Scientists have noticed that our eyes are not just sensory organs, but they also provide for the others clues about our thoughts and feelings.

Human eyes are the most conspicuous amongst the eyes of all monkeys and apes; they see, but they are also meant to be seen. The model of our eyes, colored irises posed over white background and rounding black pupils is not found in any ape. Researchers formulated a new "The cooperative eye hypothesis" that claims that our conspicuous eyes evolved as an increased ability to communicate, following each other's gaze when cooperating on tasks requiring close contact and silence (like hunting for example).

On a test to check this theory, a team at the Max Planck Institute for Evolutionary Anthropology in Germany verified the effect of head and eye movements on attracting the attention in great apes versus human infants. A human experimenter created four situations: eyes closed with head tilted toward the ceiling; head stationary looking at the ceiling; both head and eyes turned toward the ceiling and head stationary looking straight ahead.

The apes ( represented by the species closest genetically to humans: chimps, bonobos, gorillas) were more likely to follow the experimenter's gaze on the moments associated with head movement, while human infants looked up mostly when the experimenter moved only his eyes. Thus, apes are influenced more by head movement than eyes when trying to follow another's gaze, while humans rely more on eyes under the same circumstances.

Human eye lacks some pigments found in primate eyes, so the outer fibrous layer (sclera) of the human eyeball appears white. Most primates have uniformly brown or dark-hued sclera, thus it's much more difficult to follow the gazing direction of an ape.

Also, humans present a powerful color contrast between facial skin, sclera and irises, which is low in apes. And we are the only primates with the eye's outline and the position of the iris clearly visible. Human eyes are more horizontally elongated and proportionately larger than any ape eye.

Human mothers and infants are deeply reliant on eye contact during their interactions: researchers found that human infants look at the face and eyes of their caregiver twice as long on average compared with the apes.

Another theory says that white sclera could signal good health and mating potential.

A recent study at UCLA suggested that conspicuous eyes promote cooperative and altruistic behavior in individuals. People donated more money if they felt being watched, even in situations when the watching eyes were just drawings. Researchers noticed that "these hypotheses are not mutually exclusive, and highly visible eyes may serve all of these functions."

The cooperative eye hypothesis is an indication about the roots of the human highly social behavior. "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," said Michael Tomasello from Max Planck Institute.

Photo credit: stock.xchng. Eyes of a gorilla (top), chimpanzee (middle) and a human infant