Dusk starts to settle across the landscape. The dirt trail, lit by the last licks of sunlight, winds through the trees. The sound of your shuffling footfalls fills the quiet.

Suddenly, you stop. Something isn’t right. 

As you scan the path, you see it. No more than two paces ahead of you is a coiled rattlesnake ready to strike. Its rattle sounds. 

Fear of snakes is common. In a 2001 Gallup poll, 51% of American respondents listed snakes as their top fear, outranking public speaking and heights. 

Why are so many of us afraid of snakes? And more curiously, why does our unconscious mind recognize them as a threat before our conscious mind? 

Fear or threat detection?

According to Distinguished Professor Emerita of Anthropology Lynne A. Isbell, our relationship with snakes is an ancient one that reaches back to the evolutionary origins of primates. 

“Primates are really differentiated from other mammals by their heavy reliance on vision as the primary sensory modality with the environment,” said Isbell, a primatologist and fellow of the American Association for the Advancement of Science. “If you want to understand why primates evolved, you have to address why they have such good vision.” 

Isbell’s book The Fruit, the Tree, and the Serpent: Why We See So Well (Harvard University Press, 2011) centers around her Snake Detection Theory, which argues that the predator-prey relationship between snakes and primates across tens of millions of years enhanced our visual acuity.

“Avoiding predators and getting food are the two main selective pressures operating on organisms,” Isbell said. “The idea is that the unique conditions under which primates were living, that is, being active at night like other mammals but resting during the day in trees where sunlight penetrates, instead of in caves or burrows, allowed them to expand their visual sense as a way to avoid being eaten by snakes.” 

Molecular evidence bolsters the idea of ancient snake predation on primates. Researchers have found that primates in Africa and Asia, where cobras live, have evolved some immunity toward cobra venom. But primates in Madagascar and South America, where there are no cobras, have no immunity. 

Even today there seems to be a biological tendency for primates to perceive snakes as a threat. 

“If a young, naïve monkey watches a video of older monkeys reacting carefully to a snake, they will learn to do that,” Isbell said. “But if you splice in a flower where the snake was, they don’t learn to react carefully toward the flower.” 

Even captive-born-and-raised rhesus macaques, who likely haven’t been harmed by snakes, respond with “fearful fascination” to them. 

Primate vision is highly snake-sensitive 

Isbell’s decades-long research to formulate the Snake Detection Theory has taken her far beyond primatology to fields like neuroscience, evolutionary theory, genetics and molecular biology, to name a few. 

Her hypothesis spans the overall evolution of primates, but she homes in on anthropoids — monkeys, apes and humans — which have the most advanced visual systems among primates. 

While constricting snakes were instrumental in the origin of primates and the initial changes in their vision, venomous snakes, appearing later, were very important in facilitating the changes in vision that led to anthropoid primates, according to Isbell.

Two mammalian visual systems are key to this idea: the superior colliculus-pulvinar visual system and the lateral geniculate nucleus visual system. The first system enables our unconscious detection of an object in our environment. The second, slightly slower, system allows for conscious recognition of the object and the ability to assign intent to it. Both visual systems are more developed in primates than in other mammals, and even more so in anthropoids. 

“If you’ve ever had the experience of walking on a trail and coming across something that could be a snake, you might suddenly freeze or jump away before you actually recognize that there is a snake in front of you,” Isbell said. “The recognition taps into the conscious visual system, but the unconscious visual system gives you the ability to get out of danger more quickly.”