That's one theory. Some others are that there is more disease in the warmer areas or that since early agricultural/domestication took place further north it gave those communities a head start
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.
Cultures vary. Also individuals vary. That’s true now, and it was very likely true 50000 years ago.
Maybe not all the rugged individualists survived, but quite likely a few wandering souls travelled far away from their original homes, and got to see different places and experience different things.
They'd also have the situational awareness to decide that life there in France sandwiched between coastal tribes A and B and inland tribe C sucks, so they're going to try their chances on that fuzzy bit of land that looks promising on the horizon.
It only takes one small group over a timescale of thousands of years to make that decision. Dispersal is even more likely if people or stories are able to get back and forth; the issue of "we only have 8 people and not enough genetic diversity" is moot if the story or the rumor spreads that there's lots of land and lots of food on that fuzzy spot over there.
It is way more complicated than that. Even within Africa different groups moved around and exchanged with each other, then out of Africa there was intermingling along with returns to Africa and yet more migrations out of Africa.
The article is no better. It says there was free movement between tribes and over large distances, but there's no real proof of the first one, and the last one is only mentioned for large communities, not for individuals.
Just because the author used only confirmatory evidence doesn't mean that there isn't a correlation or a causal factor at play. The author didn't posit a theory; he merely pointed out that the maps are interesting. Which they are.
Sure, you could probably pick arbitrary political boundaries from different times and create maps that show the opposite trend. However, I would be surprised if there weren't correlative or causal factors, given that historical people reproduced and, in the case of many agrarian peoples, didn't migrate. It's possible that our collective ignorance of history makes this appear to be a novel fact, when we shouldn't be too surprised.
I'd be interested if this were also the case in other countries with high plains populations like Tibet, Nepal, Bolivia, Switzerland, etc. Some places have had stable populations for a long period of time, other populations (and individuals) may be newer, relatively speaking --that might be something to investigate as well, if it hasn't.
Intra-species competition would also have been my first guess. Homo erectus seems to have been quite a successful species, given the archeological evidence. And given the time frame of the migration, I'm betting on a slow but steady expansion as the families and tribes grew larger and split to explore new territories.
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