From Scientific American:
Natural selection argues against cooperation. If all organisms, including humans, are pitted in a ceaseless struggle for survival and sex, those who help others would quickly find themselves swamped in a rising tide of selfishness, especially if those they helped bore no relation to them. Yet, most humans reflexively help another person in need even if there are no family ties or a direct benefit to be gained. This conundrum has puzzled evolutionary biologists since the time of Darwin, but a new study shows how internecine warfare among early humans might have allowed for the spread of a dominant group of altruistic tribes.
Economist Samuel Bowles of the Santa Fe Institute examines the evolutionary forces at work on early human populations. He posits two distinct groups: the altruistic and the selfish, divided into many different tribes, which Bowles refers to as demes. Altruists are disposed to take an action helping others, but such actions have a specific cost. For example, an altruist might jump into the river to save a drowning child at the cost of her own life but to the overall benefit of the tribe. Reducing these sets of conditions to a mathematical equation reveals that altruists can only prosper if their altruism enables their group to acquire more territory.
More here.