Genealogists have worked out most American Presidents 800 years or so. A couple interesting facts include (1) about everyone one of them is at least 10th cousin to every other one and (2) a large fraction descend from King John of England. Probability suggests that after a millennium that that someone becomes a super-ancestor, i.e. everyone is descended from them, or no one is descended from them. Very little middle ground. (A lot of northwest European Americans probably have the same statistics as Presidents.)