Prehistoric Humans are Likely to Have Formed Sex Networks to Avoid Inbreeding
Early humans seem to have recognized the dangers of inbreeding at least 34,000 years ago, and developed surprisingly sophisticated social and mating networks to avoid it, new research has found.
The study, reported in the journal Science, examined genetic information from the remains of anatomically modern humans who lived during the Upper Paleolithic, a period when modern humans from Africa first colonized western Eurasia. 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, in order to avoid becoming inbred.
This suggests that our distant ancestors are likely to have been aware of the dangers of inbreeding, and purposely avoided it at a surprisingly early stage in prehistory.
The symbolism, complexity and time invested in the objects and jewelry found buried with the remains also suggests that it is possible that they developed rules, ceremonies and rituals to accompany the exchange of mates between groups, which perhaps foreshadowed modern marriage ceremonies, and may have been similar to those still practiced by hunter-gatherer communities in parts of the world today.
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, second cousins, indicating that they had developed sexual partnerships beyond their immediate social and family group. Credit: By José-Manuel Benito Álvarez (Own work) [Public domain], via Wikimedia Commons
The study's authors also hint that the early development of more complex mating systems may at least partly explain why anatomically modern humans proved successful while other species, such as Neanderthals, did not. However, 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 Sunghir, a famous Upper Paleolithic site in Russia, which is believed to have been inhabited about 34,000 years ago.
The human fossils buried at Sunghir represent a rare and highly valuable, source of information because very unusually for finds from this period, the people buried there appear to have lived at the same time and were buried together. 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 in the case of two children who were buried head-to-head in the same grave.
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Artist’s reconstruction of one of the Sunghir burials. Illustration © Libor Balák
Professor Eske Willerslev, who holds posts both as a Fellow at St John's College, Cambridge, and at the University of Copenhagen, was the senior author on the study. "What this means is that even people in the Upper Paleolithic, 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 small hunter-gatherer bands were mixing at random, we would see much greater evidence of inbreeding than we have here."
Early humans and other hominins such as Neanderthals appear to have lived in small family units. The small population size made inbreeding likely, but among anatomically modern humans it eventually ceased to be commonplace; when this happened, however, is unclear.
"Small family bands are likely to have interconnected with larger networks, facilitating the exchange of people between groups in order to maintain diversity," Professor Martin Sikora, from the Centre for GeoGenetics at the University of Copenhagen, said.
Museum of Natural History - Early Hominids (Credit: Strannik45 / flickr )
Sunghir contains the burials of one adult male and two younger individuals, accompanied by the symbolically-modified incomplete remains of another adult, as well as a spectacular array of grave goods. The researchers were able to sequence the complete genomes of the four individuals, all of whom were probably living on the site at the same time. These data were compared with information from a large number of both modern and ancient human genomes.
They found that the four individuals studied were genetically no closer than second cousins, while an adult femur filled with red ochre found in the children's' 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."