Evolution and natural selection can explain why the phases of the human life cycle are as long as they are.

The human life cycle begins with a remarkably long childhood and a long period of adolescence, followed by child bearing years as an adult, followed by a remarkably long "empty nest" phase ending in death. If villages did not benefit from having its adults teach its children and adolescents, "survival of the fittest" would penalize such villages for allocating scarce resources to that purpose. In the same way, if villages did not benefit from having elders preserve and pass on the village's wisdom and knowledge to the adults, evolution would penalize such villages for wasting scarce resources keeping unproductive elders around. The length of human childhood and adolescence is strong evidence that, over the 1/2 million years since villages (specialized social roles) appeared, the teaching of children and adolescents by adults benefits the village. Similarly, the length of the "empty nest" elder phase of the human life cycle is strong evidence that having elders around to teach and guide benefits the village.