How did Stone Age hunter-gatherers avoid inbreeding?

If you’re part of a small hunter-gatherer tribe, you can really limit your options when it comes to finding a mate, especially if most of the people in your clan are your own siblings. According to the results of a new genetic study, the last Stone Age foragers in Europe avoided this problem by ensuring they were among unrelated hunter-gatherers, preventing inbreeding while keeping out the growing Neolithic farming population Remained.

Researchers have sequenced the genomes of ten skeletons found at the iconic Stone Age sites of Hoedic, Téviec and Champigny in France. According to the study authors, these ancient settlements are known for their “unusually well-preserved and rich burials.”

Radiocarbon dating of the collagen in the bones confirmed that all the skeletons were about 6,700 years old and therefore dated from the period when the Mesolithic gave way to the Neolithic, when hunter-gatherer groups were replaced by farming communities. “These conditions could have pushed these groups into a corner of severe genetic drift due to the extremely small population size, leaving no alternative to consanguinity and its deleterious consequences,” the researchers write.

This bleak prospect is compounded by the fact that all three sites contain shared graves, with multiple individuals buried next to each other. Such a practice was unusual for this period of the Stone Age and was previously interpreted as evidence that these people were blood relatives.

However, contradicting this story, study author Dr. Amélie Vialet explained in a statement: “Our results show that in many cases – even in the case of women and children in the same grave – the individuals were not related. This suggests that there were strong social ties that had nothing to do with biological kinship and that these relationships remained important even after death.”

Isotopic data was then used to confirm that the residents of each site existed as separate groups. For example, higher levels of marine proteins in Hoedic’s bones indicate that the subsistence and dietary habits of these individuals differed from those of Téviec and Champigny.

Based on this isotopic data, the study authors determined that some women in Hoedic were raised on terrestrial proteins before switching to a heavier marine diet later in life. This suggests that women were exchanged between different hunter-gatherer groups, probably as a means of preventing inbreeding.

“Our genomic analyzes show that although these groups consisted of few individuals, they were generally not closely related,” says study author Luciana G. Simões. “In addition, there were no signs of inbreeding.”

“However, we know that there were different social units – with different dietary habits – and a pattern of groups is emerging that was probably part of a strategy to prevent inbreeding,” Simões added.

It was previously suggested that some of the women buried at these sites had actually grown up in farming settlements – where they would have consumed more terrestrial animals – before later being absorbed into hunter-gatherer groups. However, based on their genetic analysis, the researchers confirm that “these females […] did not come from Neolithic populations, as they occurred within the [hunter-gatherer] genetic variation and show no traces of Neolithic descent from farmers.”

“Thus, contrary to previous conclusions based on stable isotope data from the same sites, the Late Mesolithic foraging community was limited in partner exchange to neighboring hunter-gatherer groups, to the exclusion of Neolithic farmers,” they write.

In line with these results, the researchers can paint a more accurate picture of the interactions between the last hunter-gatherers and the first farmers who occupied Western Europe. More specifically, they show that any gene flow that may have occurred between the two groups “was unidirectional and resulted from individuals with [hunter-gatherer] ancestors joining farming groups and not the other way around.”

The research was published in the Proceedings of the National Academy of Sciences.

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