Writing in the Proceedings of the National Academy of Sciences, the researchers report they found specific nerve cells in the brains of rhesus macaque monkeys that respond to images of snakes.
The snake-sensitive neurons were more numerous, and responded more strongly and rapidly, than other nerve cells that fired in response to images of macaque faces or hands, or to geometric shapes, the researchers said.
"We're finding results consistent with the idea that snakes have exerted strong selective pressure on primates," said anthropologist Lynne Isbell of the University of California, Davis, who first proposed the theory being tested by the Japanese and Brazilian researchers.
In a hypothesis first put forward in 2006, Isbell argued our primate ancestors evolved good, close-range vision primarily to spot and avoid dangerous snakes.
Japanese researchers studied the neural mechanisms responsible for emotion and fear in rhesus macaque monkeys, especially instinctive responses that occur without learning or memory.
"The results show that the brain has special neural circuits to detect snakes, and this suggests that the neural circuits to detect snakes have been genetically encoded," Hisao Nishijo of Toyama University said.
The monkeys in the study were reared in a walled colony and none had previously encountered a real snake, the researchers said.
"I don't see another way to explain the sensitivity of these neurons to snakes except through an evolutionary path," Isbell said in a UC Davis release.