An extensive archaeological and genetic study at Newgrange found signs of incest in early Irish society.
The study, led by researchers at Trinity College Dublin, shed new light on the earliest period of Ireland's human history.
Among the finds are signs of incest in an adult male buried in the heart of the Newgrange passage tomb, implying he was among a ruling social elite. Similar inbreeding was an aspect of other ancient societies, like the Inca god-kings in South America and the Egyptian pharaohs.
Older than the pyramids, Newgrange passage tomb is world famous for its annual solar alignment where the winter solstice sunrise illuminates its inner chamber in a blast of light. However, little is known about the society which built it more than 5,000 years ago.
The study of ancient Irish genomes, published in the leading journal Nature, was carried out by a team from Trinity in collaboration with researchers from UCC, NUIG, Sligo IT, Queens, Belfast and several UK universities, with support from the National Museum of Ireland and National Museums Northern Ireland.
It suggests a man buried in the chamber belonged to a dynastic elite.
Trinity's Dr Lara Cassidy said, "We all inherit two copies of the genome, one from our mother and one from our father. This individual’s copies were extremely similar, a tell-tale sign of close inbreeding. In fact, our analyses allowed us to confirm his parents were first-degree relatives."
Such inbreeding practices were most commonly found in elite circles, typically royal families, as it intensified hierarchy.
A 'powerful extended family'
The research also unearthed distant family relations between this man and others buried in the mega-cemeteries of Carrowmore and Carrowkeel, Co Sligo, indicating a powerful extended family which had access to burial sites all over the island for "at least half a millenium", Dr Cassidy added.
The genome survey studied a period of more than 2,000 years, uncovering some other remarkable finds.
Within the Poulnabrone portal tomb in Co Clare, the earliest case of Down Syndrome was discovered in a male infant buried some 5,500 years ago.
Additionally, the analyses showed the monument builders were not hunter-gatherers but early farmers who migrated to Ireland and replaced them.
The genomes also showed that while Irish hunter-gatherers closely resembles British and European ones, they showed signs of island isolation too.