Tokyo Institute of Technology pointed out that human eyes are unique in having a white sclera (the white outer part that surrounds the coloured iris in the centre): all other primates have brown or dark brown ones that are much closer in colour to both the iris and the surrounding fur. Our sclera is also much larger, relative to the size of the iris, than in any other species. Our eyes really do stand out in our faces. The white surround with the dark iris means that it is much easier to see whether or not someone is looking directly at you.
