Over the course of their roughly 400,000-year existence, Neanderthal populations were not static. Glacial cycles repeatedly contracted and expanded their habitat: during glacial periods, ice sheets advanced across northern Europe and forced Neanderthal populations southward into refugia; during warm interglacial periods, Neanderthal populations reexpanded northward and eastward into newly habitable terrain. Each cycle of contraction and expansion mixed populations that had been separated and split populations that had been united.
The Vindija Neanderthals from Croatia date to approximately 55,000 to 44,000 years ago—among the most recent Neanderthals in the fossil record—and belong to a western European Neanderthal population. The Denisova Cave Neanderthal, by contrast, represents a much earlier time period and an eastern population that may have been partially isolated from western groups.
The genetic similarity between Denisova 11’s maternal chromosomes and the Vindija Neanderthals suggests that, at some point in the maternal lineage, the Neanderthal ancestry being carried was more similar to western than eastern Neanderthal populations. How to interpret this phylogenetically is not entirely clear: it could mean that Denisova 11’s mother was herself from a western-origin Neanderthal population that had moved eastward, or that the maternal lineage reflects deeper ancestral connections between eastern Neanderthal populations and western ones. The researchers interpret it as evidence for Neanderthal migration between western and eastern Eurasia, which is the most parsimonious explanation given the data, but the specific details of when and how that movement occurred cannot be reconstructed from a single genome.
What is clear is that the Altai Mountains, where Denisova Cave is located, were not a terminal, isolated location cut off from the rest of the Neanderthal world. They were reachable from the west. Neanderthal populations that had their origins or deeper genetic connections in western Eurasia could, and apparently did, travel or spread eastward far enough to reach the Altai.