Researchers from the U.S., Japan and Brazil say they have found the first brain-based evidence backing a theory that we are destined from birth to be scared of snakes.

The theory maintains that natural selection killed off all the primates unable to spot snakes out of the corner of an eye.

Hominids, chimps and monkeys which were too slow in detecting predator snakes tended not to live long enough to leave descendants.

To test the theory that primates can detect snakes faster than innocuous objects, the researchers, led by Lynne Isbell, a professor of anthropology at the University of California, Davis, implanted electrodes into the brains of two Japanese macaques (Macaca fuscata).

The monkeys, a male and a female, had been born and raised in captivity and had never encountered snakes.

The researchers showed them images of macaque faces with either angry or neutral expressions, macaque hands in various positions, geometric shapes and ... snakes either coiled or uncoiled.

While doing so, they measured the electrical spikes from individual neurons in the monkeys’ brains.

The images of the snakes elicited “the strongest, fastest responses,” the researchers wrote in the Proceedings of the National Academy of Sciences of the U.S. And the responses were not reduced when the images were blurred.

The researchers suspect a contest has been going on for millions of years in tropical ecosystems between monkeys and the constrictor or venomous snakes that feed on them.

So the existence of snakes has been one reason for the primates getting smarter.

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