Original in Spanish: Tests de ADN y el pedigree collapse
Those DNA ethnicity tests usually say they can tell you where your ancestors may have been up to about the year 1200.
And I didn’t understand where that number came from. Of course it’s all approximate, but why 1200?
Well, turns out there’s a concept called “pedigree collapse” that explains it. It’s so simple and so obvious that I’m even a little embarrassed never to have stopped to think about it. I’ll tell you in case it slipped past you too.
You have 2 parents, 4 grandparents, 8 great-grandparents… and so on. Each generation is 2^n people. In my family we reach the year 1800 in 7-8 generations, which means in 1800 there were 256 ancestors of mine walking around.
That’s assuming people limited themselves to mating with non-family, which is a lot to assume. The further back in time you go, the less people moved around and the more likely you’d end up having children with a cousin or someone in your family, so that 2^n is somewhat… “exaggerated”.
Really, the numbers give it away much earlier. If you sketch the 2^n function over time and compare it with the world population of each moment… around 1200 your ancestors coincide with the entire world population, so from there upward everyone was going to be your ancestor anyway, and the family tree “narrows”, because the same people play several roles.
Here you can see the chart you get for any family tree. In mine, from what I know so far, in the year 1251 my 360M ancestors would coincide with the world population.
https://learnforeverlearn.com/ancestors/?file=royal92.txt
And here someone who tells it much better than me and with more drawings:
https://waitbutwhy.com/2014/01/your-family-past-present-and-future.html
That we all come from the same little monkeys was clear, but intuitively I didn’t imagine we could be so related to people from such recent times. Of course I haven’t touched the “more or less isolated populations” factor, but you get the idea.
Cool, right? If you can think of someone born early enough, you’re family with that person in some way, be it Genghis Khan or Cleopatra.
To my arms, sibling :)
(Facebook, 2017-12-30)