The report in the Proceedings of the National Academy of Sciences offers new evidence to support the notion that primates evolved vision skills that enable them see snakes quickly in order survive the threats snakes pose in the jungle.
"It really strengthens the argument that snakes are very important for the evolution of primates," lead co-author Lynne Isbell, a professor in the anthropology department at the University of California Davis, said.
Certain neurons respond "selectively" to images of snakes, and they outpace comparable neurons that react to visuals of faces, hands or geometric shapes, the researchers said.
"Snakes elicited the strongest, fastest responses," said the study, co-authored by Quan Van Le of the University of Toyama and researchers at the University of Brasilia.
The research was done using two young macaque monkeys that were born on a national monkey farm in Japan. Researchers said that they believe the monkeys had no chance to encounter snakes prior to the experiment.
Scientists surgically implanted micro-electrodes in a part of the brain known as the pulvinar, which is involved in visual attention and the fast processing of threatening images.
Then they showed the monkeys various colour images on a computer screen, including snakes in various positions, threatening monkey faces, pictures of monkey hands and simple shapes like stars or squares.
Seeing a snake caused the brain to fire off rapid fear responses that were unparalleled by those seen in reaction to faces, hands or shapes.
Researchers found that of about 100 neurons that fired off when presented with at least one of the image types, 40 percent had the largest response to snakes. That was the biggest group, followed by almost 29 percent that were superior at responding to faces.
While researchers have long known that primates have an uncanny ability to see snakes even in cluttered surroundings, the latest data adds a new answer to the question of why.
"Snakes are largely responsible for the origin of primates. Vision is what separates primates from other mammals. A lot of the structures in our brain are devoted to vision," said Isbell, who wrote a book on the topic in 2011 called "The Fruit, the Tree, and the Serpent: Why We See So Well."


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