Primates have incredibly complex and accurate visual systems. There’s no shortage of evolutionary reasons why good vision might have evolved among primates; it may have helped our ancestors locate colorful fruits in trees, catch insects, or identify other group members. But one idea in particular taps into what is still a major fear for humans: snakes.

According to “Snake Detection Theory,” these slithering creatures have played a major role in the evolution of our brains, and, more specifically, our visual systems. Over more than 50 million years, primates have had a complicated relationship with snakes as predators, competitors, and prey. Since both snakes and primates flourish in tropical regions, there have been plenty of opportunities for primates and snakes to interact—and adapt to each other's presence—on an evolutionary time scale. Snakes are hard to spot, so if their threat was strong enough, natural selection might have favored primates with keen eyesight and quick reaction times.

Of course, until there’s solid evidence that we have snakes to thank for our keen vision, it’s just a lot of evolutionary hand waving. But in this week’s issue of PNAS, an international group of researchers lend some solid support to the Snake Detection Theory.

An area of the brain called the pulvinar nuclei is disproportionately large in humans and other primates compared to other mammals. This region is thought to be involved in “selective spatial attention,” which is the task of picking important visual stimuli out of an otherwise cluttered environment. If the threat of snakes helped shape primates’ brains, it’s likely that this area would have been affected.

To test this idea, the researchers inserted probes into the brains of two Japanese macaques. These monkeys—one male and one female—were captive-bred and had never encountered snakes. As the activity of neurons in the pulvinar region was monitored, the macaques were shown various images, including pictures of monkeys’ faces, monkeys’ hands, simple geometric shapes, and snakes. The brightness and size of all these stimuli were kept constant in order to control for any visual biases. In total, 91 individual neurons were tested with each type of image.

First, the neurons were categorized by the type of image that triggered the most activity. Out of the four categories, more than forty percent of the tested neurons responded most strongly to the snake pictures. About thirty percent of the neurons showed the highest activity in response to the faces, and almost twenty percent responded most strongly to the hands. Only about 12 percent showed the most activity in response to the geometric shapes.

When the researchers looked at the average size and speed of the response, they found similar results: snakes provoked the strongest, fastest responses. On average, the neural impulses triggered by snakes were much larger than those triggered by the other types of images. Furthermore, the average speed of a neuron’s response to a snake image was just 55.4 milliseconds, which was significantly shorter than response latencies to the other three types of images.

In short, the pulvinar region appears to be especially attentive to images of snakes. It didn’t matter whether the snakes were coiled up or stretched out; the macaques’ neurons responded similarly to snakes in each position. Since these monkeys had likely never seen a snake before, the neural responses appear to be hard-wired rather than a result of the experience.

These results don’t mean that a fear of snakes is innate in humans or other primates; instead, the study suggests that being able to quickly notice snakes may have been an advantageous trait in our distant past. Of course, many other selective pressures had a hand in shaping primates’ brains as well. For instance, in this study, angry monkey faces also elicited relatively strong responses in the macaques' pulvinar neurons, suggesting that the recognition of facial expressions is an important skill for primates.

Follow-up studies are needed to tie up some loose ends; for example, maybe monkeys have a completely different reason to react to snake-shaped things. But in light of the results, even the snake-haters among us should accept that we may owe a debt to our long, skinny, slithering friends for giving us the gift of great vision.

PNAS, 2013. DOI: 10.1073/pnas.1312648110 (About DOIs).