Neuronal energy use and brain evolution.

Consider how advantageous it might be to have eyes on our hands, rather than on our faces: depth perception would be improved by the greater distance between the eyes, and it would be easy to look into relatively inaccessible spaces by appropriate movement of the hands. The absence of mammals that use this visual strategy draws attention to constraints on how evolution is able to 'design' the nervous system. Energy use in particular, in this case the large amount of energy that would be needed to send visual information along the ∼106 optic nerve axons over the length of the arms to the brain (instead of along the much shorter optic nerve), imposes significant design constraints on the nervous system.