Why are the whites of human eyes white?

A: The tough outer covering of the eyeball is called the sclera. In humans, the sclerae (in both eyes) are white. This allows someone standing a distance away to easily say which way a person is looking.

Scientists are not sure why human sclerae are white. Chimpanzees may have white, brown or darker sclerae. Bonobos and Sumatran orangutans have light brown sclerae. On the other hand, researchers have documented pale sclerae in golden langurs and southern pig-tailed macaques and some leopards and sea lions.