In a recent experiment, our research team has shown that even infants — at around their first birthdays, before language acquisition has begun — tend to follow the direction of another person’s eyes, not their heads. Thus, when an adult looked to the ceiling with her eyes only, head remaining straight ahead, infants looked to the ceiling in turn. However, when the adult closed her eyes and pointed her head to the ceiling, infants did not very often follow.

Our nearest primate relatives, the African great apes (chimpanzees, bonobos and gorillas) showed precisely the opposite pattern of gaze following. When the human pointed her eyes only to the ceiling (head remaining straight ahead), they followed only rarely. But when she pointed her head only (eyes closed) to the ceiling, they followed much more often.

It has been repeatedly demonstrated that all great apes, including humans, follow the gaze direction of others. But in previous studies the head and eyes were always pointed in the same direction. Only when we made the head and eyes point in different directions did we find a species difference: humans are sensitive to the direction of the eyes specifically in a way that our nearest primate relatives are not. This is the first demonstration of an actual behavioral function for humans’ uniquely visible eyes.

Why might it have been advantageous for some early humans to advertise their eye direction in a way that enabled others to determine what they were looking at more easily? One possible answer, what we have called the cooperative eye hypothesis, is that especially visible eyes made it easier to coordinate close-range collaborative activities in which discerning where the other was looking and perhaps what she was planning, benefited both participants.

If we are gathering berries to share, with one of us pulling down a branch and the other harvesting the fruit, it would be useful — especially before language evolved — for us to coordinate our activities and communicate our plans, using our eyes and perhaps other visually based gestures.

Infant research, too, suggests that coordinating visual attention may have provided the foundation for the evolution of human language. Babies begin to acquire language through joint activities with others, in which both parties are focused on the same object or task. That’s the best time for an infant to learn the word for the object or activity in question.

We are still a long way from figuring out why humans evolved to do so many complicated things together — from building houses to creating universities to fighting wars. But the simple fact that we have evolved highly visible eyes, to which infants attune even before language, supplies at least one small piece of the puzzle of how.