Mon, Mar 14, 2011 - Page 9 News List

New view of how humans moved away from apes

A new survey suggests that human evolution was shaped by the shift to cooperative behavior, as distinct from the fierce aggression characterizing chimp groups

By Nicholas Wade  /  NY Times News Service, NEW YORK

Illustration: Yusha

Anthropologists studying living hunter-gatherers have radically revised their view of how early human societies were structured, a shift that yields new insights into how humans evolved away from apes.

Early human groups, according to the new view, would have been more cooperative and willing to learn from one another than the chimpanzees from which human ancestors split about 5 million years ago. The advantages of cooperation and social learning then propelled the incipient human groups along a different evolutionary path.

Anthropologists have assumed until now that hunter-gatherer bands consist of people fairly closely related to one another, much as chimpanzee groups do, and that kinship is a main motive for cooperation within the group. Natural selection, which usually promotes only selfish behavior, can reward this kind of cooperative behavior, called kin selection, because relatives contain many of the same genes.

A team of anthropologists led by Kim Hill of Arizona State University and Robert Walker of the University of Missouri analyzed data from 32 living hunter-gatherer peoples and found that the members of a band are not highly related. Fewer than 10 percent of people in a typical band are close relatives, meaning parents, children or siblings, they reported in Friday’s issue of Science.

Michael Tomasello, a psychologist at the Max Planck Institute for Evolutionary Anthropology in Germany, said the survey provided a strong foundation for the view that cooperative behavior, as distinct from the fierce aggression between chimp groups, was the turning point that shaped human evolution. If kin selection was much weaker than thought, Tomasello said, “then other factors like reciprocity and safeguarding one’s reputation have to be stronger to make cooperation work.”

The finding corroborates an influential new view of early human origins advanced by Bernard Chapais, a primatologist at the University of Montreal, in his 2008 book Primeval Kinship. Chapais showed how a simple development — the emergence of a pair bond between male and female — would have allowed people to recognize their relatives, something chimps can do only to a limited extent. When family members dispersed to other bands, they would be recognized and neighboring bands would cooperate instead of fighting to the death as chimp groups do.

In chimpanzee societies, males stay where they are born, and females disperse at puberty to neighboring groups, thus avoiding incest. The males, with many male relatives in their group, have a strong interest in cooperating within the group because they are defending both their own children and those of their brothers and other relatives.

Human hunter-gatherer societies have been assumed to follow much the same pattern, with female dispersal being the general, though not universal, rule and with members of bands therefore being closely related to one another. However, Hill and Walker find that though it is the daughters who move in many hunter-gatherer -societies, the sons leave the home community in many others. In fact, the human pattern of residency is so variable that it counts as a pattern in itself, one that the researchers say is not known for any species of ape or monkey. Chapais calls this social pattern “bilocality.”

Modern humans have lived as hunter-gatherers for more than 90 percent of their existence as a species. If living hunter-gatherers are typical of ancient ones, the new data about their social pattern has considerable bearing on early human evolution.

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