The Neanderthal lineage is essentially attested in the western part of Eurasia. However, probably at a late stage in its evolution, this lineage extended its territory as far east as the Altai. The separation between the Eurasian lineages, represented by Neanderthals and Denisovans, and the ancestral African lineage of modern man dates back to around 650,000 years BC. Despite a relatively short separation time, the two groups rapidly developed very different phenotypes. Neanderthals and modern humans have large brains, but differ greatly in the architecture of their craniums. The long, strongly forward-projecting medial face of Neanderthals is also very different from the short, flat, retracted frontal lobes of modern humans. These divergent anatomical evolutions resulted in marked differences in mandibular and skull base morphology.
The emergence of Neanderthal morphology is synchronous with the amplification of glacial/interglacial cycles affecting the Middle Pleistocene. These cycles are the main cause of a succession of genetic bottlenecks and reduced morphological and genetic variability throughout the evolution of the Neanderthal lineage. In this "accretion model", gene drift probably played an essential role in the fixation of Neanderthal-derived traits.
