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 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, 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 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 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.
This new population, which lived by hunting and gathering, survived a sharp cold spell called the Younger Dryas that lasted from 12,300 to 11,000 years ago. Much later, some 6,000 years ago, agriculture finally reached the British Isles from its birthplace in the Near East. Agriculture may have been introduced by people speaking Celtic, in Oppenheimer's view.
Although the Celtic immigrants may have been few in number, they spread their farming techniques and their language throughout Ireland and the western coast of Britain. Later immigrants from Northern Europe had more influence on the eastern and southern coasts. They, too, spread their language, a branch of German, but these invaders' numbers were also small compared with the local population.
In all, about three-quarters of the ancestors of today's British and Irish populations arrived 15,000 to 7,500 years ago, when rising sea levels split Britain and Ireland from Continental Europe and from each other, Oppenheimer calculates in a new book, "The Origins of the British: A Genetic Detective Story."
Ireland received the fewest of the subsequent invaders; their DNA makes up about 12 percent of the Irish gene pool, Oppenheimer estimates. DNA from invaders accounts for 20 percent of the gene pool in Wales, 30 percent in Scotland, and about a third in eastern and southern England.
But no single group of invaders is responsible for more than 5 percent of the current gene pool, Oppenheimer says on the basis of genetic data. He cites figures from the archaeologist Heinrich Haerke that the Anglo-Saxon invasions that began in the fourth century A.D. added about 250,000 people to a British population of one million to two million, an estimate Oppenheimer notes is larger than his but considerably less than the substantial replacement of the English population assumed by others. The Norman invasion of A.D. 1066 brought not many more than 10,000 people, according to Haerke.
Other geneticists say Oppenheimer's reconstruction is plausible, though some disagree with details. Several said that genetic methods did not give precise enough dates to be confident of certain aspects, like when the first settlers arrived.
"Once you have an established population, it is quite difficult to change it very radically," said Daniel Bradley, a geneticist at Trinity College, Dublin. But he said he was "quite agnostic" as to whether the original population became established in Britain and Ireland immediately after the glaciers retreated 16,000 years ago, as Oppenheimer argues, or more recently, in the Neolithic Age, which began 10,000 years ago.
Bryan Sykes, another Oxford geneticist, said he agreed with Oppenheimer that the ancestors of "by far the majority of people" were present in the British Isles before the Roman conquest of A.D. 43. "The Saxons, Vikings and Normans had a minor effect, and much less than some of the medieval historical texts would indicate," he said.
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