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Prehistoric humans formed mating networks to avoid inbreeding

last modified Oct 05, 2017 08:49 PM
A new study, reported in the journal Science this week, demonstrates that, by at least 34,000 years ago, human hunter-gatherer groups had developed sophisticated social and mating networks that minimised inbreeding.

Science pr image
Detail of one of the burials from Sunghir, in Russia. The new study sequenced the genomes of individuals from the site and discovered that they were, at most, first cousins, indicating that they had developed sexual partnerships beyond their immediate social and family group.

Animal species have mating systems that minimise the dangers of inbreeding. In the case of humans, such systems include complex socio-economic strategies that magnify the advantages of sharing resources with non-kin. Such strategies shape social and cultural diversity among hunter-gatherers and other small-scale societies today, but the timing of the transition from simple inbreeding-avoidance mating systems to complex hunter-gatherer residence structures and trade networks occurred remains unclear. A new study, reported in the journal Science this week, demonstrates that, by at least 34,000 years ago, human hunter-gatherer groups had developed sophisticated social and mating networks that minimised 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 colonised western Eurasia, eventually displacing the Neanderthals who lived there beforeThe 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.

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, although more ancient genomic information from both early humans and Neanderthals is needed to test this idea.

The research was carried out by an international team of academics, led by the University of Cambridge, UK, and the University of Copenhagen, Denmark. They sequenced the genomes of four individuals from Sunghira famous Upper Palaeolithic site in Russia, which was inhabited about 34,000 years ago. 

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. 

Sunghir burial reconstruction 1
Sunghir burial reconstruction Image Credit: Libor Balák, Anthropark

Professor Eske Willerslev, who holds posts as a Fellow at St John’s College and Prince Philip Professor of Ecology and Evolution at Cambridge and as Lundbeck Professor at the University of Copenhagen, was the senior author on the study. “What this means is that people in the Upper Palaeolithic, who were living in tiny groups, understood the importance of avoiding inbreeding,” he said. “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.”

The small family bands were likely interconnected within larger networks, facilitating the exchange of peoples between bands in order to maintain diversity,” Professor Martin Sikora, from the Centre for GeoGenetics at the University of Copenhagen, said. 

Most non-human primate societies are organised around single-sex kin (matrilines or patrilines) where one of the sexes remains resident and the other migrates to another group, thus minimising 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 formalised 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 other hominins.

Sunghir burial reconstruction 2
Sunghir burial reconstruction. Image credit: Libor Balák, Anthropark

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.

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” says Prof. Marta Mirazón Lahr, from the Centre for Human Evolutionary Studies, Department of Archaeology, University of Cambridge.

Willerslev also highlights a possible link with the unusual sophistication of the ornaments and cultural objects found at SunghirSuch 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 unitsThe 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.”

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 behaviour, and the impact this has had on our success as a species. This study takes us a step further towards pinpointing when and why the things that make humans unique evolved”, says Prof. Robert Foley, also from the Centre for Human Evolutionary Studies, Department of Archaeology, at the University of Cambridge.

The research paper, Ancient genomes show social and reproductive behaviour of early Upper Paleolithic foragers, is published in the October 5 issue of Science.

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