primates in Africa, "the leopards kept eating my monkeys," she says. So when the data she was collecting suggested a new theory involving primate predators, she was willing to spend six years of her life delving into subjects she admits she initially knew little about. Like neurology.

Here, briefly, is what she found:

Constrictor snakes, like the modern boa constrictor, emerged in Africa around 100 million years ago and found mammals, including the tiny primates that had just appeared on the scene, easy prey.

Over millions of years the primates made adjustments, evolving more forward-facing eyes and better eyesight. That made it easier for them to see the snakes, and the arms race hit high gear.

Not to be outdone, about 60 million years ago snakes evolved the ability to poison their prey, and the venomous reptiles posed a new threat to the primates.

The primates answered that challenge by evolving even better eyesight, including the ability to distinguish between prime colors, something few animals are able to do even today. That gave the monkeys and apes of Africa the ability to see through camouflage, even at very close distances.

Armed with all that, Isbell began a global search, at least a library-based global search.

Primates migrated to South America about 35 million years ago, she says, based on fossil evidence. And lemurs in Madagascar have never suffered predation by venomous snakes. Is their eyesight different?

Isbell says primates in Africa, who have had to fuss with snakes the longest, have the best eyesight. And South American primates, which fall between those two extremes of exposure to predation, fall between them in terms of vision.

"It was a nice fit," she says. It seemed likely that primates developed better vision because of snakes, not to reach out and grab a piece of fruit.

The problem with the old theory, she says, is it doesn't mesh well with current neurological evidence. The structure of the brain's visual system differs among primates.

Among primates with the best eyesight, the part of the brain that is associated with fear is more "expanded" than the part that is associated with "visually guided reaching and grasping," Isbell says. That suggests that the evolutionary changes in the primate brain resulted from predation.

But she admits neurology is not her cup of tea, and she's not confident that neurologists will embrace her ideas any time soon.

But others have, including psychologists who see her theory as a good explanation for why so many humans hate snakes, and herpetologists, who know first hand of the awkward relationship between primates, including humans, and snakes.

So who's the winner in all this?

Us, if she's right.

"Primates have the best vision of all mammals," she says. "And we have the best vision of all primates."

"We're very lucky."

If it hadn't been for snakes, your eyes couldn't even read all this good stuff.