I believe there have been numerous waves of Proto-Europeans across the Russian/Asian steppes like the Ainu, Jomon and most recently these: The Takla Makan Mummies.
I believe some of these early waves made it across the 'top' and we're finding them now as Kennewick Man, Spirit Cave Man and etc.
I'm beginning to think there was a time when there were more proto-Europeans in Asian than there were Asians.(as we know them today.)
There's an excellent book by Victor Mair about these mummies titled: The Tarim Mummies. I believe the Tarim Mummies are refugees from the Black Sea flood in 5,600BC.
That makes sense, when you consider the immediate effects of geocatastrophes (The Toba eruption comes to mind, although not the best example) would tend to be limited by mountain ranges and other geographic features, as would disease outbreaks.
We also have to escape from the limiting vice of attempting to approximate prehistoric climates based on today's climates.
While most understand this at higher latitudes, where the effects of glaciation/warming were more imminently obvious (there was a kilometer thick ice sheet where I am sitting now, just a 10-12,000 years ago--only an inch of snow now), the effects also shifted more temperate and tropical regions as well. They were not the same then, and have varied since, both in short (catastrophic) spurts, and in long termed trends.
It is entirely possible that what we regard as Asian populations today had either not migrated into the region yet or had been severely thinned out by any of a number of natural killers.