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.