It's hard to respond to these questions without thousands of words, but I'll try some scattered responses to various points above.
From what we understand, all of your scenarios were true at various points. There are times when humans expanded at a lightning pace and times when we seem to have been at a standstill. We don't really know to what degree these are accurate reflections of the reality vs illusions caused by an imperfect material record.
One well-known obstacle to human expansion was the Arctic itself, which seemingly resisted human inhabitation (even by archaic humans) until relatively recently.
As for how fast we could have expanded, modern humans are incredibly specialized to rapid, optimized adaptations. Our language capabilities, high order thinking, sociality and other rapidly adaptive traits evolved at least partially in response to periods where climates could shift by tens of degrees in mere decades during the pleistocene. On top of this, the kinds of nomadic foragers that would have been these early pioneers live a lifestyle that's not only adaptive to variable environments, it's designed to exploit that variability. If the winter is going to be too cold in this valley, you move over to the next one. If there are dangerous animals too close, you pick up camp and go. You're using the resources from multiple environments as those resources are accessible, which allows you to exploit landscapes that any other lifestyle would find completely inhospitable.
Happy to talk more about any of the above in a more focused way.
Big questions and questions we don't have a clear answer to are difficult to answer well without spending a lot of time to organize everything into a digestible narrative. Writing Askhistorians answers usually takes me a few hours each, for example.
If I don't do that and write extemporaneously, you get the garbage above.
Gotcha. So could you first point me to the one or two AskHistorians answers that you are either most proud of personally or that you think people should read? I think I’d like to read those!
Sure, [0] has held up the best of my answers over time in that I don't think rewriting it would substantially improve the content. [1] has the problem that it's answering a "big question" and is thus inherently incomplete, but I'm happy with the discussions it's generated and other people continue to find it useful based on the backlinks I see.
From what we understand, all of your scenarios were true at various points. There are times when humans expanded at a lightning pace and times when we seem to have been at a standstill. We don't really know to what degree these are accurate reflections of the reality vs illusions caused by an imperfect material record.
One well-known obstacle to human expansion was the Arctic itself, which seemingly resisted human inhabitation (even by archaic humans) until relatively recently.
As for how fast we could have expanded, modern humans are incredibly specialized to rapid, optimized adaptations. Our language capabilities, high order thinking, sociality and other rapidly adaptive traits evolved at least partially in response to periods where climates could shift by tens of degrees in mere decades during the pleistocene. On top of this, the kinds of nomadic foragers that would have been these early pioneers live a lifestyle that's not only adaptive to variable environments, it's designed to exploit that variability. If the winter is going to be too cold in this valley, you move over to the next one. If there are dangerous animals too close, you pick up camp and go. You're using the resources from multiple environments as those resources are accessible, which allows you to exploit landscapes that any other lifestyle would find completely inhospitable.
Happy to talk more about any of the above in a more focused way.