> It would make sense for a species near extinction to have small population clusters migrating and not bumping into each other
What evidence do you suggest supports this for 50k years?
The group Thorin was from may have been close to extinction when he died, but when genetic isolation started they were 50k years away from being that close to extinction. It seems remarkable based on what we know of hunter gathers or even animal population structure and movement, that there would be no mixing for that long.
>What evidence do you suggest supports this for 50k years?
there is no evidence that they were near to each other for 50K years, only that their genomes diverged 50k years ago which was the last time they were near each other.
At the separate-times-and-places that they died in "France", the recent migrants may only have arrived in the last few months, having spent the previous 50K years over in that other far away place, and died only somewhat near each other a decade apart
the mixed genome of early modern humans and the lack of mixing of Neanderthals is a separate piece of evidence, and it may point to Neanderthals not being as sexy-social, but that's just a "may".
But the other contemporary lineages were mixing more than Thorin, and relatedness broadly correlates with geographic proximity [1]. But the contemporary later lineages in the area split off from samples as far as from Siberia and the Caucasus than to Thorin, as this paper demonstrates.
It isn't out of the question that groups could have moved quite a bit since there is evidence of turnover in either Caucasus or Western Europe later on, but I am not sure coming from somewhere else solves the puzzle. Existing evidence suggests that the MRCA of known late Neanderthals including those predating Thorin was in Europe [2].
So Thorin's lineage could have traveled from somewhere without other hominids and beelined for this site ten years prior, but it is not very parsimonious considering he was in a layer with the same PNII style artifacts for thousands of years before and after him? However, the PNII artifacts don't appear to be rooted in the previous ones of the region so perhaps there was an older exotic origin.
What evidence do you suggest supports this for 50k years?
The group Thorin was from may have been close to extinction when he died, but when genetic isolation started they were 50k years away from being that close to extinction. It seems remarkable based on what we know of hunter gathers or even animal population structure and movement, that there would be no mixing for that long.