In-laws transformed early human society

Give it up for in-laws. Those much-maligned meddlers helped spur an ancient social revolution that propelled human groups from savannas to cities, a new study suggests.

That conclusion stems from an analysis of genealogical and marital data indicating that, among modern hunter-gatherers, monogamous sexual unions between men and women from neighboring groups create networks of in-laws that spawn widespread cooperation and cultural learning, says a team led by anthropologist Kim Hill of Arizona State University in Tempe. Social groups organized in this way distinguish humans from other primates, Hill and his colleagues propose in the March 11 Science.

“Alliances between foraging groups are facilitated because unrelated males all associate with the same female, who may be their daughter, sister, wife, mother or daughter-in-law,” Hill says. “By friendly association with her, males begin to associate with each other.”

A social system of this type, which encourages collaboration among genetically unrelated individuals, originated approximately two million years ago as human ancestors began to hunt and gather foods that youngsters could not obtain for themselves, Hill hypothesizes. In this situation, females would have had an incentive to seek mates willing to stick around and provide food for offspring.

Monogamous relationships between males and females began even earlier, some scientists suspect, perhaps more than 3 million years ago (SN: 6/11/05, p. 379).

In contrast, female chimps mate with many partners. Males in adjacent chimp groups try to kill each other on sight, making cooperation between communities impossible.

“Differences in social structures, not necessarily cognitive advances, allowed our species to cross the barrier to cumulative cultural evolution,” remarks anthropologist Joseph Henrich of the University of British Columbia in Vancouver. In this process, cultural inventions become increasingly complex from one generation to the next.

Based on limited observations, researchers have thought that modern foraging communities, and by inference prehistoric groups, consist of many male relatives with women migrating into groups as marriage partners. Hill’s analysis of a range of hunter-gatherer populations shatters that assumption, in Henrich’s view.

Hill and his colleagues analyzed previously collected data on more than 5,000 individuals from 32 modern hunter-gatherer societies worldwide. Each society consists of two or more bands of people that live together while moving about the landscape. Bands range in size from five to 64 individuals.

Three social features characterize hunter-gatherer societies and are unique to humans, the researchers conclude. First, both men and women are as likely to stay in the bands they were born into as to move to new bands to find marriage partners. Second, adult brothers and sisters frequently reside together, along with lots of in-laws. Third, a majority of band members are genetically unrelated.

In a comment published in the same issue of Science, anthropologist Bernard Chapais of the University of Montreal argues that this monogamy-based social structure encourages males to circulate freely among bands in which they have kin and in-laws. Cultural innovations and traditions thus spread rapidly and unite bands into larger social units called tribes, Chapais proposes.

Cultural learning among hunter-gatherers led to the rise of agriculture around 10,000 years ago and the ensuing formation of states and complex institutions, Hill hypothesizes.