Man, in the achievement of a unique gift—a thinking brain capable of weighting stars or atoms—cannot grow that brain in the nine months before birth. It is, moreover, a peculiarly plastic brain, intended to receive impressions from the social world around it. Instinct, unlike the case in the world of animals, is here reduced to a minimum. This brain must grow and learn, be able to profit by experience. In man much of that growth and learning comes after birth. The result is that the human infant enters the world in a peculiarly helpless and undeveloped condition. His childhood is lengthy because his developing brain must receive a large store of information and ways of behavior from the social group into which it is born. It must acquire the complicated tool of speech.
~ Loren Eiseley, An Evolutionist Looks at Modern Man