Garter snakes Credit: Bruce MacQueen
While studying monkeys in Africa, Lynne Isbell practically stumbled upon an unlikely hypothesis: The evolutionary development of the primate brain may have been significantly impacted by venomous snakes.
It certainly wasn't a conclusion the anthropologist from the University of California, Davis set out to find.
"It was pretty accidental, actually," Isbell said. "I was working on a different issue. My focus is on primate behavioral ecology."
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Still, her paper, published in the July issue of the Journal of Human Evolution, almost immediately ended up in the media and on personal blogs, sparking online discussions about snake phobia, human evolution, and trends in popular science.
In the evolutionary conflict between predatory snakes and their prey, many mammals responded by developing a physiological resistance to snake venom. Isbell postulates that instead of building up immunity, primates responded by evolving a visual system that allows them to detect a snake before it makes a deadly strike.
"Within the visual system, there are cells that seem to be very responsive to the cues that snakes give off, like their scale patterns and their long shapes," Isbell said.
Structures in the mammalian brain involved in vigilance, fear, learning and memory are particularly responsive to frightening stimuli, such as predators. In Old World monkeys—which have had a long, continuous coexistence with venomous snakes—these brain structures are bigger than in mammals like lemurs, which do not coexisted with the reptiles.
Isbell posits that snakes were likely to have been the first predators of placental mammals, and therefore ultimately guided the development of the primate brain.
This hypothesis contradicts a popular alternative, which states that primates' ability to reach for and grasp food, like insects and fruit, is what prompted their brain expansion. (Isbell says the grasping for food hypothesis is not supported by neurological evidence.)
Isbell stumbled upon her hypothesis six years ago, when researching the evolution of leopards, which "really wiped out" the monkeys she was studying in Africa. She happened to come across cat retrovirus research describing an analogous retrovirus in snakes. Curious about the evolution of these reptiles, she studied their long coexistence with primates and began her building her hypothesis, which now includes evidence from paleobiogeography, neuroscience, ecology and immunology.
"I dug and I dug and I dug," Isbell said, adding that she had to learn neuroscience in order to fully understand what she was uncovering. "I never knew it would take as long as it did. I said, 'I can't drop it until I find something that tells me it's not right.' And I never did."
But according to Todd Preuss, a neuroscientist at Emory University, Isbell's hypothesis may depend upon ambiguous data. For example, much of her evidence is drawn from developments in a visual pathway that scientists still don't know much about.
"On the other hand, I'm really impressed," Preuss said. "It's audacious, and not simply because it presents a new hypothesis. What's really audacious about it is the willingness to go beyond her narrow area of expertise."
Isbell admits that her hypothesis still needs to be tested further. For example, scientists could examine whether primates can distinguish between venomous and non-venomous snakes.
Even if her theory turns out to be wrong, Isbell said, it's fine with her.
"What I hope is that it will spark enough interest in people from different disciplines that they would want to collaborate with each other," she said. " I'm just throwing the idea out, and I hope that someone else will do some work on it."
Isbell is sure of one thing: The public response has been overwhelming. But she noted that, contrary to speculation, the close timing of her paper's release to the opening of the movie Snakes on a Plane is purely coincidental.
"People have this thing about snakes," she said. "The hypothesis can explain this."