Steve Olson in Why We're All Jesus' Children has a gimmicky exposition on the reticulated character of our genealogies. But Olson tries to pull a fast one here:
It gets even stranger. Say you go back 120 generations, to about the year 1000 B.C. According to the results presented in our Nature paper, your ancestors then included everyone in the world who has descendants living today. And if you compared a list of your ancestors with a list of anyone else's ancestors, the names on the two lists would be identical.
The reality is that quantity, not quality, counts, you need to know how many times someone was your ancestor, not that they were your ancestor. More here.