The first primates, like most present day species, were arboreal, or tree-living. They were small, defenseless animals, and their survival depended on their ability to escape their enemies. Forelimbs ending in strong, flexible digits enabled them to get a secure grip on high branches where no big cat could follow. Thus, hands were an evolutionary adaptation to tree living. Among primate tree dwellers even the feet begin to look and function like hands. Some of lemurs, monkeys, and even the bulky orangutan can hang by their feet. It seems apparent that tree-living, over many millions of years of evolution, favored the development of the hand. But what about the face? What kind of head is best suited for tree-dwellers?
On the ground a sense of smell conveys a lot of information; above ground very little. In the trees you have to rely on your eyes, not your nose, to catch and pick off food, such as fruit, and hang onto a branch to keep out of danger. And for that you have to be able to see in depth, that is to say, you must have a three-dimensional vision. So, evolution led the primates to have eyes located one beside the other, not wide apart on each side of the head, like cow's or horse's eyes. Thus, during the long course of the evolution of the primates, their sight grew keen at the expense of their sense of their smell. While their snouts dwindled in size, their eyes moved forward in their heads. In other words, they developed faces. Like hands, faces seem to be an adaptation to an arboreal way of life.
Stereoscopic vision made life safer for the tree primate because it made it a better judge of distance. One miscalculated jump to a far bough, after all, and it might land in the jaws of the long-snouted non-stereoscopic creatures waiting patiently below. Small, weak, and defenseless, the early primates had none of the fighting equipment of ground animals such as claws, spines, tusks, and horns. In order to stay alive, they had to rely on their agility and good sight. As their eyes developed, so did the part of their brain that controls seeing. In turn, the growth of the brain gave their intelligence a great boost. The development of hands, face, eyes, and brain were all interrelated, each depending on the other.