3 ms·
Your intuition on this is wrong, and there's no need to fly off the handle. Ancestry is infectious. Take two populations, A and B, completely isolated from eac
by titzer 6y ago
Your intuition on this is wrong, and there's no need to fly off the handle.
Ancestry is infectious. Take two populations, A and B, completely isolated from each other. (nevermind that they all have a common ancestor long before the populations diverged). Now one day in 1000 B.C. a single individual from population A migrates over to population B, mates, and has offspring (more than 1) with an individual from population B. Just keeping things simple with 2 children per generation. The next generation, those 2 individuals go off and have offspring with other members of population B. That's 4 descendants. Next generation 8. And so on. Of course the characteristics of population A (it's phenotype) may become less, because proportionally, that individual's genes make up less and less with each generation. So it might not even be detectable by looking at someone that they have a tiny bit of A's genes.
The key is that the propagation of ancestry never stops, because ancestry doesn't round down to zero. It's a mathematical relation. Now, genetically, those genes keep spreading (thinly) over the entire population, generation after generation, so it is actually possible that those genes get rounded down to 0, and you actually have no DNA in common with that ancestor (except that which they had in common with your other ancestors). But mathematically, in terms of ancestry, there is a point many generations later that it becomes almost impossible to not interbreed with someone who is descended from A.
This is exactly the intuition behind this genetic isopoint--when you run the exponentials either direction in time, they overflow the population. So you either have your genes completely selected out, or they "infect" the entire population, assuming that you and your offspring consistently have more than 1 child on average.
- jsnk 6y ago"The key is that the propagation of ancestry never stops, because ancestry doesn't round down to zero." I get your other points but this assumption seems obviously wrong. Plenty of people historically did not live to leave children behind. Then of course, we can have complete stops in the propagation of ancestry.
- magicalhippo 6y agoBut they would then not become an ancestor, and thus will not appear in a chain of ancestors.
- simonh 6y agoIt only takes one single person to migrate and have surviving descendants for everyone to end up their descendant. So yes it's possible that people migrate and not have descendants, but that has to be true for every single one of them. In reality even over very long geographic distances we have evidence of hundreds or even thousands of people migrating. Marco Polo first visited China with his uncles who had been there before. He recoded that there were so many European traders in Beijing that the Italians, Spanish, etc each had their own districts. Even before that Rabban bar Sauma was an Armenian Christian monk born in the Armenian district of Beijing in about 1220, who travelled to Europe. He appears in the court records at the Vatican and in England. As for sub-Saharan Africa, Middle Eastern traders have been plying the seas of eastern Africa, and running caravans through central Africa for thousands of years. Their cultural and genetic fingerprint is all over Africa. There is a small, but persistent and measurable level of sub-Saharan African genetic markers in southern Europe dating to the Roman era.