It is very difficult to evolve by altering the deep fabric of life; any change there is likely to be lethal. But fundamental change can be accomplished by the addition of new systems on top of old ones…Thus evolution by addition and the functional preservation of the preexisting structure must occur for one of two reasons-either the old function is required as well as the new one, or there is no way of bypassing the old system that is consistent with survival.
Carl Sagan, Dragons of Eden: Speculations on the Evolution of Human Intelligence
evolution, human-evolution, intelligencence