Well we have human ancestors on both sides of that barrier, so clearly it's not insurmountable in absolute terms, either from taking the souther route or migrating with herds or some other thing I haven't thought about (I'm not sure waterways would get you the whole way, but it'd get you from the urals to either side). The question is why this would pose a barrier to some populations and not others.
EDIT: also, I forgot that a lot of the steppe is forested. Surely that would make it significantly less of a barrier.
No terrain is insurmountable considering chance and a hundred thousand years. The question is how likely will any given group be able to pass through and flourish on the other side? We have many examples of isolated human populations on islands or across inhospitable barriers that it shouldn't be surprising to find isolated populations on either side of the Eurasian continent. Off the top of my head, Australian aboriginals, the Andamanese and New Guinea islanders are examples of isolated populations with essentially no gene flow between larger populations over 10s of thousands of years. Even the fact that we talk about how many populations ultimately left Africa in prehistory implies there must have been some barrier to overcome.
> also, I forgot that a lot of the steppe is forested. Surely that would make it significantly less of a barrier.
Also consider the ice age and how that affected the degree to which Eurasia was hospitable.
Yes, its a continent-spanning grassland. But humans don't eat grass. Sourcing water is also a problem. It's inhospitable to humans without some reliable means of turning grass into nutrition. If we're talking about early humans it would be a significant barrier.
EDIT: also, I forgot that a lot of the steppe is forested. Surely that would make it significantly less of a barrier.