This is the maths:
You have got two organic mother and father, 4 grandparents, eight great-grandparents, and so forth. The quantity doubles each era – after six generations, for instance, you’ve got 2⁶, or 64, ancestors. This quantity grows so shortly that it hits roughly a billion ancestors by the point you get 30 generations down the road. However world inhabitants did not attain a billion till the 1800s, and that’s far fewer than 30 generations again in time.
Basically, as you return in time, you quickly attain some extent at which everybody who was alive at a given level is the ancestor of both everyone (if that they had kids, and their kids had kids, and many others) or no one (if their line died out).