Are you startled by snakes? You're in good company. A new study by Masahiro Shibasaki, PhD, and Nobuyuki Kawai, PhD, of Nagoya University in Japan, has found that lab-reared macaque monkeys that had never before seen a snake pointed more quickly to pictures of snakes than to pictures of flowers—a result that disinters the deep evolutionary roots of snake phobias among humans.

The researchers showed macaques a grid of nine pictures of flowers, one of which was hiding a snake. Later, they showed the monkeys a grid of nine pictures of snakes, one of which also included a flower. The monkeys were about 0.1 second faster at finding the hidden snake than the hidden flower. That difference may seem small, but it can give monkeys just enough extra time to evade a snake's split-second strike, says Kawai. Primates' brains may do this by routing images of snakes directly to the amygdala, short-circuiting the usual path through the cerebral cortex and allowing them to respond to threats without thinking. "You can see these two roads in action when you are startled by a snake, and then realize it's just a rope," says Kawai.

Humans, too, are quick to see snakes in pictures, and for millions of people that inborn startle response develops into a full-fledged phobia. But while 30 million years ago humans and macaques probably had a common ancestor whose main predator was snakes, modern humans have more to fear from electrical outlets, Kawai notes. "Evolution requires a long period of time," he says.

The research was published in the May Journal of Comparative Psychology (Vol. 123, No. 2).

—S. Dingfelder