Take a stroll through a zoo and, if you're paying attention, you might notice that most animals can be sorted into two groups. There are those animals with eyes on the sides of their heads – chickens, cows, horses, zebras – and then there are those with eyes that are closer together on the front of their face, like monkeys, tigers, owls, wolves. All the humans visiting the zoo are obviously in the latter camp. What's behind this divide?
There's a trade-off when it comes to eye placement. As the eyes move forward along the face, two fields of vision overlap. It's that overlap – the slightly different perspective on the scene in front of you that each of your two eyes sends to your brain – that allows you to perceive depth. Animals with sideways-facing eyes may not have this well-developed depth perception, but they are able to see an extremely wide panorama instead.
Eye placement probably evolved for different reasons in different groups of animals. Some turtles, for example, have eyes on the side of their heads but process optical information as if their eyes were forward facing – probably because when they retract their heads into their carapaces their eyes only receive light from the front, as if their eyes faced forwards. But how did our corner of the family tree, the primates, wind up with forward-facing eyes? There are lots of ideas.
In 1922, Edward Treacher Collins, a British ophthalmologist, wrote that early primates needed sight that would "enable them to swing and spring with accuracy from bough to bough…to grasp food with their hands and convey it by them to their mouths". As our primate ancestors moved into the trees to escape their predators, the needs to navigate the tree branches and to grab rapidly fleeing prey with their hands, he argued, meant that evolution favoured a visual system with good depth perception.
Collins' idea has become known as the "arboreal locomotion hypothesis" – arboreal meaning living in trees. 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. After all, the stakes for failing to work out the true distance between trees were high. “The price of failure was to drop many metres onto a ground inhabited by carnivorous beasts," wrote visual psychotherapist Christopher Tyler in 1991.
The problem with Collins’ hypothesis is that many animals that thrive in trees have eyes on the sides of their heads – squirrels, for instance. So, in 2005, biological anthropologist Matt Cartmill proposed a different idea: the "visual predation hypothesis". Predators are best served, ostensibly, by having extremely good depth perception. 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. Cartmill thought his explanation was the most elegant, because it also explained other evolutionary changes that are distinctive to primates. Early primates, for example, hunt by sight rather than by scent. 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.
Neurobiologist John Allman picked up on Cartmill's hypothesis and expanded it to focus on nocturnal predation. Not all predators, after all, have forward facing eyes. Cats, primates and owls do, but not mongooses, tree shrews, and robins. Allman's contribution was to suggest that forward-facing eyes proved beneficial for creatures that hunt at night, such as owls and cats, because they can take in more light than sideways-facing eyes. As it happens, early primates were also nocturnal hunters, and their adaptation for nighttime predation may have granted forward-facing eyes to all their descendants, including our own species.
Theoretical neurobiologist Mark Changizi has yet another idea. In 2008 in the Journal of Theoretical Biology, he offered up the "X-ray vision hypothesis". In short, forward-facing eyes allowed our ancestors to see through the dense leaves and branches in their forest habitat. The catchy name for his hypothesis comes from a curious phenomenon. "When you hold up your finger vertically and fixate your eyes on something far beyond it," he writes, "you perceive two copies of your finger, and both copies of your finger appear transparent." Thus, you have the ability to "see through" your finger, as though you were seeing with X-ray vision.
The clutter problem is unique to large-bodied animals in forests, such as primates. Smaller animals, such as squirrels, suffer less clutter because their heads are small enough to see in between branches and leaves. And large animals in non-forest environments can do just fine with eyes that face sideways.
So the reason we all have peepers on the front of our heads is by no means settled. Each hypothesis has its own strengths and weaknesses. But whether our eyes moved forward to account for leaping through branches, chasing after tasty bugs, or to see through leaves, at least one thing is certain: it all comes down to life in the trees.
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