These narrative simplifications end up just being confusing.
Neanderthals from 400kya are often classed as Heidelbergensis. These guys were less Neanderthal-ish and more similar to us... being closer to and less divergent from the sapiens-neanderthal LCA. Neanderthal-Denisovan divergence occurs at this time.. so calling them Neanderthals rather than Neanderthal ancestors is kind of messy.
There is a shortage of fossil evidence from this and earlier periods... It's called the "muddle in the middle."
In any case.... Sapiens also had ancestors at this time. We don't have fossils, but something has to be our ancestors. So if we are calling Neanderthal ancestors from this period Neanderthals... it would be more consistent to call sapien ancestors sapiens.
Individual populations may have been insular, small and most died out. But... there were people everywhere.
Humans existed over a vast range. From south Africa to Northern Eurasia. East to west. At this point in time... I think it's confusing to think of neanderthal/denisovan/sapiens as different species.
Individuals may have been inbred... but the overall genetic diversity across the whole range was greater than the genetic diversity we have today. In some sense, we are the inbred ones.
Also... population estimates are pretty dicey. We don't really know. Could have been booms and busts. Could have been ideal habitats with higher populations.
We still have a fairly poor grasp of human "natural history"
It is also true that inbreeding for extended periods weeds out both dominant and recessive bad genes very effectively. As long as at least one good or not-so bad alternative is maintained.
So not as surprising that small groups can last a long time, once they reach a threshold, as implied by the article.
It’s a brutal way to improve the stock, as lots of individuals suffer until (and in service of) a debilitating gene going “extinct”. And every new maladaptive mutation restarts the process, but it works.
On the upside, any adaptive mutation can just as quickly become pervasive.
The biggest downside in the long term is a lack of genetic diversity as a shield against new diseases.
I know this is not unique to this population, but I also always wondered if it correlated to the fact that it is one of the historic Neanderthal populations. I have a photo of a dude I used to play soccer with that looks like I put a Neanderthal model from the natural history museum in a jersey, and I have met very few people like that in the states. The Basque Country is a very small population.
In my country, there is an area with archaeological sites of Neanderthal villages and their mammoth hunting grounds. In one area, there are thousands of mammoth bones. Imagine having only wooden spears and ordinary stones at your disposal. Maybe flint spears, maybe not. In this area, flint is too rare and is mainly used for cutting, because the nearest flint deposit is 400 km away.
I would assume there were local populations that lived for a long time. But then they were gone too.
Some DNA is in modern humans, so there must have been some inter-breeding, but that in itself alone can not explain why the Neanderthals went extinct.
Maybe Enkidu from the Epic of Gilgamesh was one.
Was it a knifes edge at times? Sure. Was it abundant at times? Also true.