The ability to detect threatening snakes may have shaped the visual system of our primate ancestors, a new study says.
In a sort of evolutionary arms race, primates kept improving their eyesight to help spot and avoid snakes as the snakes became more dangerous, suggests Lynne Isbell, a behavioral ecologist at the University of California, Davis.
(Related story: "Fear of Snakes, Spiders Rooted in Evolution, Study Finds" [October 2001].)
"The initial change in primate [eyes] ... occurred when they had to deal with constricting snakes, probably about 90 million years ago," Isbell said.
"That ended up with primates that have forward-facing eyes, whereas other mammals tend to have eyes on the sides of their heads." Forward-facing eyes allow better depth perception.
When poisonous snakes evolved about 60 million years ago, primates further specialized their visual systems.
"That resulted in the anthropoid primates—which we are one of—which had better vision all around, compared to the earlier primates that only had to deal with constricting snakes," Isbell said.
The study is published in the July issue of the Journal of Human Evolution.
All the Better to Eat You With
Modern mammals first appeared about a hundred million years ago. Snakes were probably their first major predator—species with mouths big enough to eat mammals show up in the fossil record at about this time.
"Snakes evolved a variety of ways to get their food, including widening their mouths enough to eat some mammals," Isbell said.
And venomous snakes raised the stakes even further, forcing primates to get better at detecting the predators by, for example, developing excellent color vision and pattern recognition.
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