According to Darwin...

According to Darwin, it is such things as social interactions which would make intelligence, learning, and cooperation optimal characters to possess under the conditions experienced by the early hominids.  Such social interactions would include learning how to provide parental care, and forming cooperative relationships within and  amongst other groups.

 The attainment of such advanced cognitive abilities would be important in order to be able to detect so-called cheaters in social exchanges.  This idea is upheld by those authorities who believe in the theory of reciprocal altruism.

The selection for greater intelligence involves attaining a bigger brain size.  This required several anatomical changes for the lineage as well.  A larger birth canal was needed in the female pelvis.  A longer time interval between an individualís birth and the time at which he or she reached sexual maturity was also needed.  This extended time period allowed for prolonged growth, learning and dependence on parents.  It is believed by some that the prolonged juvenile stage in human development is responsible for such human features as skull shape and sparse body hair.