"Some mammals evolved a physiological resistance to venom, but it was only the primates that went the visual route, strengthening the visual system [to detect snakes]," Isbell said.
"Of course, the [improved] visual system is also good for seeing [other] things, but the early parts of the visual system seem particularly sensitive to seeing and detecting snakes."
Seeing Is Believing
In addition to good vision, primates have large brains and grasping hands and feet.
Scientists thought that these characteristics evolved together as early primates used their hands and eyes to grab insects and other small prey.
But neurological studies suggest that the visual system is actually connected to the "fear module," a group of brain structures in mammals involved in vigilance, fear, learning, and other behaviors, Isbell says.
"If you look at the brains, sometimes you find cells that respond to things that might represent snakes, like diamond-shaped patterns," she said.
Cats, for example, have cells in their visual system that are preferential to such shapes—which resemble snake scale patterns.
"All mammals have a sensitivity to snakes, and their brains are shaped in ways that allow them to detect snakes," Isbell said.
Harry Greene, a herpetologist at Cornell University in Ithaca, New York, is intrigued by the evolutionary connection between snakes and the visual system of primates.
"Isbell's study is really a wonderful piece of synthetic scholarship, since she pulls together snake and primary evolutionary history, neurobiology, and so forth," Greene said. "And I'll bet she's right."
The Human Brain
Primates that were less threatened by snakes have had less pressure to evolve better vision, Isbell says. This is additional evidence that snakes may have played an important role in the evolution of primates' visual systems.
"Old World monkeys have always had to live with venomous snakes, so there's been continuous selection to [evolve] visual systems that are better at detecting snakes," she said.
"New World primates arrived in South America millions of years before venomous snakes and show less specialization in their visual system compared with Old World monkeys and apes, which all have good vision."
But Isbell warns that her hypothesis should not be extended to fully explain human eyesight.
"I'm stopping at the difference between Old World and New World monkeys," she said. "If you're talking about [humans], there are so many other things that are important to [them].
"I'm not saying the human brain is all about having to avoid snakes," she added.
"We certainly use our brains for other things, and many of us have never seen a snake in our lives, but the basic part of our visual system that we share with other primates is what I'm saying changed as a result of exposure to snakes."
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