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.

One of the primary ways that humans--indeed all primates--acquire territory is through "contests," or war. By sharing the costs of war, as well as its benefits, a group of altruists typically outnumbers and therefore defeats a less cohesive band of individuals. Thus, whereas individual natural selection would argue for the rise of the selfish, larger group dynamics showcase the triumph of the altruists. This latter type of selection also relies on that group sharing a large proportion of similar genes, because, in that case, altruists' genetic material persists in some form if they sacrifice themselves for others in war. This is the solution offered by Darwin in The Descent of Man and Bowles in a paper published in the December 8 Science.

Bowles examines the genetic interrelatedness of hunter-gatherer groups that persist to this day, assuming that they are at least somewhat indicative of the behaviors of our remote ancestors. Many of them show high degrees of interrelatedness--a bit less than cousins. In addition, Bowles points out that abrupt climate change happened several times during recent geologic history, subjecting our ancestors to even more rigorous competition--and potential population extinctions for those who couldn't band together to survive. Indeed evidence of warfare in archaeological remains increases in times of environmental stress. Plus, the proclivity to wipe out subjected populations continued to reinforce our newly developing altruistic ways.

None of this evidence, of course, proves that altruism evolved in this manner, but it does provide an intriguing argument and some nice mathematical equations for describing human behavior. Plus, Bowles demonstrates how the effect of leveling mechanisms such as shared access to scarce resources, enables altruism to become a very persistent way of life when coupled with territorial expansion. History isn't just written by the winners, the people reading that history are probably their descendants. "Language or culture may have led to the evolution of leveling mechanisms, which then potentiated the spread of prosocial genes because those mechanisms reduced the costs of cooperation," writes anthropologist Robert Boyd of the University of California, Los Angeles, in a commentary on the research. "It is certainly fair to invoke reproductive leveling to explain the stability of extended altruism among humans, but whether it is sufficient to explain its origin is not yet clear."