If You have 2 parents, 4 grandparents, 8 great parents, so if you go back 40 generations you’d have a supposed 1 Trillion ancestors, But only 100B humans have ever existed. Explain.
In: Biology
People have shared ancestors. They might not be mating with their close relatives, like shared grandfathers or even great-grandfathers, but if you look far enough back everyone is going to be related.
This seems obvious from a practical standpoint, how are you going to be able to know that a prospective mate shares no ancestors with you *for all time*? There is no way of knowing, so such relationships occur. In fact how far removed one must be to mate varies by culture, with some middle eastern cultures viewing first cousins as ideal mate prospects.
Simple: a lot of those 1 trillion people happen to be the same people. There is unavoidably a lot of n-th degree cousin fucking all through history, the important thing is to not to fuck too close relatives too often or you get interesting congenital defects. A couple villages with a population above a thousand or two can get by fine marrying among themselves.
One word – interbreeding.
The further you go back the more common ancestors you’ll find between any two random people.
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