One of the most complex human mysteries involves how and why we became the most progressive species in terms of biological success.

Findings by a team of noted anthropologists, including several from Arizona State University, suggest that human ancestral social structure may be the root of cumulative culture and cooperation and, ultimately, human uniqueness.

Because humans lived as hunter-gatherers for 95 percent of their species’ history, current foraging societies provide the best window for viewing human social evolution, according to the authors, the journal Science reports.

Researchers focused on co-residence patterns among more than 5,000 individuals from 32 present-day foraging societies around the globe which include the Gunwinggu, Labrador Inuit, Mbuti, Apache, Aka, Ache, Agta and Vedda.

Professor Kim Hill of Arizona State led the study along with Robert Walker of the University of Missouri, among others.

Their findings showed that across all groups, adult brothers and sisters frequently live together, making it common for male in-laws to co-reside.

They also found that it was equally common for males or females to move from or remain with family units. This is in contrast to other primate species, where either males or females move to another group at puberty.

A major point in the study is that foraging bands contain several individuals completely unconnected by kinship or marriage ties, yet include males with a vested interest in the offspring of daughters, sisters and wives.

This organisation mitigates the group hostility frequently seen in other apes and also promotes interaction among residential groups, thereby leading to the development of a large social network.

“The increase in human network size over other primates may explain why humans evolved an emphasis on social learning that results in cultural transmission,” Hill said.

“Likewise, the unique composition of human ancestral groups promotes cooperation among large groups of non-kin, something extremely rare in nature.”

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