Comment by samatman | Hacker News Reader

Comment by samatman | Hacker News Reader

Parent Full thread

So I was wondering if this article was just being incredibly misleading or straightforwardly lying, and I hit this:

> “If you were alive at the genetic isopoint, then you are the ancestor of either everyone alive today or no one alive today,”

Yeah. That's a lie. There's no way around it, this is a dishonest statement.

I'm in no kind of mood to explain why that is, for pretty predictable reasons this conversation goes to places I'm not fond of.

But it ain't true, and I'm willing to explain why people might wish it was: 12 generations back, you have absolutely no DNA from some of your ancestors. That's how recombination works, the deck is shuffled in a "chunky" way and some of your ancestors get shuffled out.

So all those Europeans have no (dig this: zero) genes in common with Sub-Saharan Africans that weren't conserved 90,000 years ago either because: they don't have an African ancestor (the answer) or because they are descended from all Africans alive in 5000BCE and the genes all got shuffled out because reasons, which is, not the answer.

It's a nice story and I'd love to believe it, if it was true, which it isn't.

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.

"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.

It 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.

But they would then not become an ancestor, and thus will not appear in a chain of ancestors.

Most of our ancestors, even in recent history have migrated a lot.

For example Vandals, a tribe living in Poland, moved west and south, and established kingdoms on the Iberian Peninsula, Mediterranean islands and North Africa in the 5th century! Many tribes living in Europe had a similar migration pattern at that time [0].

This was a part of humans migrations from Asia to Europe, pushing people ahead of them. Another incredible migration is the Bantu expansion, where people consistently moved eastward and southward for at least a millennia.

And we changed a lot in the process, New arriving people in Europe from Africa, at the end of the ice age, had a much darker skin than today.

[0] https://en.wikipedia.org/wiki/Migration_Period

Take a deck of cards, and heavily shuffle it. Is it the same deck or an entirely new one?

I think most of us understand that there is a difference between sharing DNA and biological ancestors.

The statement you're arguing against says nothing about having DNA from that ancestor, just about their existence as an ancestor. Instead, the article seems to agree with you, saying: "Not everyone of European ancestry carries genes passed down by Charlemagne, however. Nor does every Jew carry genes from their Sephardic ancestors expelled from Spain. People are more closely related genealogically than genetically".

Do you have a citation? Because that’s not what Wikipedia says https://en.m.wikipedia.org/wiki/Identical_ancestors_point

Well the most common ancestor is used to push a certain narrative about all people "being the same", while conveniently putting aside hundreds of thousands years of evolution, which are precisely the source of the present day differences, but that doesn’t mean it’s false either, as there is indeed a common ancestor if we look back far enough.

I think it's clear from history that narrative of us "being the same" hasn't caused nearly as much trouble as the opposite, the complete and utter lie that we are not all human.

I believe for all people to be the same, ability of interbreeding should be considered enough. I agree pushing agenda on half truth is harmful.

This is idiotic.If this was true, Humans wouldnt share 90%+ genetic similarity with great apes (depending on which genes you count) which would go against every scientific consesus.

And you are claiming humans share no genes each other over a span less than 100K years.

Reply on news.ycombinator.com