Snakes Key to Primate Evolution Suggests Study by Viet Researcher in Japan

A study by a Vietnamese researcher working at a Japanese university adds support for the theory that the threat posed by snakes may have been key to the evolution of the primate brain.

In a study authored by Toyama University researcher Quan Van Le two young macaque monkeys were surgically implanted with micro-electrodes in the brain’s pulvinar, the center of visual attention and quick processing of threatening images.

The monkeys had been born on a national monkey farm in Japan and are thought not to have had any prior encounters with snakes. They were shown various color images on a computer screen, including non-threatening images like shapes of stars or squares, monkey faces with threatening expressions and images of various other parts of monkeys, as well as of snakes in various positions.

The snake images caused the macaques’ brains to fire off fear responses whose intensity and speed far exceeded reactions to any other images. Of 100 neurons that fired in response to all the various image types, 40% showed the most marked response to snakes. The second biggest number, 29%, showed stronger responses to faces.

Primates have long been known to exhibit an uncanny ability to spot snakes in even the most visually complex environments. That acuity has suggested to some that snakes had been a major threat to primates during tens of millions of years of evolution.

The researcher best known for advancing the importance of snakes to the evolution of the primate brain is UC Davis anthropology professor Lynne Isbell who authored the book The Fruit, the Tree and the Serpent: Why We See So Well.

“Snakes are largely responsible for the origin of primates,” she said. “Vision is what separates primates from other mammals. A lot of the structures in our brain are devoted to vision.”

Having supported her book with what she calls “indirect evidence”, Isbell is delighted by the results of Le’s study.

“Here is the first time that somebody has come along and actually tested some of the predictions in the book and I am really gratified that it was supported,” she said. “It really strengthens the argument that snakes are very important for the evolution of primates.”

She expressed hope that future research could focus on responses to other predators by primates and possibly reveal more insights into how the human brain responds to such threats.

However, some prior research suggests that a deep fear of snakes isn’t necessarily innate to all primates. One showed that Malagasy lemurs of Madagascar, which has no venomous snakes, don’t exhibit the usual primate fear of snakes.

The study, which also involved researchers at the University of Brasilia, was published in the scientific journal Proceedings of the National Academy of Sciences.