I was in a schol bio tutorial the other day & one of the students asked a really intriguing question - one that I hadn't really thought about before. Apparently the class had watched the series Walking with cavemen a few weeks ago, & at some point (she couldn't remember which species it was) the narrator said that this particular species was the first to have whites to their eyes. The student said, I was wondering what the significance of that is. Do you have any ideas?
And of course I said - no, I honestly haven't got a clue. But I could offer an hypothesis, & try later to find out more. But first - I don't know how the narrator (well, the people writing the script) could actually know if a particular species, one known only from reasonably scanty fossil evidence, had eyes like ours. And yes, our eyes are different. If you look at any of the great apes, you'll find that the whole of the visible part of the eye (both sclera & iris) is pigmented. Other animals may have a white sclera but (as for my puppy) normally all you see is the coloured parts of the eye. Ours, on the other hand, have that prominent while sclera surrounding the coloured iris. But things like that don't fossilise, so it's surely just conjecture to say that a particular species had what we'd regard as fully 'human' eyes...
Anyway, what about that hypothesis? Well, I suggested to the class that having that expanse of white around the iris really makes our eyes stand out. You can see which way someone's looking, or if they're surprised or startled (in which case your eyes often open wider, so that white ring would be even more obvious). Perhaps, I said, this is an adaptation that aids communication & social interaction. (Hard to cast deep & meaningful glances if the object of your attention can't really be sure that you're looking at them!) The class agreed that this might be a possibility & we left things at that.
But now I've done a bit of research :-) First up was a news report that suggests that maybe I was thinking along the right lines: it describes research examining the effect of head & eye movements on redirecting the gaze of apes (gorillas, common chimpanzees, and bonobos) & 12- to 18-month-old human infants (Tomasello, Hare, Lehmann & Call, 2007). Tomasello & his colleagues noted that humans seem more likely than other primates to spend time in collaborative activities that see each individual keeping an eye (pardon the pun!) on what the other individuals are looking at - this may allow better coordination of their joint activity. They devised an experiment to see whether humans were in fact more likely than apes to follow the direction of someone's gaze.
Working first with the different apes, they used the following set of experimental conditions. One of the team (the 'experimenter') sat before one of the apes (the subject), and did one of the following:
- closed his eyes and looked immediately to the ceiling
- kept his head stationary & glanced with his eyes to the ceiling
- looked to the ceiling with head & eyes
- stared straight ahead at the subject
- sat with his back to the subject & looked up to the ceiling ('back of head' condition)
- sat with his back to the subject & stared straight ahead ('back' condition)
WIth children, the infants sat on their parents' laps, & were exposed to only 5 of the treatments - the team found that the babies became upset if the the experimenter completely turned his back on them, & so didn't use the 'back' condition. (For some reason, 'back of head' didn't have the same effect). They found that the apes were more likely to look to see what the experimenter was looking at when he moved only his head - but human infants would follow the direction of his gaze alone. The researchers wrote that these results suggest that eyes evolved a new social function in human evolution, most likely to support cooperative (mututalistic) social interactions. They also comment that the ability to detect gaze direction with a fair degree of accuracy might also help to deter cheating!
M.Tomasello, B.Hare, H.Lehmann & J.Call (2007) Reliance on head versus eyes in the gaze following of great apes and human infants: the cooperative eye hypothesis. Journal of Human Evolution 52(3): 314-320 doi:10.1016/j.jhevol.2006.10.001
(Incidentally, here's an interesting item on the genetics of human eye colour.)