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
You share a lot of those ancestors with other people. I have 2 parents, and my sister has 2 parents, but that doesn’t mean that 4 people needed to exist. It’s just the same 2 people for both of us. Likewise your last 500 ancestors are also the ancestors of thousands of other people too.
You are almost correct, but you are forggeting a simple fact:
The more you go back the more likely it is that some of you ancestors were “on both sides of the family”. As in, some generations ago there was a couple who is the great-great-…-grandparent of both you father and you mother. The more you go back, the more such couples there are.
Your numbers are correct, but the missing piece of the puzzle is that some of them are the same people. Your great to the n^th grandparent on your mother’s side might also be your great to the m^th grandparent on your father’s side.
People die. A lot. Like ALL the time. Before you’re done reading this reply, people died.
Humans have been around for couple hundreds of thousands of years. Probably been a few hundred billion people in those years; to put that in perspective, it would take you roughly 3 days to count to 1 million. It would take you 30+ years to count to 1 billion.
There are two parts to the answer:
People share ancestors. Sibling have the same parents. They have parents who may have had siblings. People breed with cousins of various levels of remove. Some people didn’t breed at all. It’s not just a simply forever dividing tree.
Secondly, there are ~8 billion people alive today. There have been ***~100 billion*** homo sapiens since the species arrived.
There’s a lot of inbreeding, people marrying nth cousins.
An extreme example: It’s estimated that the 12 million Ashkenazi Jews alive today are descended from apx. 350 individuals who lived around 1350 A.D. with < 0.5% outside DNA per generation. All of them are no more distant than 6th cousins.
If you marry your first cousin, then your child has six great-grandparents, not eight, because two are duplicates. Expand as necessary.
> Demographer Kenneth Wachtel estimates that the typical English child born in 1947 would have had around 60,000 theoretical ancestors at the time of the discovery of America. Of this number, 95 percent would have been different individuals and 5 percent duplicates. […] At the time of the Black Death, he’d have had 3.5 million — 30 percent real, 70 percent duplicates. The maximum number of “real” ancestors occurs around 1200 AD — 2 million, some 80 percent of the population of England.
https://www.straightdope.com/21341588/2-4-8-16-how-can-you-always-have-more-ancestors-as-you-go-back-in-time
I believe your numbers are wrong. You don’t have kids every 50 years. The kids happen from 10 to 50. Yeah, 10, we hopefully are mostly civilized now, but go back to the past and people where having children a lot earlier. And dying, a lot earlier. So 20 generations could be as early as merely 200 years.
You get to many of the same ancestors via various roots, so the same person may both be your mothers great great grandfather and also your fathers great great great grandfather.
Most people would be surprised to find that they probably don’t have to go very far back in their lineage before ancestors start to “overlap” (to put it nicely)
Travel wasn’t really any significant part of people’s lives until the last century or so. Most of human history, people didn’t go very far and only interacted with relatively small populations. Most people in these smaller areas were somewhat related.
Not to mention, your ancestor count would actually shrink at critical mutation points. For example, all of your ancestors would converge at mitochondrial eve, then again at Y chromosomal Adam, etc.
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