Ancient Irish DNA Reveals Secrets of Inbreeding and Down Syndrome in Early Irish Society

In the earliest periods of Irish history, archaeologists and geneticists led by those from Trinity have shed new light.

The amazing findings of this study include an adult male genome hidden in the heart of the Newgrange passage tomb points to first-degree incest, implying he was among a ruling social elite akin to the similarly inbred Inca god-kings and Egyptian pharaohs.

Newgrange passage tomb in Ireland is world-famous for its annual solar alignment where the winter solstice sunrise illuminates its sacred inner chamber in a golden blast of light. However, little is known about who was interred in the heart of this imposing 200,000 tonnes monument or of the Neolithic society that built it over 5,000 years ago.

The survey of ancient Irish genomes, published in Nature, suggests a man who had been buried in this chamber belonged to a dynastic elite.

The research, led by the team from Trinity, was carried out in collaboration with colleagues from University College London, NUIG, University College Cork, University of Cambridge, Queen’s University Belfast, Sligo Institute of Technology and the National Monuments Service, with support from the National Museum of Ireland and National Museums Northern Ireland.

“I’d never seen anything like it,” said Lara Cassidy, professor at Trinity College Dublin, first author of the paper. “We all inherit two copies of the genome, one from our mother and one from our father; well, this individual’s copies were extremely similar, a tell-tale sign of close inbreeding. Our analyses allowed us to confirm that his parents were first-degree relatives.”

Matings of this type (e.g. brother-sister unions) are a near-universal taboo for entwined cultural and biological reasons. The only confirmed social acceptances of first-degree incest are found among the elites – typically within a deified royal family.

By breaking the rules, the elite separates itself from the general population, intensifying hierarchy and legitimizing power. Public ritual and extravagant monumental architecture often co-occur with dynastic incest, to achieve the same ends.

“Here the auspicious location of the male skeletal remains is matched by the unprecedented nature of his ancient genome,” said Professor of Population Genetics at Trinity, Dan Bradley. “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.”

The team also unearthed a web of distant familial relations between this man and other individuals from sites of the passage tomb tradition across the country, including the mega-cemeteries of Carrowmore and Carrowkeel in Co. Sligo.

“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,” added Cassidy.

Remarkably, a local myth resonates with these results and the Newgrange solar phenomenon. First recorded in the 11th century AD, four millennia after construction, the story tells of a builder-king who restarted the daily solar cycle by sleeping with his sister.

The Middle Irish place name for the neighbouring Dowth passage tomb, Fertae Chuile, is based on this lore and can be translated as “Hill of Sin.”

“Given the world-famous solstice alignments of Brú na Bóinne, the magical solar manipulations in this myth already had scholars questioning how long an oral tradition could survive,” said Ros Ó Maoldúin, an archaeologist on the study. “To now discover a potential prehistoric precedent for the incestuous aspect is extraordinary.”

The genome survey stretched over two millennia and unearthed other unexpected results. Within the oldest known burial structure on the island, the Poulnabrone portal tomb, the earliest yet diagnosed case of Down Syndrome was discovered in a male infant who was buried there five and a half thousand years ago.

Additionally, the analyses showed that the monument builders were early farmers who migrated to Ireland and replaced the hunter-gatherers who preceded them. However, this replacement was not absolute; a single western Irish individual was found to have an Irish hunter-gatherer in his recent family tree, pointing toward a swamping of the earlier population rather than an extermination.

Genomes from the rare remains of Irish hunter-gatherers themselves showed they were most closely related to the hunter-gatherer populations from Britain (e.g. Cheddar Man) and mainland Europe.

However, unlike British samples, these earliest Irelanders had the genetic imprint of prolonged island isolation. This fits with what we know about prehistoric sea levels after the Ice Age: Britain maintained a land bridge to the continent long after the retreat of the glaciers, while Ireland was separated by sea and its small early populations must have arrived in primitive boats.