New View of How Humans Moved Away From Apes
Published: March 10, 2011
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 five 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 S. Hill of Arizona State University and Robert S. 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 report 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, Dr. 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 book “Primeval Kinship” (2008). Dr. 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. But Dr. Hill and Dr. 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. Dr. 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.
On a genetic level, the finding that members of a band are not highly interrelated means that “inclusive fitness cannot explain extensive cooperation in hunter-gatherer bands,” the researchers write. Some evolutionary biologists believe that natural selection can favor groups of people, not just individuals, but the idea is hotly disputed.
Dr. Hill said group selection, too, could not operate on hunter-gatherer bands because individuals move too often between them, which undoes any selective effect. But hunter-gatherers probably lived as tribes split into many small bands of 30 or so people. Group selection could possibly act at the level of the tribe, Dr. Hill said, meaning that tribes with highly cooperative members would prevail over those that were less cohesive, thus promoting genes for cooperation.
The new data on early human social structure furnishes the context in which two distinctive human behaviors emerged, those of cooperation and social learning, Dr. Hill said. A male chimp may know in his lifetime just 12 other males, all from his own group. But a hunter-gatherer, because of cooperation between bands, may interact with a thousand individuals in his tribe. Because humans are unusually adept at social learning, including copying useful activities from others, a large social network is particularly effective at spreading and accumulating knowledge.
Knowledge can in fact be lost by hunter-gatherers if a social network gets too small. One group of the Ache people of Paraguay, cut off from its home territory, had lost use of fire when first contacted. Tasmanians apparently forgot various fishing techniques after rising sea levels broke their contact with the Australian mainland 10,000 years ago.
Dr. Chapais said that the new findings “validate and enrich” the model of human social evolution proposed in his book. “If you take the promiscuity that is the main feature of chimp society, and replace it with pair bonding, you get many of the most important features of human society,” he said.
Recognition of relatives promoted cooperation between neighboring bands, in his view, allowing people to move freely from one to another. Both sons and daughters could disperse from the home group, unlike chimp society, where only females can disperse. But this cooperation did not mean that everything was peaceful. The bands were just components of tribes, between which warfare may have been intense. “Males could remain as competitive and xenophobic as before at the between-tribe level,” Dr. Chapais writes.