A mere look can lead another individual to a doorway, implement, or threat without head movement or verbal communication. The human eye renders this signal particularly noticeable due to the contrast of a dark iris against a widely visible, light background.
The sclera forms the robust outer layer of the eye. In humans, the visible portion is typically depigmented and appears white under the clear conjunctiva. Other great apes may exhibit light or even white patches of sclera, yet humans are distinctive in exhibiting a horizontally elongated eye opening with sclera that is consistently light from the iris to the outer corners in nearly all cases.
The cooperative eye hypothesis suggests that this prominent feature was advantageous as humans greatly depend on shared focus and coordination. It serves as a significant evolutionary rationale, rather than a verified sequence found in fossils. Subsequent research backs the notion of a perceptual advantage from having white sclera while challenging the older assertion that white sclera is unique to humans.
Reasons behind the clarity of human gaze
Determining gaze direction involves more than just color. An observer must discern the shape of the eye and the placement of the iris within. Human eyelids reveal a relatively broad horizontal area, and the dark iris and pupil typically contrast sharply with the light sclera. When the iris moves left or right, the two visible sections of sclera adjust instantly.
A 2001 study comparing external eye morphology across nearly half of extant primate species highlighted three notable characteristics unique to humans: unpigmented visible sclera, a high proportion of exposed sclera, and an unusually elongated eye shape. The researchers suggested that prominent eyes could assist others in detecting gaze, while darker eyes may obscure attention in more competitive contexts.
This foundational work, however, did not survey every individual but rather included limited species photographs. Later, more extensive samples revealed greater variability within nonhuman apes. Hence, the most justifiable distinction for humans is not that no other ape displays white sclera. Rather, it is that human sclera is consistently and uniformly light across individuals, instead of being white only in patches, at the edge of an averted gaze, or during specific life stages.
The 2007 cooperative eye study
Michael Tomasello, Brian Hare, Hagen Lehmann, and Josep Call formalized the hypothesis in a 2007 Journal of Human Evolution publication. They assessed 19 great apes residing at the Leipzig Zoo research facility: 11 chimpanzees, four gorillas, and four bonobos. The human comparison included 20 infants aged 12 months and 20 aged 18 months.
A human experimenter directed attention to the ceiling under four specific conditions: only eyes while the head stayed still, only head while the eyes were shut, both head and eyes, or neither. Both apes and infants followed when head and eyes aligned. When those signals conflicted, the groups exhibited notable differences.
The great apes primarily followed the direction of the experimenter’s head, though eye direction had some influence. Human infants predominantly depended on the eyes. The authors interpreted this difference as evidence that eyes evolved a specialized social role in humans, facilitating close-range joint attention and communication.
The study focused on a single human experimenter, captive apes with specific upbringing histories, and infants from one German city, without altering scleral color, identifying specific genes under selection, or observing ancestral humans. Its behavioral outcomes were clear within the task; its evolutionary implications were inherently inferential.
From gaze following to shared attention
Gaze following involves orienting towards where another individual is looking. Joint attention introduces a shared understanding: two individuals focus on the same object or event and acknowledge their mutual focus. A child observes an adult’s face, follows the adult’s gaze to a toy, then looks back. This interaction can clarify the meaning of a word, demonstrate an action, or solve a problem jointly.
ScienceBlog has previously detailed how infants and adults synchronize their brains during play based on joint attention. In typical development, gaze works alongside pointing, expressions, speech, and prior knowledge. Eyes seldom convey the whole message independently.
Teaching, cultural learning, and coordination all benefit from shared attention. The stronger assertion in the headline, that readable gaze facilitated large-scale collaboration, connects these repeated minor interactions to a wider human social framework. This hypothesis is reasonable. The 2007 ceiling experiment did not