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
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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