Ireland is particularly interesting to geneticists, because it seems like a place where many ancient peoples may have converged. For starters, the pre-historic residents there showed a smooth transition from hunting and gathering to farming, and then from stone to metal working. It's likely that changes like these were driven by newcomers with new ideas, but we can't assume that the original inhabitants of Ireland didn't just come up with these life changes on their own.
But even the genes of the modern Irish hint at a melting-pot past. They have some of the highest levels of certain genetic mutations, including the one that allows adult humans to tolerate dairy. Several mutations that promote dangerous illnesses, like haemochromatosis (excessive iron retention) and cystic fibrosis are also more prevalent than they are elsewhere in the global population.
Study author Dan Bradley, professor of population genetics at Trinity College Dublin, explained that recent technological and methodological advances in ancient DNA analysis allowed his team to produce full genomes for the four skeletons used in their research. They were surprised to see how different the Neolithic woman, who was found in Belfast in 1855 and lived over 5,000 years ago, was from the three male skeletons analyzed, who were found off of Rathlin Island in 2006. With just 1,000 years separating them, their genomes shouldn't have looked so strikingly different - which suggests that some major migration really must have occurred.
"It was a surprise to see several genetic elements typical of the modern Irish genome, both of interesting genes but also of more anonymous DNA fragments, appearing in the Bronze Age specimens," Bradley said of the more recent skeletons. "These genomes when taken as a whole are more like modern Irish, Scottish and Welsh - insular Celtic populations. This suggested some large degree of establishment of the genetics of these populations 4,000 years ago."
The Bronze Age men even had the genetic mutation for haemochromatosis, which is now so common in Irish populations that it's sometimes called a Celtic disease.
The differences between these men and the ancient farming woman speak of a "profound migratory episode" in the 1,000 years between their lifetimes, Bradley said. Based on the men's DNA, the researchers suspect that their ancestors may have come to Ireland from the Pontic Steppe - the area of Eastern Europe that sits over the Black Sea, including what's now the Ukraine.
For now, this probable migration is still quite mysterious. We know it must have occurred sometime between about 5,000 years ago and 4,000 years ago, but scientists will have to sequence the genomes of more skeletal remains from before, during and after that period to confirm just how and when the migration took place.