A project, headed by researchers from Trinity College Dublin, has sequenced the DNA of more than 40 individuals excavated from both Mesolithic and Neolithic funerary contexts across Ireland. The results (published in Nature: https://doi.org/10.1038/s41586-020-2378-6) illuminate not only the Irish transition to an agrarian way of life but also the social hierarchies that might have formed during this time.
The two Mesolithic genomes sequenced by the team represent the first from this period in Ireland. They formed a distinct cluster when compared with the genomes of their contemporaries from Britain and the Continent, indicating that interaction between Ireland and the rest of Europe was probably minimal during the Mesolithic. Despite a lack of gene-flow from abroad, however, there appeared to be little evidence of inbreeding. This means that, even though there were probably only 3,000-10,000 individuals living in Ireland at this time, they were still able to avoid close intermixing – evidence perhaps of a well-developed communication network.
This Irish isolation ended by at least 3800 BC. The Neolithic genomes sequenced by the team suggest large-scale migration from the Continent around this time, with these new inhabitants probably bringing with them the agricultural practices that define the period. Genetically affiliated with the Neolithic population of Britain, it appears that similar communities migrated to both islands.
Once they had spread out across Ireland, it seems that these Neolithic peoples managed to maintain a link across several centuries. The data suggest ‘a web of relatedness’ which connects individuals buried at the mega-cemeteries of Carrowmore (see CA 355) and Carrowkeel along the Atlantic Coast with those found at the megalithic structures 150km to the east at Newgrange, as well as at the Millin Bay megalith 230km away on the north-east coast.
Despite the apparent genetic-relatedness between these major funerary areas, though, on the whole there was little evidence of close inbreeding. There was one notable outlier, however. One man, buried within a chamber of the Newgrange passage tomb, was the result of a first-order incestuous relationship – meaning that his parents were either full siblings or parent and child.
Parent/child incest seems to have almost always been completely taboo, but examples of sanctioned sibling incest, while not common, have been documented in several more recent cultures (such as in pre-contact Hawai’i, the Inca Empire, and ancient Egypt), usually in order to cement the power of a particular ruling family. The team proposes that this might have also been the case at Newgrange, particularly as this individual was buried in such a prominent location, suggesting high status.
This discovery, in addition to the familial ties seen among several other major tombs, suggests that these newly formed agrarian societies may have been maintained by a ruling elite. Other key discoveries within the paper include information on the probable appearance of several of the individuals examined in this study. In particular, the Neolithic group included a male infant from the Poulnabrone dolmen (or portal tomb; see CA 298) in County Clare who had a trisomy of chromosome 21 – making him the earliest known case of Down syndrome found in the archaeological record.