Figure 1 from Dalén et al. 2012. Original caption: "Figure 1. Phylogenetic relationships and geographic distribution of Neandertals. Recent (<48 kyr) western Neandertals are placed within a well defined monophyletic group (blue box), whereas specimens older than 48 kyr constitute a paraphyletic group together with eastern Neandertals (red box). The sampling locations for the specimens are shown with corresponding colour coding."
Close examination of the phylogeny suggests that a Neanderthal group migrated westward into Europe from the east sometime before 49kya and subsequently populated most of western Europe. Other Western European Neanderthal groups predating 49kya (those at Monte Lessini, Valdegoba and Scladina) seem not to have survived in Western Europe after 49kya and appear to have been replaced.
It helps to look at a map when pondering the implications of the phylogeny:
Map from the Science article "The Neandertal Genome" illustrates the geographic range of Neandertals.
If there was migration and replacement, one has to wonder at the driving force behind it. The likelihood that Neandertal groups in Western Europe were under pressure during OIS 3 (MIS 3) due to climate has been suggested. John R. Stewart states:
"The cold-adapted mammals as a whole, such as the extinct mammoth and extant species like the reindeer and arctic fox, did not retreat from the north as the climate cooled like the Neanderthals."
"... Neaderthals were presumably a warmer-continental adapted species, based on their geographical distribution through time. Neanderthals seem to have been better-suited to the Mediterranean during the Late Pleistocene than to the more northern parts of Europe."
"The environmental changes that affected the earlier Late Pleistocene extinctions, including the Neandertals, contributed to the eventual disappearance of the steppe-tundra and all the later Late Pleistocene animals that relied upon it. The transition from Late Pleistocene to the Holocene may also have been similar to the changes that took place earlier at the approach of the LGM. Certainly, we have evidence for a reduction in carrying capacity through OIS 3 in the form of decrease in small carnivore occurances as well as a decrease of mammoth numbers on non-archaeological sites towards the LGM. The steppe-tundra appears to have reached a peak during the earlier part of OIS 3 and had deteriorated by the start of OIS 2, at the end of which it was replaced by the latitudinally banded vegetation zones of the Holocene still seen today. Therefore, the terminal Late Pleistocene extinctions may have been caused by the combination of the LGM together with the eventual warming and stabilising of the climate at the onset of the Holocene. It seems likely that other relatively large climate oscillations such as the Dryas events and other stadials may also have contributed."
Figure 1 of the Meerbeeck et al paper, which I posted a few days ago, describes extremely oscillatory climate in Europe after 50ka and throughout the remainder of the OIS 3. Specifically, there are mini ice ages at 48ka and 38ka which may be corrolated with the population narrowing of Neanderthals described in the Dalén paper. It is also interesting that there is a period of relatively stable but cooling climate from 55ka to 50ka, which could account for the range expansion of the "red box" Neanderthals into Western Europe in the period prior to 50ka.
What is as yet unknown is when, which and how many of these Neanderthal populations were absorbed into the populations of Homo Sapiens. Because no Neanderthal mitocondrial DNA was passed on to us, the answer to that question will have to come from whole genome sequences of Neanderthal samples. In any case, the answer cannot be far away.
Neanderthals: Victims of their Own Success
The Age of Upper Paleolithic Sites in the Middle Dnieper River Basin of Eastern Europe