system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: 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.
gpt
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.