I thought that by now we had a very good idea on how extensive the level of mixing between modern humans and Neanderthals thanks to dna studies. The Homo sapiens that got out of Africa interbred with them at such an early stage that all Europeans and Asians have some Neanderthal admixture in them. Asians have more because their ancestors mixed with Neanderthals again (after european and Asian lineages split) and finally south eastern Asians plus oceanic people also have Denisovan admixture.