Lynne Isbell was running through a glade in Kenya back in 1992, when she spotted a cobra. She froze in her tracks before she realized what she’d seen. Isbell is an anthropologist and behavioral ecologist at the University of California, Davis, and she believes that her life-saving reaction was the result of millions of years of evolution:
You can see large carnivores from afar, but the same is not always true for snakes: To pick out camouflaged snakes, you need great close-range vision. So to spot snakes better, primates evolved to have color vision and forward-facing eyes, which improves depth perception and allows 3D vision. They also evolved to have the best visual acuity among mammals, Isbell said. These visual features, which required the enlargement of some parts of the brain, were co-opted for other purposes, such as social interactions and reaching and grasping for objects.
Interestingly, the evolutionary interaction between snakes and mammals was not a one-way street, according to the snake detection theory. As mammals became better able to evade snakes — which till this point relied on squeezing their prey to death — the reptiles needed a new, easier way to kill. So they evolved venom. In response, primates evolved even better vision. Indeed, primates that live in areas without venomous snakes, such as on Madagascar, have poorer vision than other primates.
For the snake detection theory to be true, primates would have to be amazing snake detectors. And there is some research that appears to support this idea.
For example, a 1993 study by Arne Öhman, a psychologist at the Karolinska Institute in Sweden, showed that people are able to visually detect snakes before they’re even consciously aware of the reptile, just as Isbell learned firsthand.
More recently, Öhman and his colleagues compared how quickly people detected snakes and spiders. Based on Isbell’s snake detection theory, they predicted that participants would detect snakes more rapidly than spiders because the arachnids were historically less of a threat to primates — and this is exactly what they found. They also discovered that snakes are more distracting than spiders, and concluded that “attending to snakes might constitute an evolutionary adaptation.”