Snakes on a Plane, snakes in a can, a rubber snake under someone's chair—just thinking we see a snake is enough to make some of us leap away like a suddenly expert dancer. And it might be those slithery serpents that helped us evolve to see as well as we do.
What is it that makes us react to snakes so quickly? The combined efforts of neuroscientists and one anthropologist may provide some answers about the place of the snake in our brain and our hissssstory.
Lynne Isbell, an anthropologist at the University of California, Davis, had theorized back in 2006 that snakes were the main reason that primates developed the best vision of all mammals. She published her findings in the book The Fruit, the Tree, and the Serpent: Why We See So Well.
Isbell says there were two "pulses of selective pressure" that brought about the unusually sharp vision we have as human beings.
The first pulse was about 100 million years ago on the supercontinent Gondwana, when constricting snakes were evolving at the same time as our ancestors, the rodent-like mammals who would become primates. Those mammals rested in the trees in the light of day, where they fell prey to the snakes. The combination of access to bright light and the presence of those constricting snakes created the selective pressure for those early mammals to develop better vision.
The second pulse, Isbell says, was when venomous snakes began to evolve 40 million years later, putting pressure on Old World primates to refine and improve their vision so they could avoid the even-more-dangerous reptiles.
Primate vision varies, and it's not nearly as good in Madagascar, where there are no venomous snakes, Isbell says. The primates there "don't have a fovea, a pit in the retina that allows our central vision to be very sharp so that we can see fine detail. So their visual acuity isn't the best. It's still better than other mammals'; it's just not as good as ours," she says. This is likely because primates in Madagascar did have the constrictors but not the venomous snakes to contend with, which would have improved their vision even further.
A series of emails, which also involved scientists at the University of Brasilia, eventually led to the study that appeared this week in the Proceedings of the National Academy of Sciences. The study shows that neurons in a very specific area of the brain—pulvinar neurons—in macaque monkeys responded selectively to images of snakes, and responded more quickly and strongly to snake images than to images of monkey faces and hands or geometric shapes.
The Japanese macaques involved in the study lived in a high-walled enclosure and hadn't had exposure to snakes, Isbell says, suggesting that the quick neural response is a hardwired one.
These findings "provide neuroscientific evidence in support of the Snake Detection Theory, which posits that the threat of snakes strongly influenced the evolution of the primate brain," according to the study.
What Doesn't the Study Show?
Yes, primates have a quick response to snakes—we can see them and get the heck out of their way, Isbell says. But that doesn't explain the paralyzing phobia some people have of the slithering reptiles.
"People want to connect what we've done with fear and phobias, but in fact we haven't addressed emotion in this study," Isbell said. "We've only looked at vision. So we've looked at the very first step. How does the image of the snake get into our brains to begin with?"
Fear, she says, is something that comes later for some people. "There's another step that leads to fear, but our study doesn't address the emotional aspect of it."
What Does It Mean?
If you live in an area where there are venomous snakes, you might have had instant empathy with the speedy reaction of those neurons to the images of a snake. People still die of snakebite, especially in the tropics, says Isbell, who encounters snakes in her fieldwork.
"It's important to be able to see them before they see us so we can stop in time to do that acrobatic jumping away," she said.