Jonathan Shaw in Harvard Magazine explains that studies of population DNA suggest that an effective policy of sexual apartheid practiced by the newly arrived Anglo-Saxons could have eliminated British male Y chromosomal DNA in as few as five generations. The Spanish conquistadores in Colombia and the Vikings in Scotland and Ireland left similar DNA patterns, in which the male heredity of the modern population is overwhelming traceable to the invaders, but female mitochondrial DNA predominantly descends from the conquered population.

Moral? Successful invaders get the girls. At some level, history amounts to a contest over who gets to reproduce his DNA, and who does not.

There are no signs of a massacre–no mass graves, no piles of bones. Yet more than a million men vanished without a trace. They left no descendants. Historians know that something dramatic happened in England just as the Roman empire was collapsing. When the Anglo-Saxons first arrived in that northern outpost in the fourth century a.d.–whether as immigrants or invaders is debated–they encountered an existing Romano-Celtic population estimated at between 2 million and 3.7 million people. Latin and Celtic were the dominant languages. Yet the ensuing cultural transformation was so complete, says Goelet professor of medieval history Michael McCormick, that by the eighth century, English civilization considered itself completely Anglo-Saxon, spoke only Anglo-Saxon, and thought that everyone had “come over on the Mayflower, as it were.” This extraordinary change has had ramifications down to the present, and is why so many people speak English rather than Latin or Celtic today. But how English culture was completely remade, the historical record does not say.

Then, in 2002, scientists found a genetic signature in the DNA of living British men that hinted at an untold story of Anglo-Saxon conquest. The researchers were sampling Y-chromosomes, the sex chromosome passed down only in males, from men living in market towns named in the Domesday Book of 1086. Working along an east-west transect through central England and Wales, the scientists discovered that the mix of Y-chromosomes characteristic of men in the English towns was very different from that of men in the Welsh towns: Wales was the primary Celtic holdout in Western Britannia during the ascendance of the Anglo-Saxons. Using computer analysis, the researchers explored how such a pattern could have arisen and concluded that a massive replacement of the native fourth-century male Britons had taken place. Between 50 percent and 100 percent of indigenous English men today, the researchers estimate, are descended from Anglo-Saxons who arrived on England’s eastern coast 16 centuries ago. So what happened?