For humans, the eyes are more than just windows to the outside world. They are also portals inward, providing others with glimpses into our inner thoughts and feelings.
Of all primates, human eyes are the most conspicuous; our eyes see, but they are also meant to be seen. Our colored irises float against backdrops of white and encircle black pupils. This color contrast is not found in the eyes of most apes.
According to one idea, called the cooperative eye hypothesis, the distinctive features that help highlight our eyes evolved partly to help us follow each others' gazes when communicating or when cooperating with one another on tasks requiring close contact.
In a new study that is one of the first direct tests of this theory, researchers from the Max Planck Institute for Evolutionary Anthropology in Germany looked at what effect head and eye movements had on redirecting the gaze of great apes versus human infants.
In the study, a human experimenter did one of the following:
- Closed his eyes, but tilted his head up toward the ceiling
- Kept head stationary while looking at the ceiling
- Looked at the ceiling with both head and eyes
- Kept head stationary while looking straight ahead
Results showed that the great apes — which included 11 chimpanzees, four gorillas and four bonobos — were more likely to follow the experimenter's gaze when he moved only his head. In contrast, the 40 human infants looked up more often when the experimenter moved only his eyes.
The findings suggest that great apes are influenced more by head than eyes when trying to follow another's gaze, while humans are more reliant on eyes under the same circumstances.
The study, led by Michael Tomasello, will be detailed in an upcoming issue of the Journal of Human Evolution.
The small things
Kevin Haley, an anthropologist at the University of California at Los Angeles, who was not involved in the study, told LiveScience he thinks the cooperative eye hypothesis is quite plausible, especially "in light of research demonstrating that human infants and children both infer cooperative intentions in others and display cooperative intensions themselves."
Comparisons of human eyes to those of other primates reveal several subtle differences that help make ours stand out. For example, the human eye lacks certain pigments found in primate eyes, so the outer fibrous covering, or "sclera," of our eyeball is white. In contrast, most primates have uniformly brown or dark-hued sclera, making it more difficult to determine the direction they're looking from their eyes alone.
Another subtle aid that helps us determine where another person is looking is the contrast in color between our facial skin, sclera and irises. Most apes have low contrast between their eyes and facial skin.
Humans are also the only primates for whom the outline of the eye and the position of the iris are clearly visible. In addition, our eyes are more horizontally elongated and disproportionately large for our body size compared to most apes. Gorillas, for example, have massive bodies but relatively small eyes.
The cooperative eye hypothesis explains these differences as traits that evolved to help facilitate communication and cooperation between members of a social group. As one important example, human mothers and infants are heavily reliant on eye contact during their interactions. One study found that human infants look at the face and eyes of their caregiver twice as long on average compared with other apes.
Clue to our humanity
Other ideas have also been proposed to explain why humans have such visible eyes. For example, white sclera might signal good health and therefore help signal to others our potential as a mate.
Or, as one other recent study suggested, visible eyes might be important for promoting cooperative and altruistic behavior in individuals that benefit the group. The study, conducted by Haley and Daniel Fessler, also at UCLA, found that people were more generous and donated more money if they felt they were being watched — even if the watchful eyes were just drawings resembling eyes on a computer screen.
Tomasello and his team note in their paper that "these hypotheses are not mutually exclusive, and highly visible eyes may serve all of these functions."
If correct, the cooperative eye hypothesis could provide a valuable clue about when we became the social beings that we are. “It would be especially useful to know when in evolution human's highly visible eyes originated, as this would suggest a possible date for the origins of uniquely human forms of cooperation and communication,” Tomasello and colleagues write.