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Did Mammals Evolve Snake Eyes?

Our cross-cultural aversion to slithering, bastard snakes isn't just hard-wired into our brains, it could also be the very reason our pre-primate ancestors developed such sophisticated vision. A new study helps confirm what some scientists call the...

Image from Flickr, via USFWS/Southeast

Back when Adam and Eve were letting it all hang out in the Garden of Eden, a dastardly serpent convinced Eve to eat the forbidden fruit by telling her, “your eyes will be opened” by its sweet, sweet taste. A recent study suggests it wasn’t the apple that helped Eve—or whatever ancient humanoid you consider to have been our forebearer—to open her eyes, but it was that snake seducer that may indeed be responsible for what—and how—humans see.

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Anthropologist and behavioral ecologist Dr. Lynn Isbell of the University of California, Davis, author of The Fruit, the Tree, and the Serpent, has long argued the Snake Detection Theory of brain development, which posits that the need to avoid slithering predators was responsible for the evolution of keen eyesight in our primate ancestors.

Ophidians, the group of squamatereptiles including snakes, first appeared more than 110 million years ago, when dinosaurs still ruled the earth. In the eons since, they have hardly changed basic form: they have inhabited the same long, scaly, coldblooded bodies since the humid, tropical climate of the Cretaceous era, when terrifying monsters like the 40-foot, 2,000 pound Titanoboa roamed the earth. That dude died out (Thank GOD), but snakes are still around, doing what they do—they now wiggle around every continent on Earth except for Antarctica, where it’s too cold for them to survive.

Millions of years before they started producing fast-acting venom, snakes used their coils to suffocate their prey. When our nocturnal pre-primate ancestors left their burrows and began to inhabit trees and bushes around 100 million years ago, those squeezer snakes were our only real predator—birds of prey and large cats didn’t start feeding on mammals until millions of years later. In order to survive, animals needed to quickly identify snakes and get the hell away, before taking the time to make a conscious decision to escape. Natural selection for fast reactors resulted in a huge leap in visual acuity, the theory goes, making possible vast improvements in depth perception; increased ability to see colors; and perhaps the development of early forms of communication via pointing, i.e. making someone else look at the important thing you’re seeing. That development possibly spurred the evolution of deadly venom in snakes as well, as new techniques were needed to attack mammals that had gotten so much better at evading the hug of death.

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Psychologists have long documented the fact that humans and primates see snakes faster than we see other things, like spiders or flowers, and that this perception bias is probably responsible for our ancient and cross-cultural phobia of snakes (thank you, Eve). This month, Isbell, along with neuroscientists in Japan and Brazil, announced new evidence for the Snake Detection Theory. For their study, published in The Proceedings of the National Academy of Sciences, they monitored the brains of two rhesus macaque monkeys raised in captivity, neither of which had ever had a recorded encounter with snakes. The monkeys were shown four kinds of pictures: macaque faces that were either agitated or neutral; primate hands; abstract shapes like circles or squares; and both coiled and uncoiled snakes. By monitoring the electrical pulses in the monkey brains, they found that a greater percentage of the neurons in the pulvinar, which takes up 40 percent of the thalamus in humans and primates and is related to optical attention and eye movement, reacted strongly to snakes than to the other visual cues. The neurons that were “snake-best” fired 25 milliseconds faster than the neurons that were “shape-best” and 15 milliseconds faster to the “angry face-best.”

I spoke to Dr. Isbell about innate versus learned fear, having dinosaurs on the brain, and future experiments that may provide even more evidence for the Snake Detection Theory.

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VICE: How does the speed and intensity of the firing neurons correlate to the fear of snakes that is pretty widespread among humans?
Dr. Lynn Isbell: We didn’t examine fear. What we did do was look at the first step in the process of registering fear, and the common phenomenon that people have experienced where they see a snake, and somehow, before they even know what they are doing, they manage to evade it. The pulvinar has connections to another brain structure, the superior colliculus, that, when you stimulate, causes animals to dart or freeze or otherwise react through movement.

In order to experience fear the brain has to somehow get the image into the brain. So for primates that would be through visual detection, since we’re so dependent on vision. We’re trying to understand how the brain incorporates that vision, that image of the object in front of it. In this case, that is the image of a snake. Once the brain picks that up, then the question is what does it do with it after that. For some it might go to fear. In others it might not. We don’t all have fear of snakes.

A lot of people get confused about innate fear and learned fear and I want to point out that I don’t really like the word “innate.” When we talk about “innate,” we mean that in our study, we found a mechanism that enables us to avoid snakes quickly. Other scientists have shown that primates have an ability to learn to fear snakes very quickly. And that makes sense. Because we’re social animals and we learn from others around us. So if we observed somebody expressing fear of a snake when we are very young, we’re more likely to express those fears ourselves. If we don’t see that happening we don’t necessarily learn that fear. But it does appear that we have an easier time acquiring fear of snakes than fear of other things that might be deadlier, like cars. But we didn’t evolve around cars—the selection pressure hasn’t been around long enough to give us “car phobia,” for instance. It seems that we are evolutionarily prepared to be able to fear snakes quickly. That’s the distinction.

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So, you’ve established that there’s a special place where snakes dominate our brain.
There’s at least one special place. The rest of the brain hasn’t been examined. It wasn’t just guessing that made us look at the pulvinar. The pulvinar has been known to be involved with directing attention to important objects in the environment. So that seemed to be a good place to look for these neurons that are highly sensitive to snakes.

Both snakes and our pre-mammalian ancestors were around at the time of the dinosaurs. Do you think that there are neurons in our brains that are dedicated to recognizing T. Rex?
The difference between dinosaurs and snakes is that dinosaurs aren’t around any more. So whatever visual cues animals might have used to detect them wouldn’t necessarily be helpful to them now. Because dinosaurs disappeared 65 million years ago. Snakes have pretty much looked the same as they have since they started eating mammals. So cues for detecting snakes have always been there and still are.

This recent experiment is just the first step in providing evidence for your theory. What’s next?
One of the most obvious questions that people have raised as a result of this recent study was OK, maybe you can detect snakes very quickly, but what about other predators? Why not compare the reaction to snakes with what happens in the brain when primates see large cats and birds of prey? My argument is that if snakes are largely responsible for the visual system of primates, then they should have a special place in the primate brain. So primates ought to be able to detect them more quickly than they could detect leopards or eagles. My guess is that there actually is a special place in the primate brain—where the neurons will be faster acting, with a stronger response to snakes. That’s because snakes are not a threat when they are far away. We probably wouldn’t even see them then, and they are only a problem when they are close up. But when other predators are as close as a snake, you’re already dead. So there would be no selection favoring animals that could respond to those predators with the same urgency that mammals respond to snakes, with short latencies and quick responses. There would be stronger selection for animals to see cats and eagles from farther away when they could do something about it. I’m not saying that there won’t be neurons that fire in response to big cats. I’m sure there will be. Just as there are neurons that fire to when primates recognize other primate faces, these things are all important in the lives of animals, of prey animals. I just think that the behavior of the neurons might be a little bit different. @roseolm

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