ABSTRACT
It is astonishing that the use of the two eyes for perceiving the positions and forms of objects in the third dimension – depth – was not appreciated until modern times. No doubt, if the world suddenly looked flat with one eye, the importance of both working together would always have been obvious; but this does not happen. The world continues to look much the same with one eye as with two, except for rather special situations. This limited use of two eyes for seeing depth raises the question of why the elaborate neural mechanisms for combining signals from the two eyes developed through evolution to provide what is only seldom noticed as an improvement over vision by each eye alone. The answer must lie in improved precision of depth perception for specially important skills, such as for birds landing or catching fish on the wing; and primates catching branches as they leap high up in trees.
