Researchers found evidence of inbreeding in the genome of a man buried at Newgrange passage tomb, which was built more than 5,000 years ago, a team from Trinity College Dublin said in a press release.
This suggests the man belonged to a ruling elite that practiced first-degree incest -- for example, brother-sister unions -- in the same way as the pharaohs in ancient Egypt or Inca god-kings, the researchers said.
"I'd never seen anything like it," said lead author Lara Cassidy, a geneticist from Trinity College.
"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. In fact, our analyses allowed us to confirm that his parents were first-degree relatives," she said.
The full paper was published Wednesday in the journal Nature.
The discovery was an "unexpected finding" and nobody had any idea about the practice in Neolithic Ireland, paper author Dan Bradley, professor of population genetics at Trinity, told CNN.
"We had no anticipation that it would be the case at Newgrange," he said.
The man was part of a mysterious Neolithic society that built the Newgrange passage tomb in County Meath, eastern Ireland, which is known for the annual solar alignment that sees the sacred inner chamber illuminated by the winter solstice sunrise.
First-degree incest is a near-universal taboo, with the only confirmed social acceptances recorded among elites such as deified royal families, according to the researchers.