Also, if you live in a massively 3-D space, like a tree canopy—where up and down matter as much as back and forth and side to side—and you’re trying to catch bugs that keep flying away from you, your ability to judge depth and direction suddenly matters a lot. Your brain might have to get bigger, too, since processing a lot of 3-D visual data takes a lot of computational firepower. Indeed, when paleontologists measure primate fossils’ skulls, the more stereoscopic the eye placement, the bigger the brainpan.
