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It's hard to ignore a snake. Even the word provokes a reaction. From serpents in the Bible to "Snakes on a Plane," these carnivorous reptiles have been objects of both fear and fascination for centuries. Well, now, researchers think that they've discovered a reason for our involuntary reaction.

As NPR's Jon Hamilton reports, snakes seem to occupy a special place in the primate brain.

JON HAMILTON, BYLINE: In 1992, an anthropologist named Lynne Isbell was running through a glade in central Kenya. She was hurrying to get a photograph of Mount Kenya before the sun set. And then, Isbell froze her in her tracks.

LYNNE ISBELL: I stopped just in front of a cobra that had detected me before I detected it. And it was raised with its hood spread out.

HAMILTON: Another stride and Isbell might not have lived to talk about her experience. Yet, something had caused her to stop before her conscious brain even had a chance to think cobra. Isbell, who is now at the University of California, Davis, says she's spent the past couple of decades trying to understand that reaction.

ISBELL: At first, I thought it was luck. But now I'm pretty sure that it's not luck. It's a reflection of 60 million years of evolutionary history working on my visual system.

HAMILTON: Isbell developed a theory that snakes are a major reason that humans and other primates evolved really good vision.

ISBELL: We have our forward-facing eyes. We have our excellent depth perception. We have very good visual acuity, the best in the mammalian world. We have color vision. So there has to be some sort of explanation for it.

HAMILTON: Her research showed that primates in parts of the world with lots of poisonous snakes evolved better vision than primates elsewhere. Other scientists showed that people and monkeys can detect snakes before becoming consciously aware of them.

Isbell thought that there should be some evidence of this ability conferred by evolution in the brain. So she teamed up with researchers in Japan to study the brains of two macaque monkeys that had probably never seen snakes.

ISBELL: They showed the monkeys images of snakes, faces of primates, hands of primates, and simple geometric shapes to see which neurons would be most selective for those images.

HAMILTON: And they found something remarkable in a part of the visual system that's unique to people, apes and monkeys.

ISBELL: There are neurons that are very sensitive to snake images and much more sensitive to them than the faces of primates.

HAMILTON: Which is surprising because monkeys and other primates have brains that are highly sensitive to faces. Isbell says the finding appears to explain her experience in Kenya of stopping mid-stride, before she even realized she was seeing a cobra.

ISBELL: This part of the visual system appears to be the sort of quicker, automatic visual system that allows us to respond without even being consciously aware of the object that we're responding to.

HAMILTON: The finding was not a complete surprise to other researchers who study primates' fear of snakes.

SUE MINEKA: There have been a lot of people suspecting that there must be something like this must be going on. And these people found out what it was.

HAMILTON: Sue Mineka at Northwestern University says the new study appears to explain her own research showing that even monkeys raised in labs, where there are no snakes, can quickly learn to fear the reptiles.

MINEKA: Humans, we've heard all sorts of bad things about snakes. But what's interesting is that a lab-reared monkey, who has never had any encounter with a snake before, never heard anything bad about a snake before, quickly picks up a fear.

HAMILTON: Mineka says it's not clear whether the brain response of the monkeys in this study showed they were truly afraid of snakes or just had an innate ability to recognize them. But Mineka says the study does suggest that both monkeys and humans have evolved brains that are well prepared to learn to fear snakes.

MINEKA: It's identifying a possible mechanism by which this prepared learning could occur because there is a distinct neural signature that could then be associated with threat.

HAMILTON: The new research appears in the Proceedings of the National Academy of Sciences.

Jon Hamilton, NPR News.

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