An analysis of remains unearthed in southern Siberia shows that Neanderthals were heavily inbred, most likely due to their living in small, scattered communities.

Neanderthals are an extinct species of hard-browed, archaic hominins that lived across Eurasia from around 430,000 years ago until some 40,000 years ago, when our encroaching ancestors drove them extinct. It is unclear exactly when this lineage split from that of modern humans, but we estimate it happened sometime between 800,000 and 315,000 years ago. That being said, they did mingle with our ancestors, and we still carry the genetic legacy of the Neanderthals.

Neanderthal remains have been recovered from multiple sites in Europe, Russia, and southwest Asia. Although these discoveries definitely did help us better understand the Neanderthals, we still had unknowns about how these ancient humans structured their communities. However, one new discovery in Siberia is helping us piece that image together.

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“Neanderthal remains in general, and remains with preserved DNA in particular, are extremely rare,” said Benjamin Peter, a senior author on the study. “We tend to get single individuals from sites often thousands of kilometres, and tens of thousand of years apart.”

The study is based on DNA samples recovered from a group of 13 Neanderthal men, women, and children. These samples were obtained from fragments of bone and teeth found in caves in southern Siberia. But the story they tell of the Neanderthals’ family relations is… tangled.

Among the web of relationships identified by the study is one between a father and his daughter, one between the same daughter and a man related to the father, and one between two second-degree relatives, possibly an aunt and her nephew. These were couples; Neanderthals found here were heavily inbred.

It was likely not an isolated case. Based on what we know of Neanderthals, they never really reached large population numbers. As such, they most likely lived in small communities scattered over the huge swathes of land the species inhabited. Most groups, the team explained, likely numbered between 10 to 30 individuals.

Laurits Skov, a researcher at the Max Planck Institute for Evolutionary Anthropology in Leipzig and the first author of the study, says that the fact that these Neanderthals were alive at the same time heavily implies that they were members of the same social community — which is a “very exciting” thing to discover. The genetic closeness between these individuals discovered further reinforces the view that they were members of a single community.

“We find that the community we study was likely very small, perhaps 10 to 20 individuals, and that the wider Neanderthal populations in the Altai mountains were quite sparse,” Peter said. “Nevertheless, they managed to persevere in a rough environment for hundreds of thousands of years, which I think deserves great respect.”

The team could also determine that the mitochondria contained in these bones — energy-generating cell structures with their own genetic material that are passed down only from mother to offspring, not from the father — were more genetically-diverse than the Y chromosomes recovered from the bones (these are only passed down from the father).

What this most likely means is that Neanderthal women left their own families or communities to go live with those of their male partners. To what degree this was a voluntary choice or the product of coercion, the current data cannot show.

The findings nevertheless give us an unprecedented glimpse into Neanderthal society, and the way their communities functioned.

Similar research techniques could yield insight into the history of our own ancestors, clues that might not have been preserved in other sources of evidence. It might even help us understand why the Neanderthals have gone extinct while our own ancestors didn’t.

The paper “Genetic insights into the social organization of Neanderthals” has been published in the journal Nature.