Genetic diversity across modern Britain and Ireland largely mirrors the tumultuous invasions and ethnically defined kingdoms of the Dark Ages, research has found.
Although Britain and Eire dominate the region by area, as well as by influence, geographically they are but two parts of a scattered archipelago, which comprises scores of islands, of which about 134 are inhabited.
Stretching from the Shetlands in the north to the Scilly Isles in the south-west, these small islands were tempting targets for invasion or nonviolent settlement by European seafarers throughout the period between the fall of the Roman Empire and the rise of the Renaissance.
Now genetic analysis reveals that the interbred cohorts thus produced have remained largely distinct ever since.
In a paper published in the journal PNAS, a large team of researchers led by Edmund Gilbert, of the Royal College of Surgeons in Ireland, sequenced the genomes of 2554 people selected from geographically isolated areas across Britain, Ireland and the islands.
Of particular focus were folk from the north of England, Scotland and the islands beyond, because, the researchers note, these areas have been historically underrepresented in existing genetic studies.
The results, they explain, reveal that the Vikings had a strong influence on the population make-up of the Scottish islands, and that people born in isolated areas tended not to mix with outsiders.
As far as the northern regions were concerned, genetically distinct, geographically defined sub-populations had formed as early as the eighth century, when the Norse people came ashore. (In southern England, the dilution of the relatively stable population that had been in place since 3000 BCE began earlier, from 400 CE, with the arrival of the Anglo-Saxons.)
“Many genetic boundaries are consistent with Dark Age kingdoms of Gaels, Picts, Britons, and Norse,” write Gilbert and colleagues.
“Populations in the Hebrides, the Highlands, Argyll, Donegal, and the Isle of Man show characteristics of isolation.”
This was especially the case in the Shetland islands, where comparisons between local and Scandinavian genomes found between 24% and 28% shared heritage. The genetic signatures were strongly associated with two Norwegian counties – Hordaland and Sogn og Fjordane – historically linked to outward Viking migration.
The Norse contribution dropped substantially for people in North Scotland, Argyll, the Isle of Man, and Ireland, and disappeared almost completely further south.
But genetic traffic into the north of the archipelago wasn’t all one way.
Gilbert and colleagues also found that people with Gaelic genotypes were among the first settlers in Iceland. The likelihood, they suggest, is these travellers originated from either north-western Scotland or the north of Ireland.