Human eyes have white sclerae, but scientists are not sure why. The cooperative eye hypothesis suggests this helped our ancestors signal where they were looking. However, a 2025 review in Biological Reviews said the idea lacks evidence. Researchers now suggest we should consider other reasons like sunlight protection or evolution.
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.
One leading idea is called the cooperative eye hypothesis, which says we evolved conspicuous eyes because our ancestors could benefit from signalling to others which way they were looking. So some of them could follow another person's glance and know if there was food or danger in a particular direction.
However, a review published in 2025 in Biological Reviews said the hypothesis has not accumulated enough evidence. For example, in 2023, scientists found that the chimpanzees they were studying in Uganda were able to say where other individuals were looking even with darker sclerae. Instead, the review asked scientists to consider other possibilities, such as protection from sunlight and random evolutionary changes.