Whites of Human Eyes vs. Animal Eyes

[EdLincoln]

Why do humands have visible whites in their eyes and other animals don't?

[Noweia]

Animals do have whites in their eyes, you just can't see them most of them time as the coloured part of the eye - the iris - is usually behind the eyelids.

[EdLincoln]

That last answer is quite true, but not really what I'm asking. Why are the whites of human eyes so much more apparent then animal eyes? Why do the whites of human eyes take up most of the visible surface of the eye, while in animals you can rarely see more than a little white on the edge? Is there an evolutionary purpose?

[dalinian]

I have just asked a similar question at:
• http://uk.answers.yah...20091105141554AAb5liB
• http://wiki.answers.c..._the_white_of_the_eye
...having been unable to find an answer online. If the question generates a good answer, I'll post it here.

"Why are humans so distinctive in displaying so much of the white of the eye?

Other mammals, including our great ape evolutionary cousins, have eye openings that frame the iris when the eyes are at rest. What selective advantage led our ancestors to evolve eye openings that expose so much of the white of the eye?"

[dalinian]

One hypothesis is that human-type eyes evolved in the context of pressures for enhanced cooperative-communicative abilities of the kind needed in mutualistic social interactions involving joint attention and visually based communication such as pointing. This is known as the cooperative eye hypothesis.
Source(s):
'Reliance on head versus eyes in the gaze following of great apes and human infants: the cooperative eye hypothesis' Journal of Human Evolution 52 (2007) 314-320. M.Tomasello et al

(cross-posted from Yahoo Answers)