Thread: Geometric expansion of ancestors?

This must be really easy but I'm stepping around it...


We have;
2 parents (1 gen)
4 grand-parents (2 gen)
8 3 generations past
16 4 gen
32 5 gen
64 6 gen
128 7 gen
... ...

if we continue to 100 generations back we get
1,267,650,600,228,229,496,703,205,376
I dont even know how thats called a gazillion billion,
(but everything points to the fact the population was smaller before agriculture)

I conclude theres a gigantic flaw in my reasoning, what is it im missing?

Inbreeding. People marry distant cousins without realising.

Your reasoning assumes a succession of ancestors who are completely unrelated going back 100 generations.