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Few sights strike fear in the heart as surely as the slithering of a snake. The odds that an adult is afraid of snakes are 1 in 1.96 (51%), making it the most common phobia. Yet very few of us will ever have an experience to justify that fear, also known as ophidophobia: in a single year, the chances that a person will die from contact with a venomous snake or lizard are just 1 in 37,420,000. 1 in 16,630 accidental deaths is caused by contact with venomous snakes or lizards.

Why does something with a relatively low chance of killing us fill us with such terror? The answer may lie in our biology. Lynne Isbell, a professor of anthropology and behavior at University of California, Davis, argues that fear of snakes is a phenomenon common to all primates that evolved deep within our evolutionary history.

In her book The Fruit, the Tree, and the Serpent: Why We See So Well, she makes the case that our primate ancestors were evolutionarily shaped by a fundamental fear of snakes; this fear, she believes, may be the reason we evolved such excellent vision. Those primate species that spent their time in trees eating fruit—the same habitat of many venomous snakes—are the ones that developed keen vision and larger brains. This adaptation not only allowed them to spot snakes more easily but to pick the ripest and sweetest fruit based on color. These fruit-loving species include the branch of primates that eventually gave rise to humans.

Isbell believes this long-held phobia, which once helped our ancestors survive, can explain why snakes are such a common theme in human mythology. Consider the Genesis story of Eve in the Garden of Eden: it was Eve's ability to spot the snake, and her attraction to the fruit, that led her to the Fall. Isbell notes how well this story matches evolutionary history: our ancestors' propensity for eating fruit, in addition to the threat of snakes, led them to evolve keener vision and hence a larger neocortex, the part of the brain responsible for vision processing. Incidentally, Eve's punishment for eating the fruit was pain in childbirth, which Isbell points out is a consequence of giving birth to large-brained babies.

For more on snakes see this entry in the Staff Blog.