Britain and Ireland are so thoroughly divided in their histories that there is no single word to refer to the inhabitants of both islands. Historians teach that they are mostly descended from different peoples: the Irish from the Celts and the English from the Anglo-Saxons who invaded from northern Europe and drove the Celts to the country’s western and northern fringes.

But geneticists who have tested DNA throughout the British Isles are edging toward a different conclusion. Many are struck by the overall genetic similarities, leading some to claim that both Britain and Ireland have been inhabited for thousands of years by a single people that have remained in the majority, with only minor additions from later invaders like Celts, Romans, Angles, Saxons, Vikings and Normans. The implication that the Irish, English, Scottish and Welsh have a great deal in common with each other, at least from the geneticist’s point of view, seems likely to please no one. The genetic evidence is still under development, however, and because only very rough dates can be derived from it, it is hard to weave evidence from DNA, archaeology, history and linguistics into a coherent picture of British and Irish origins.

That has not stopped the attempt. Stephen Oppenheimer, a medical geneticist at the University of Oxford, says the historians’ account is wrong in almost every detail. In Dr. Oppenheimer’s reconstruction of events, the principal ancestors of today’s British and Irish populations arrived from Spain about 16,000 years ago, speaking a language related to Basque.

The British Isles were unpopulated then, wiped clean of people by glaciers that had smothered northern Europe for about 4,000 years and forced the former inhabitants into southern refuges in Spain and Italy. When the climate warmed and the glaciers retreated, people moved back north. The new arrivals in the British Isles would have found an empty territory, which they could have reached just by walking along the Atlantic coastline, since the English Channel and the Irish Sea were still land.