This seems a strained theory as they do not actually have evidence of this happening but reason it might have happened – and anything can happen of course. William sent in the link – see for example https://phys.org/news/2019-11-reveals-humans-migrated-europe-levant.html … where it is clear, and the idea revolves around accepting the various C14 dates of Neanderthal remains undertaken in Europe and the Levant. These are somewhat ambiguous as archaeology has shown that modern humans always follow Neanderthals and it is a false assumption to think they overlapped. There is a clear divide and the problem is related to a C14 plateau event (one of many but on this occasion one that rides on the cusp of the limits of C14 methodology). One may note these dates are close to the onset of the Late Glacial Maximum – which should be telling us that something unusual was going on.
The early humans in question are the Aurignacians. The idea, on the face of it, seems to fly in the face of mainstream thinking as modern humans are thought to have entered Europe around 40,000 years ago. They have also been blamed for the extinction of the Neanderthals – by various means such as out competing them at hunting, or by introducing a new disease that decimated them (the latest offering a few weeks ago) and so on. Genetics has put some of this uniformitarian thinking to bed as modern humans in Europe have inherited a certain amount of Neanderthal lineage (reduced to a small percentage over the last 40,000 years as no new Neanderthals genes have been available to top up the genome). The theory now is that a certain amount of admixture took place – some Neanderthals being absorbed by the newcomers. This is of course contradicted if at all sites Neanderthals die out and Modern Humans occupy a strata above them – which is said to be so. The new study therefore seeks to further substantiate the mainstream contention. Scientists from Tell Aviv University have demonstrated that Aurignacians arrived in what is now the Levant from Europe – and they were an admixture of Neanderthals and Modern Humans. See the Journal of Human Evolution (October 11th 2019). However, a word of caution. The assumption is there was a difference between Neanderthals and Modern Humans (and therefore admixture) rather than the possibility that Neanderthals already had Modern Human traits prior to their disappearance – and a bottleneck in population numbers may have caused the disappearance of one and the emergence of the other. In other words, did Neanderthals evolve into Modern Humans in both Europe and the Levant. Indeed, Neanderthals may have disappeared completely from Europe (in tandem with large numbers of other mammals) but those Neanderthals in the Levant may have possessed links to Modern Humans and after the LGM may then have gone on to expand into Europe. However, the thrust of the research is also a possibility. They say that because a cave in the Levant has Neanderthal remains that clearly display the characteristic genes of Modern Humans and then they must have come from elsewhere. As Europe is the place where prehistory has been mostly studied (rather than the Levant) it is speculated that the admixture arrived from Europe as it is thought to be the most likely place of origin (or the only place which has been extensively studied). Is this roundabout thinking. In doing so they have suggested the Aurignacians were responsible – somewhat convenient as they appear to have been around for a very short period of time, around 3000 years (before disappearing themselves). Did they all vacate Europe and seek warmer climes with the advent of the Late Glacial Maximum (a nice idea) or were they in turn wiped out like the Neanderthals as a double whammy event has been speculated as occurring between 40 and 30,000 years ago. Interesting points of interest in this study but the claim that Aurignacians arrived in the Levant is hypothetical – and even the idea they were the group that hybridised with Neanderthals.