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Comment by amanj41 | Hacker News Reader

Can you explain why that is the case? I would have thought that the large amounts of immigration from many places to countries like the U.S. would lead to greater genetic diversity.

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Humans have been living in Africa for as long as humans have existed--let's call that a round 300,000 years. The populations around where humanity first existed therefore reflects accumulated genetic diversity for all 300,000 years.

Let's say 100,000 years ago, a small group of humans left Africa for the other places of the world. That small group of humans represent a very small fraction of genetic diversity--it's effectively genetically identical. Assuming no more admixture over the millennia, the out-of-Africa humans will get more genetically diverse. But so are the original Africans, who also have the genetic diversity they started with--they're getting diverse no less quickly than the out-of-Africans, and since they started more diverse, the entire out-of-Africa can't ever catch up.

Real genetics is of course more complicated than this simple picture, but the basic principle holds that you find more diversity the closer you get to the origin.

Also - Africa is huge. It's the second largest continent after Asia, and has 1.4 billion people living there.

And it’s a north south orientation which leads to more diversity in species and perhaps people too (my hunch)

Think about how much you know about a random country, say Hungary. Probably only one composer, maybe one movie. But in Hungary there are millions of books, movies, etc most of which never left.

Same for humans during migrations. Only a small percent of humans left Africa, and then only a smaller percent kept going, etc. So as long as not much time has passed to generate new variation in the areas they settle, only a small fraction of local variation ever leaves is low. And that's what we see, Africa is extremely diverse, Europe/Central Asia less, east Asia even less, etc. It's neither bad nor good, just a number.

There are some other factors, such as mixing with people already present at the destination for a long time (Neandertals, Denisovans) which balance it out slightly.

The US has lots of diversity, but there were a lot of groups in Africa for a very long time (maybe 10k+ separate groups). It's unlikely that people from each one have made it here, or even out of Africa at all, in significant numbers.

Not the OP but I’m guessing that it’s because humans in Africa were isolated in thousands of communities/regions for hundreds of thousands of years and evolved separately. Meanwhile the diaspora outside of Africa spread via select communities in a much shorter time. Somewhat counterintuitive.

It's the founder effect. While on the surface it looks like "large amounts" of migration, it in fact represents a very small percentage of the gene pool. That group effectively becomes an isolated breeding population (though it's far more complex than that because groups would often encounter other groups and there would be interbreeding and such).

Humans originated in africa, and only a subset of our species genetic diversity ever left africa. So all immigration from "many places" still represents only a subset of the existing diversity.

There is also some immigration directly from africa, but that can only increase it to at most the same as exists there. Almost certainly somewhat less, in practice.

Interestingly, even within Africa, the human genome notably lacks variability, to a point where it’s hypothesized that there was a fairly severe population bottleneck at some time in the last couple hundred thousand years.

oh gee i wonder what that bottleneck could have been

cough mysterious 1:4:9 monolith cough

I don’t remember exactly where I read that, but from mitochondrial DNA it was calculated that at some point we were down to 50 or so individuals.

Thank you for the clear explanation.

I've heard that humans were at some point reduced to a very small number, like thousands of individuals.

So how did a few thousand individuals become such great genetic diversity? Does genetic diversity come from being isolated, instead of mingling with other migratory groups?

Human genetic diversity on the whole is very, VERY low compared to other species. The superficial physical variation we associate with ethnic diversity (skin tone, nose shape, lip shape, hair color/texture, eye folds and angles) are genetically insignificant compared to their visual impact.

It's worth noting that this is not the only hypothesis for the origin of modern humans: https://en.wikipedia.org/wiki/Multiregional_origin_of_modern...

It's the only correct one though.

There is small amounts of "multiregionality" since some peoples are part Denisovan and most everyone is part Neanderthal, but it's not even a plurality in anyone we've ever found.

You can do a search for that, I read that article a few years ago but never saved the link.

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