Humans may have evolved altruistic traits as a result of a cultural 'tax' we paid to each other early in our evolution, a new study suggests. The research also changes what we knew about the genetic makeup of our hunter-gatherer ancestors.
     The origin of human altruism has puzzled evolutionary biologists for many years. In every society, humans make personal sacrifices for others with no expectation that it will be reciprocated. One theory to explain how human altruism evolved involves the way we interacted as groups early in our evolution. Towards the end of the Pleistocene period – about 12,000 years ago – humans foraged for food as hunter-gatherers. These groups competed against each other for survival. Under these conditions, altruism towards other group-members would improve the overall fitness of the group. If an individual defended the group but was killed, any genes that the individual shared with the overall group would still be passed on.
     Many researchers reject this model, however. One reason is that competition between individuals is likely to increase if a group becomes isolated, and any altruistic behaviour would then decrease an individual's level of fitness compared with other members. Biologists also assume that hunter-gatherer groups around this time period would have been insufficiently genetically related to favour altruism. In other words, die when defending the group and your genes die with you.
     Now a new study by Samuel Bowles at the Santa Fe Institute in New Mexico (USA), breathes new life into the model. Bowles conducted a genetic analysis of contemporary foraging groups. The genetic variation found within these modern-day groups was analysed and then used to estimate the kind of genetic variation that would have existed in ancestral populations of hunter-gatherer from the Pleistocene and early Holocene (150,000 to 10,000 years ago, combined). Bowles calculated that early human individuals were likely to be substantially more related to each other than previously thought. But he found bigger genetic differences than expected between discrete groups of ancient peoples. These conditions would have favoured altruistic behaviour, says Bowles. Bowles also worked out that early customs such as food sharing or monogamy could have levelled out the 'cost' of altruistic behaviour, in the same way that income taxes redistribute income in society.
     In his model, members of a group bearing genes for altruistic behaviour pay a 'tax' by limiting their reproductive opportunities to benefit from sharing food and information, thereby increasing the average fitness of the group as well as their inter-relatedness. Bands of altruistic humans would then act together to gain resources from other groups at this challenging time in history. For example, an injury may be one of the costs of defending the group during an intergroup conflict: a broken leg could be fatal for an individual who may starve through being unable to obtain his own food. But food sharing would make it less of a risk for individuals to participate in these conflicts, Bowles says.
     The archaeological and ethnographic data he used showed that 13% to 15% of foragers died from wars, which were common between groups. Bowles’s mathematical models suggest that altruism must have been a significant factor in these populations. Although Bowles admits that he has found no evidence for any gene for human altruism, he says that if such a genetic disposition were to exist, group conflict would have played an important role in its development.
     Monogamy would also level the playing field within the group, he showed in his statistical analysis. "Monogamy limits the ability of the stronger or more aggressive males to monopolise copulation," says Bowles. "Humans are very unusual in this way."

Sources: NewScientist.com (7 December 2006), The Philadelphia Inquirer, Herald Today (8 December 2006)