Michael Tomasello, co-director of the Max Planck Institute for Evolutionary Anthropology in Leipzig, offers an interesting speculation in a brief essay in the Op-Ed section of the Jan. 13 New York Times. Because the whites of our human eyes are large we can easily detect the direction of another person's gaze even if their head is pointing slightly away from us. In contrast, neither chimpanzees nor any of the other 220 species of nonhuman primates have whites of the eyes that can be easily seen, making it much harder to see if their eyes are looking in a direction other than the one in which their heads are pointing.
Tomasello: "Evolutionary theory tells us that, in general, the only individuals who are around today are those whose ancestors did things that were beneficial to their own survival and reproduction. If I have eyes whose direction is especially easy to follow, it must be of some advantage to me...If I am, in effect, advertising the direction of my eyes, I must be in a social environment full of others who are not often inclined to take advantage of this to my detriment — by, say, beating me to the food or escaping aggression before me. Indeed, I must be in a cooperative social environment in which others following the direction of my eyes somehow benefits me."
"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."