One aspect of human demographic expansions seems to be the fact that we often model them as a constant diffusion process, when in reality there were likely pulses (economic historians can conceive of this as the periodic gaps between land and labor factor inputs). I don't know much about the human movements prior to H. sapiens sapiens, and from what I can gather the fossil remains are too sparse to be too wedded to a specific model, but it seems clear that anatomically modern human expansion occurred through a series of rapid outward sweeps which would periodically reach a "natural barrier." Modern humans reached the Solomon Islands ~30,000 years ago, after which there was stasis for ~25,000 years. Only with the Austronesian expansion did humanity push past the Solomons. And this was no baby-step, ultimately the Austronesians went as far as the Hawaiian islands and Easter Island. The New World is similar. The initial migration out of Africa by modern humans resulted in the range expansion of the human lineage into a region which had been untouched by earlier hominins, Australasia. But after that point tens of thousands of years passed before our species pushed into virgin territory, in this case North America. The when and the how of this though is still up for debate. A new paper PLoS One attempts to construct a plausible scenario by taking archaeological data points and inputing them into a diffusion model. Archaeological Support for the Three-Stage Expansion of Modern Humans across Northeastern Eurasia and into the Americas:
We use diffusion models...to quantify these dynamics. Our results show the expansion originated in the Altai region of southern Siberia ~46kBP , and from there expanded across northern Eurasia at an average velocity of 0.16 km per year. However, the movement of the colonizing wave was not continuous but underwent three distinct phases: 1) an initial expansion from 47-32k calBP; 2) a hiatus from ~32-16k calBP, and 3) a second expansion after the LGM ~16k calBP. These results provide archaeological support for the recently proposed three-stage model of the colonization of the Americas....Our results falsify the hypothesis of a pre-LGM terrestrial colonization of the Americas and we discuss the importance of these empirical results in the light of alternative models.
It's an interesting paper because it seems to have been triggered in part by inferences made from the genetic data. I don't know how confident archaeologists are about their radiometric dates, but I think some of the molecular clock results from the genetics of Amerindians need to be taken with a grain of salt (I don't see many people repeating some of the really ancient coalescence dates for Amerindian Y lineages at this point). These data seem to indicate that modern humans made it no further than previous hominin groups for several tens of thousands of years. But something happened within the last 20,000 years, and our species made the leap across Beringia. The bottleneck here is certainly not the Bering Strait, which was spanned by land much of the time in any case. Rather, our species didn't have the biological or cultural capacity to survive in extremely frigid environments. I've read modern humans pushed the boundaries of their range in northern Europe further than Neandertals ever did, indicating our flexibility and plasticity. Since the human lineage had been resident in Eurasia for at least one million years that suggests to me that it was behavioral modernity that was key. In particular, how quickly our cultures evolve and shift. Though that flexibility itself may be a function of our biological competencies.