A study of prehistoric humans remains found in Russia has revealed why humans did not have sex with their relatives.
According to a new study, humans who lived around 34,000 years ago avoided inbreeding and developed mating networks. As per the new study led by Cambridge University and the University of Copenhagen, prehistoric humans designed social networks and pursue partners beyond their families. The study theorizes that this could explain why anatomically-modern humans proved more successful than species such as Neanderthals which did not avoid inbreeding.
The study, published in the journal Science, analysed genetic information from the remains of anatomically modern humans who lived during a period when the first people from Africa settled in western Eurasia.
Researchers led by Martin Sikora, from the University of Copenhagen, Denmark, were looking at human remains found at the Sunghir burial site. This Upper Paleolithic archaeological site represents some of the earliest evidence of Homo sapiens in Europe. The four individual males found there lived between 34,600 and 33,600 years ago.
The Upper Paleolithic burial site in Sunghir contains the complete remains of an adult male, the symbolically incomplete remains of another male, as well as those of two younger individuals. All of these people lived at this site during the same time. Unusual for similar finds from this period, all the four males were buried together. Two of them were children and had been buried head to toe.
“I think many researchers had assumed that the people of Sunghir were very closely related, especially the two youngsters from the same grave,” said Professor Eske Willerslev, a senior author on the study.
“What this means is that even people in the Upper Palaeolithic, who were living in tiny groups, understood the importance of avoiding inbreeding. The data that we have suggest that it was being purposely avoided. 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.”
When it comes to comparison, genomic sequencing of a Neanderthal individual from the Altai Mountains who lived about 50,000 years ago indicated inbreeding was not avoided. It led researchers to speculate that an early, systematic approach to preventing inbreeding may have helped anatomically-modern humans to thrive, compared with other hominins. For thousands of years, the Neanderthal population size remained small, and mating among close relatives was likely very common.
There are more harmful mutations in Neanderthal genome that lead to production of homonids who 40% less fit when compared to modern humans. Still non-african humans carry the burden of their inbreeding to this day. 2% of our DNA is Neanderthal, and due to our distantcousins’ interbreeding events, harmful gene variants continue to reduce the reproductive fitness of some populations today.
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 people live in fairly small groups of around 25 people, but they are also less directly connected to a larger community of perhaps 200 people, within which there are rules governing with whom individuals can form partnerships.
To the researchers’ surprise, the 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.
“Most non-human primate societies are organised around single-sex kin where one of the sexes remains resident and the other migrates to another group, minimising inbreeding” says Professor Marta Mirazón Lahr, from the Leverhulme Centre for Human Evolutionary Studies at the University of Cambridge. “At some point, early human societies changed their mating system into one in which a large number of the individuals that form small hunter-gatherer units are non-kin. The results from Sunghir show that Upper Palaeolithic human groups could use sophisticated cultural systems to sustain very small group sizes by embedding them in a wide social network of other groups.”
The social structure of these people may have had an impact on the development of cooperation and information transfer, the researchers note. If so, these aspects of their lives could have influenced how human culture evolved, and could therefore be “crucial” to understanding our development as a species.