Haidinger's brush: Is this a by-product of the eye's physics, or are there any evolutionary grounds for it?
The human eye is, very subtly, sensitive to the polarization of light. This is an effect known as Haidinger's brush (see Wikipedia article of this name). What, if anything, is known or at least ...
The most distinctive characteristic of a human is it's face - it is unique among each individual (with the exception of identical twins). It is uncertain to me if whether we best identify other humans ...