Look sideways without turning your head. Another person can instantly see where you are looking because your dark iris moves against a bright white background.
This may be a small but important human adaptation. The cooperative eye hypothesis proposes that visible white sclera evolved because our ancestors benefited from sharing attention.
A glance could silently direct others toward food, danger or a task—helping groups coordinate, teach and communicate.
Experiments support part of this idea.
Both humans and chimpanzees identified gaze direction more accurately from human eyes than chimpanzee eyes, particularly when images were small or shadowed. When researchers digitally gave chimpanzees human-like white sclera, their gaze became easier to read.
What is well established is that white sclera increases gaze visibility. What remains debated is why it evolved.
Some gorillas and other primates also have relatively light sclera, and a 2025 scientific review concluded that the evidence connecting human eye coloration specifically to cooperation remains insufficient.
Humans are not the only animals with white or pale sclera. It also appears in:
- Chimpanzees: In one large study, 15% had white sclera and another 41% had tan, brown or partly light sclera.
- Gorillas: Many individuals have visible pale sclera; one study found substantial variation among gorillas.
- Orangutans: Some species show strong contrast between the iris and sclera, approaching the human pattern.
- Bonobos: They can also possess relatively light, visible sclera.
- Other mammals: White sclera occurs sporadically in species outside the apes. A survey of 70 zoo species found at least one white-sclera individual in 19 species.
Dogs, horses and some other animals may briefly expose the white of the eye—especially when frightened or looking sideways—but it is usually less consistently visible.
What remains unusual about humans is the combination: uniformly white sclera, a dark contrasting iris and horizontally elongated eyes that expose the sclera continuously.
This makes our gaze especially easy to read.
Other animals have “eye whites,” but humans appear to have turned them into a highly effective communication signal.
Our eyes undoubtedly reveal attention unusually well.
But whether cooperation created the whites of our eyes—or later took advantage of them—remains an open evolutionary question.
