Humans, like most other mammals, are primarily sight oriented, which means that our eyes are our dominant sense organs. The reason we rely so heavily on vision most likely lies in our evolutionary history. Millions of years ago, the ancestors of Homo sapiens were small, tree-dwelling primates. Their way of life -- leaping from limb to limb and paying a high price for mistakes -- favored animals with good vision and selected against individuals that could not see as well.

Life in the trees required not only visual acuity, but good depth perception as well. An animal whose life depended on knowing precisely where the next handhold was had to be able to estimate the ever-changing distances it saw while moving quickly through the tree canopy. To do this, the animal needed eyes positioned near the front of its head, like an owl, for example, rather than on the sides of its head, like a deer.

What is most important about a forward-facing eye position -- often called binocular, or stereoscopic, vision -- is that it allows the visual fields of each eye to overlap. Overlapping visual fields provide our brain with simultaneous views of the same area from two slightly different angles; each eye picks up visual information that the other eye doesn't. When the two images arrive at the back of the eye, they stimulate the retina, which in turn sends nerve impulses to the brain.

By comparing the pattern of nerve stimuli produced by the retinas, the brain can determine an object's distance relative to other objects in view. The closer an object is to the eyes, the greater the difference between the two images and the nerve patterns they stimulate. The farther away the object, the more the two patterns resemble each other. You can observe this with your own eyes by alternating the eye with which you look at an object. Cover one eye, then switch and cover the other. The object will appear to move when you switch back and forth. But notice, an object at arm's length will appear to move more than an object on the other side of the room. This explains why distant landscapes often look like flat pictures.

View optical illusions in this NOVA classroom activity to investigate the human visual system.