Chang’s calculations get even weirder if you go back a few more centuries. A thousand years in the past, the numbers say something very clear, and a bit disorienting. One-fifth of people alive a millennium ago in Europe are the ancestors of no one alive today. Their lines of descent petered out at some point, when they or one of their progeny did not leave any of their own. Conversely, the remaining 80 percent are the ancestor of everyone living today. All lines of ancestry coalesce on every individual in the 10th century.