Fear of Snakes

Fear of Snakes Drove Primate Evolution, Scientist Says

An evolutionary arms race between early snakes and mammals triggered the development of improved vision and large brains in primates, a radical new theory suggests.

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To avoid becoming snake food, early mammals had to develop ways to detect and avoid the reptiles before they could strike. Some animals evolved better snake sniffers, while others developed immunities to serpent venom when it evolved. Early primates developed a better eye for color, detail and movement and the ability to see in three dimensions—traits that are important for detecting threats at close range.

Humans are descended from those same primates. ¹

Harry Greene, an evolutionary biologist and snake expert at Cornell University in New York, says Isbell's new idea is very exciting.

"It strikes me as a very special piece of scholarship and I think it's going to provoke a lot of thought," Greene said.

Isbell's work is detailed in the July issue of the Journal of Human Evolution. ²

Early snakes killed their prey using surprise attacks and by suffocating them to death—the method of boa constrictors. But the improved vision of primates, combined with other snake-coping strategies developed by other animals, forced snakes to evolve a new weapon: venom. This important milestone in snake evolution occurred about 60 million years ago.

"The [snakes] had to do something to get better at finding their prey, so that's where venom comes in," Isbell said. "The snakes upped the ante and then the primates had to respond by developing even better vision."

Once primates developed specialized vision and enlarged brains, these traits became useful for other purposes, such as social interactions in groups. ³

Isbell says her theory can be tested. For example, scientists could look at whether primates can visually detect snakes more quickly or more reliably than other mammals. Scientists could also examine whether there are differences in the snake-detecting abilities of primates from around the world. ⁴