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Prehistoric humans formed complex mating networks to avoid inbreeding

Human hunter-gatherer groups developed sophisticated social and mating networks to minimize inbreeding as early as 34,000 years ago, a new study finds.

A new study has sequenced the genomes of individuals from an ancient burial site in Russia and discovered that they were, at most, first cousins, indicating that they had developed sexual partnerships beyond their immediate social and family group.

A new study has identified when humans transitioned from simple systems designed to minimize inbreeding to more complex ones suitable for hunter-gatherer societies.

The study findings are reported in the journal Science and demonstrate that, by at least 34,000 years ago, human hunter-gatherer groups had developed sophisticated social and mating networks that minimized inbreeding.

The study examined genetic information from the remains of modern humans who lived during the early part of the Upper Palaeolithic, a period when modern humans from Africa first colonized western Eurasia, eventually displacing the Neanderthals who lived there before.

The results suggest that people deliberately sought partners beyond their immediate family, and that they were probably connected to a wider network of groups from within which mates were chosen, thus avoiding inbreeding.

The research was carried out by an international team of academics, led by the University of Cambridge, U.K., and the University of Copenhagen, Denmark. The team included SMU archaeologist David J. Meltzer, whose expertise includes the First People in the Americas.

The researchers sequenced the genomes of four individuals from Sunghir, a famous Upper Palaeolithic site in Russia, which was inhabited about 34,000 years ago.

The article, “Ancient genomes show social and reproductive behavior of early Upper Paleolithic foragers,” is published in the Oct. 5, 2017 issue of Science.

Complex mating systems may partly explain modern human survival
Among recent hunter-gatherers, the exchange of mates between groups is embedded into a cultural system of rules, ceremonies and rituals. The symbolism, complexity and time invested in the extraordinarily rich objects and jewellery found in the Sunghir burials, as well as the burials themselves, suggest that these early human societies symbolically marked major events in the life of individuals and their community in ways that foreshadow modern rituals and ceremonies — birth, marriage, death, shared ancestry, shared cultures.

The study’s authors also hint that the early development of more complex mating systems may at least partly explain why modern humans proved successful while other, rival species, such as Neanderthals, did not. More ancient genomic information from both early humans and Neanderthals is needed to test this idea.

The human fossils buried at Sunghir are a unique source of information about early modern human societies of western Eurasia. Sunghir preserves two contemporaneous burials – that of an adult man, and that of two children buried together and which includes the symbolically modified remains of another adult.

To the researchers’ surprise, however, these individuals were not closely related in genetic terms; at the very most, they were second cousins. This is true even for the two children who were buried head-to-head in the same grave.

“What this means is that people in the Upper Palaeolithic, who were living in tiny groups, understood the importance of avoiding inbreeding,” said Eske Willerslev, a professor at St John’s College and the University of Copenhagen, who was senior author on the study. “The data that we have suggest that it was being purposely avoided. This means that they must have developed a system for this purpose. If the small hunter and gathering bands were mixing at random, we would see much greater evidence of inbreeding than we have here.”

Early human societies changed ancestral mating system
The small family bands were likely interconnected within larger networks, facilitating the exchange of peoples between bands in order to maintain diversity, said Martin Sikora, a professor at the Centre for GeoGenetics at the University of Copenhagen.

Most non-human primate societies are organized around single-sex kin (matrilines or patrilines), where one of the sexes remains resident and the other migrates to another group, thus minimizing inbreeding. At some point, early human societies changed the ancestral mating system into one in which a large number of the individuals that form small resident/foraging units are non-kin, where the relations among units that exchange mating partners are formalized through complex cultural systems.

In at least one Neanderthal case, an individual from the Altai Mountains who died about 50,000 years ago, inbreeding was not avoided, suggesting that the modern human cultural systems that allows to decouple the size of the resident community from the danger of inbreeding was not in place. This leads the researchers to speculate that an early, systematic approach to preventing inbreeding may have helped modern humans to thrive in relation to with other hominins.

This should be treated with caution, however.

“We don’t know why the Altai Neanderthal groups were inbred,” Sikora said. “Maybe they were isolated and that was the only option; or maybe they really did fail to develop a network of connections. We will need more genomic data of diverse Neanderthal populations to be sure.”

Upper Palaeolithic human groups sustained very small group sizes
The researchers were able to sequence the complete genomes of all four individuals found within the two graves at Sunghir. These data were compared with information on both modern and ancient human genomes from across the world.

They found that the four individuals studied were genetically no closer than second cousins, while the adult femur filled with red ochre found in the youngsters’ grave would have belonged to an individual no closer than great-great grandfather of the boys. “This goes against what many would have predicted,” Willerslev said. “I think many researchers had assumed that the people of Sunghir were very closely related, especially the two youngsters from the same grave.”

The people at Sunghir may have been part of a network similar to that of modern day hunter-gatherers, such as Aboriginal Australians and some historical Native American societies. Like their Upper Palaeolithic ancestors, these societies lived in fairly small groups of some 25 people, but they were also connected to a larger community of perhaps 200 people, within which there were rules governing with whom individuals can form partnerships.

“The results from Sunghir show that Upper Palaeolithic human groups could sustain very small group sizes by embedding them in a wide social network of other groups maintained by sophisticated cultural systems,” said Marta Mirazón Lahr, a professor at the University of Cambridge.

Willerslev also highlights a possible link with the unusual sophistication of the ornaments and cultural objects found at Sunghir. Such band-specific cultural expressions may have been used to signal who are “we” versus who are “they,” and thus a means of reinforcing a shared identity built on marriage exchange across foraging units. The number and sophistication of personal ornaments and artefacts found at Sunghir are exceptional even among other modern human burials, and not found among Neanderthals and other hominins.

“The ornamentation is incredible and there is no evidence of anything like that with other hominins,” Willerslev added. “When you put the evidence together, it seems to be telling us about the really big questions: what made these people who they were as a species, and who we are as a result.”

Ancient genomics throw light on aspects of social life
These results show the power of ancient genomics to throw light on aspects of social life among early humans, and pave the way for further studies to explore variation in social and demographic strategies in prehistoric socieities.

“Much of human evolution is about changes in our social and cultural behavior, and the impact this has had on our success as a species. This study takes us a step further toward pinpointing when and why the things that make humans unique evolved,” said Robert Foley, a professor at the University of Cambridge.

Meltzer is Henderson-Morrison Professor of Prehistory in the SMU Department of Anthropology in Dedman College. As a scientist who studies how people first came to inhabit North America, Meltzer in 2009 was elected a member of the National Academy of Sciences in recognition for his achievements in original scientific research. In 2013 he was elected to the American Academy of Arts and Sciences. — University of Cambridge, SMU

By Margaret Allen

Senior research writer, SMU Public Affairs