I googled the amount of people that lived on earth throughout its entire history, it’s roughly 108 billions. If I take 1 person and multiply by 2 for each generation of ancestors, at the 37th generation it already outnumbers that 108 billions. (it’s 137 billions). If we take 20 years for 1 generation, it’s only 740 years by the 37th generation.
How??
(I suck at math, I recounted it like 20 times, got that 137 billions at 37th, 38th and 39th generation, so forgive me if it’s not actually at 37th, but it’s still no more than 800 years back in history)
In: 1391
(Nearly) everyone of European ancestry is a descendant of Charlemagne. If all four of my grandparents are descendants of Charlemagne, that means Charlemagne fills _at least_ four different ancestor slots on my family tree.
But it isn’t just Charlemagne, it’s many of my ancestors, meaning that if you go back far enough most of the names will be duplicated elsewhere in your tree.
Aside from mathematic principals, and the Kissing Cousins point that keeps coming up, I think you’re also forgetting that Genghis Khan, *one person* that lived almost a thousand years ago, still has ~16,000,000 living male descendants with his Y Chromosome.
Assuming that every descendant is coming from a unique pair of individuals is part of the problem. If you have a single individual procreating with multiple unique partners (like, say, a conquering warlord that razed every village he came to and took any woman he wanted as his concubine) you’ll wind up with an inverted funnel where dozens of generations wind up funneling back to one person.
the gene pool is not as deep as you think it is. it was common for cousins, second cousins etc to mate. people didn’t even travel much a long time ago. the gene pools of small villages are shallow. the family trees branch then come back together again. imagine 2000 years ago if you and your family had only ever met 1000 people. you would choose your mate from that 1000 and your kids would choose from the 1000 and their kids from 1000… we are all a lot more related than we like to think. if you’re up to the 108 billion, you’re also making assumptions that everyone in the world could choose anyone from anywhere in the world as a mate. you don’t have to go back too far to find people who’s entire existence was a 100km radius of their birthplace.
Incest is wincest. Your great-great-great grand dad from mothers side also happens to be your great-great-great grand dad from your fathers side. Further back you go in your family tree more overlap you are going to find.
Everybody in one village has the same set of ancestors from few hundred years ago and its much less than 2^n ancestors per n generations.
You’re right. Of course it would exceed the number of people who were around at the time. You have to remember how often people will either commit incest directly or marry into families they are distantly related to which is likely if the family remains in a small area for hundreds of years..
Maybe if you try thinking of it the other way around it would be more intuitive, like the biblical version of history where everyone is a descendant of Adam and Eve. In that extreme scenario every family tree would eventually merge back to a single set of parents rather than the billions and billions your assumption leads to. In reality this merging has happened at points all the way up everyone’s family tree.
In 1850 25 couples, man and woman, settled in a new community, Freefall, Kansas.
The average number of children each couple had was 4. That means the next generation has 100 children in it (50 couples). They pair up and make their own babies, again assume 4 children. So generation 3 has 200 in it. They repeat their parents 400 kids in the next generation (gen4). Gen 5 has 1600, gen 6 has 3200, gen 7 has 6400 kids, and gen 8 has 12800.
Now if we assume each generation is 20 years, then the children are born around the following years:
1850
1870
1890
1910
1930
1950
1970
1990
Now you come along being born in 1990 and you do the anscestor math and you say, wow I has 2 parents born in 1970 and 4 grandparents born in 1950, and you keep going back all the way to the founding of Freefall, KS. By your math you have 128 ancestors in 1850. But the town was founded by only 50 people.
You are correct that on your family tree going back 7 generation you have 128 slots to fill. Those 128 slots will be filled by the same 50 people.
It’s more like: by the time the nth generation rolls around pretty much anyone you choose will have a common ancestors. Not that everyone is banging their sister. More like everyone is a cousin of varying degree.
We’re all a bunch of inbred mutants
It’s because you share a lot of ancestors with a lot of people , like a lot a lot.
Think of it this way , your great great grandmother ( 3 generations back ) has one one sibling , let’s say each generation has 2 kids. That means that your grandmother has a sibling and 2 cousins , then your mother has a sibling and 2 cousins and 4 second cousins and it means you have a sibling , 2 cousins 4 second cousins and 8 third cousins. So there’s 16 people in 3 generations who share the same great grand parents on one side and it doubles every generation essentially.
Now factor in people used to have like 4 kids back in the old days so you would have absolutely enormous amounts of third cousins in just 3 generations since in just three generations back you will have 3 siblings , 16 first cousins , 64 second cousins and 256 third cousins that all share one of the same sets of great grandparents
In genealogy it’s called pedigree collapse.
The ELI5 is that we are all related and if you go back far enough you will find that you very likely have a lot of overlap in your family tree through closer cousins marrying.
In my own tree, I’ve found that in one early Colonial American branch I have five instances of 2nd cousins marrying over a period of maybe 100 years. As a result, I have a lot fewer direct ancestors than I would have otherwise. Now multiple that out over dozens of generations and you wind up with far more realistic and manageable numbers.
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