From Google+ by Terry Tao:
A counter-intuitive manifestation of a zero-one law: according to computer models. everyone who was alive in 3000 BCE is either an ancestor of everyone alive today (including people in extremely remote and isolated communities), or else an ancestor of nobody alive today; furthermore, someone alive as recently as 1000 BCE is also the ancestor of everyone alive today.
The original post is We Are All Princes, Paupers, and Part of the Human Family.
See also Charlemagne’s DNA and Our Universal Royalty.
And somewhat related Your husband is terrestrial (in Russian).