Eyes white open09 Nov 2006
There is a story from LiveScience's Ker Than on the function of white sclerae in the eyes, as an adaptation for communicating gaze direction in hominids:
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.
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.
This is Michael Tomasello's work, and it is pretty interesting if the differences are as great as implied here. We'll have to wait to read the article in JHE.
The basic idea is that directed gaze and shared (joint) attention on objects are integral to human learning, so that this would be a strategy that emerged in early hominids to facilitate teaching some adaptive behaviors to young.
We can wonder at what point in human evolution this became important. I would offer a candidate: that toolmaking is a behavior that requires this joint attention on small objects and parts of objects. Of course, another option is that the objects in question are large and distant, such as animals being hunted.
There may be no contradiction here, because these behaviors may have emerged at around the same time -- which may be the real point; the underlying learning adaptation may have been necessary for both foraging and technological strategies.
I suppose that early hominids had apelike eyes, then. Unless there is some other reason (scanning predators?) why they might have needed to communicate gaze.