![]() ![]() Neurobiologist John Allman picked up on Cartmill's hypothesis and expanded it to focus on nocturnal predation. Cartmill thought that the reduction in their ability to smell was a side effect of the eyes' convergence, simply because the space available for the nose and its connections to the brain became smaller as it was crowded out by the eyes. Early primates, for example, hunt by sight rather than by scent. Cartmill thought his explanation was the most elegant, because it also explained other evolutionary changes that are distinctive to primates. ![]() That would help them to better locate and more effectively take down their prey, whether that's a leopard stalking a gazelle or a raptor snatching a rabbit in its talons, or one of our primate ancestors grabbing an insect from the branch of a tree. ![]() Predators are best served, ostensibly, by having extremely good depth perception. So, in 2005, biological anthropologist Matt Cartmill proposed a different idea: the "visual predation hypothesis". The problem with Collins’ hypothesis is that many animals that thrive in trees have eyes on the sides of their heads – squirrels, for instance. “The price of failure was to drop many metres onto a ground inhabited by carnivorous beasts," wrote visual psychotherapist Christopher Tyler in 1991. After all, the stakes for failing to work out the true distance between trees were high. In the decades since, it has been expanded and refined, but the basic idea that our ancestors evolved forward facing eyes to accurately judge distances while leaping from tree to tree remained central for quite a while. Still, the researchers say, detecting snakes was definitely a beneficial side effect regardless of why better vision evolved.Collins' idea has become known as the "arboreal locomotion hypothesis" – arboreal meaning living in trees. The team didn’t find any correlations between snake exposure and primate vision, concluding that snake attacks did not drive the evolution of better eyesight. Led by behavioral ecologist Brandon Wheeler of the Cognitive Ethology Laboratory at the German Primate Center, the team tested the snake hypothesis by looking at variations in modern primates’ visual skills (in terms of stereoscopic vision, as measured by the closeness of the eyes) to see if the primates with the best eyesight had the longest evolutionary history of coexisting with snakes and the greatest likelihood of encountering and being attacked by them. Later, some monkeys and apes in Africa and Asia started to live alongside venomous snakes, which led to even more visual advancements.īut the idea may not hold up, according to the authors of a recent study in the Journal of Human Evolution. In 2006, anthropologist Lynne Isbell of the University of California at Davis argued that early primates were stalked by constricting snakes, and it was highly beneficial to see these camouflaged predators before it was too late. More recently, snakes came into the picture. Another hypothesis is that primates needed to see well to pluck fruits from the ends of tree branches. But that hypothesis lost favor in the 1970s after biological anthropologist Matt Cartmill, now at Boston University, pointed out that many other acrobatic, tree-dwelling animals like squirrels get by without such an advanced visual system.Ĭartmill offered his own explanation, called the “visual predation hypothesis”: early primates needed superb visual skills to hunt and grab insects. The ancestors of primates needed to accurately judge the distances between tree branches before taking a leap, so the theory went. In the early 20th century, scientists attributed primates’ keen sense of sight to their arboreal lifestyle. Compared with many other mammals, primates have more closely spaced, forward-facing eyes that allow for a lot of overlap between each eye’s visual field, which in turn gives primates 3-D, or stereoscopic, vision and a good sense of depth perception. Good vision is a hallmark of the primate order. Snakes have been preying on primates for millions of years, and some researchers think they might be the reason we-and our fellow primates-have such good eyesight. And for good reason- snakes eat primates. ![]() We humans aren’t alone in our aversion to snakes. ![]()
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