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Ancient Irish Genomes Hint at Neolithic Hierarchies

NEW YORK – A team led by investigators in Ireland used ancient genomics to identify an inbred individual who appears to have been part of a high-status group found at burial sites stretching west from Newgrange in northeastern Ireland.

"It seems what we have here is a powerful extended kin-group, who had access to elite burial sites in many regions of the island for at least half a millennium," first and co-corresponding author Lara Cassidy, a genetics researcher at Trinity College Dublin, said in a statement. She and her colleagues published their new findings in Nature on Wednesday.

Past research suggests monuments and other markers of social status started cropping up in Europe around that time, the team noted, including the appearance of complicated passage tombs found near the Atlantic coast.

"Although co-operative ideology has often been emphasized as a driver of megalith construction," Cassidy and co-authors noted, "the human expenditure required to erect the largest monuments has led researchers to emphasize hierarchy — of which the most extreme case is a small elite marshaling the labor of the masses."

To explore this possibility in Ireland during the Neolithic, including those found in passage tombs in Newgrange and other parts of the country, the researchers did low-coverage genome sequencing on two Mesolithic and 42 Neolithic samples from sites in present-day Ireland, using imputation with available ancient genome sequences from individuals in Ireland and Britain to fill in sequence gaps for 43 of the ancient individuals.

The team also did deeper sequencing on four of the individuals, including an adult male born to first-degree relatives, whose remains were found in an especially extravagant, solar-aligned passage tomb at the Newgrange site. Based on the extensive runs of homozygosity found in the latter individual, the investigators concluded that he was born to a brother-sister or parent-child pair that may have gotten the green light from the society at the time, based on the notable nature of his grave.

"The prestige of the burial makes this very likely a socially sanctioned union and speaks of a hierarchy so extreme that the only partners worthy of the elite were family members," senior and co-corresponding author Daniel Bradley, a researcher with the Trinity College Dublin Smurfit Institute of Genetics, noted in a statement.

Though somewhat unexpected, the findings line up with a local myth behind the Fertae Chuile — known in English as the "Hill of Sin" or "Hill of Incest" — where a so-called "builder-king" fornicated with his sister to reset the solar cycles.

The researchers linked the inbred Newgrange individual to related individuals at sites in other parts of Ireland, including Carrowmore and Carrowkeel. Those individuals appeared to be part of a socially elevated group that were not only laid to rest in intricate tombs, but also ate better food, according to stable isotope analyses done for the study.

In contrast, remains from Neolithic farmers and other Neolithic individuals did not show signs of incest, the team noted, suggesting that the inbreeding was relatively rare and may have been limited to individuals at the upper echelons of the ancient society. The Irish sample set also offered look at relationships between farming and hunter-gatherer populations in the region, along with ancestral patterns in these groups within a broader European context. 

"Overall, our results demonstrate the capacity of ancient genomes to shed light not only on population movements, but also on political systems and social values where no written records exist," the authors wrote, adding that "our findings support a re-evaluation of social stratification and political integration in the megalithic cultures of the Atlantic seaboard, and suggest that the passage-tomb-building societies of Ireland possessed several attributes found within early states and their precursors."