Besides Tom Klein´s comment reproduced below, I suggest the original poster to look in the Jewishgen archives for my 1994 (yes, not a typo) post titled "how many Jews have been in the history of mankind" (and replies) and further mathematical inquiries as posted by A.R. Liboff, Michael Bernet their respective replies.

A demographic timeline of Jewish history I find very interesting is available at

"at the 2002 iajgs conference (in toronto as it happens), one of the presentations touched on a very similar theme, that within a given population, is it mathematically possible to prove whether 2 individuals were related? the answer lies in reversing the question, and asking what it would take for them not to share a common ancestor: for each generation that they are not related, there would have to be 2x as many distinct ancestors. and fairly quickly, that number, 2^n, would exceed the total number of humans. assuming about 25 years per generation, 1000 years would be 40 generations; 2^40 is a trillion. that's a far bigger number than there have ever been humans on earth. (estimated at 100 billion.)

the flipside is that assuming that the person's line didn't die out, and assuming that there weren't any special taboos or physical barriers limiting their offspring, it would also mean that after a sufficient number of generations, everyone is descended from any given individual.

so either king david's line ended, and no one is, or after 1000 years everyone was his descendant.