Posted elsewhere but would like explained like I’m 5.
What I can’t get my head around is: I had 2 parents, they had 4 (in total) who would have had 8 in a geometric progression, so going back even 1000 years or 20 generations (assuming an average lifespan of 50 years) is 2,097,152 ancestors for just me, and given that there is a reported 7.9 billion people on earth alive today it seems mathematically impossible that all those people could have existed.
In: 7
Suppose each person has 2 million ancestors (at generation 20). And there are 8 billion people on earth.
Multiply these two together, and you get 16 quadrillion, You’re right, there were not that many people alive back then. But that’s the wrong way to count the total, because you’ve gone and double-counted a lot of ancestors.
Your brothers and sisters, for example, all share the same 2 million ancestors. Even if you’re an only child, many people are not. Also, you share a lot of ancestors with your cousins, second cousins, etc.
We’re all distantly related, in fact, and often not as distantly as you might think.
Also because of this, some of your “2 million ancestors” are the same person. For example, maybe your father’s father’s mother’s father’s mother’s father and your mother’s father’s father’s mother’s mother’s father’s father are actually the same person. So you don’t actually have 2 million 20th generation ancestors, you’ve double-counted a lot of them as well.
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