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.