[egjerlow]

Guns, Germs, and Steel by Jared Diamond has its fair share of critics, but one thing I feel it does well is introduce to the general populace the ways of thinking around questions like this. In the case of migration, one of the factors that slowed it down so much is the lack of properly 'domestic' peoples that actually had a population surplus. If you're a hunter-gatherer tribe that has reached an equilibrium between children born and people dying, and your hunting grounds provide the resources you need to sustain that equilibrium, there is little reason to go looking elsewhere (migrate).