Thanks fren! I went down the rabbit hole and found this too.

Actually, almost every single person alive around 1150AD or earlier, whose blood line isn't extinct, is an ancestor to almost every one of us alive today. Prophet Mohammed, lord Buddha and Genghis Khan are ancestors to almost ALL of us, including YOU.

Unbelievable? lets see... All of us have 2 (mom n dad) ancestors in the previous generation, 4 in the generation before that (2^2 = 4 grand parents) and so on. Hence if we go back N generations, the ancestor count is 2^N (not accounting for related people marrying in a large, related-population). The average generation gap now is 30 years and before a few centuries until up to a century earlier has been approx 20. Taking an average of 25 years for a generation gap, we have 4 generations per century.

So, before a century, each of us had 2^4 = 16 ancestors. Before X centuries, each of us would have 2^4X ancestors. If we go back C centuries, where the world population was <= 2^4C, every person alive at that time (and whose blood line is not extinct now) is an ancestor to each of us now (theoretically)

That value of C is 7.25 (725 years). 2^(4*7.25) = 500M = approx human population of the world 725 years back (1300AD) (source: World History for Us All: Key Theme One)

Of course there should be allowances for geographically local concentration of ancestors - so we'll go back another couple of centuries to be sure ;) That makes it between 1100AD - 1200AD.

Therefore, almost every single person alive during 1150AD or earlier, whose blood line isn't extinct, is an ancestor to almost every single person alive today.