David Reich: If you say a white British person is a pure descendant of people who lived in Britain a long time ago, "nothing could be further from the truth... And this idea that any one people is somehow purely descended from the people who lived in the past [which was something people thought prior to the genetic revolution], it’s really an untenable picture. Every time we look, it’s not true." In the 80s, the idea developed "that as people move around, they split, they don’t remix,... But the truth is, from genetic data... all of those groups are profoundly mixed, of multiple very divergent groups..." "And so there’s no one time that we’ve been able to reconstruct where there’s a single homogeneous population that’s ancestral to everybody who comes after. Rather, it’s some kind of constant change over time, with mixtures, separations, structuring, and coming back together... It used to be that we had this null hypothesis that no mixture occurred, and we would test that null hypothesis. And that is almost a silly null hypothesis at this point, because everybody is mixed." "And the thing that becomes clear from ancient DNA is that... the strategy used by these direct-to-consumer ancestry testing companies of using modern-day Irish as surrogates for who lived in Ireland in the past — maybe that works in the last thousand years, but it’s not even going to really work very well 2,500 years ago or 3,000 years ago, because there’s just too much mixing and migration." "There are interesting exceptional areas. There actually seems to be an unusually high amount of continuity over many thousands of years in many parts of the Americas, which is really fascinating. But there too, there are disruptions as well. So the pattern seems to be, for the most part, that there are periodic profound movements of people coming from somewhere else, either completely displacing or mixing with the people who were there before." youtube.com/watch?v=qS0Cf2ec…