[petrocrat]

At large timescales, the migrations most likely occurred in a way that followed Fick's diffusion patterns [1]. Some human groups inevitably even migrated backwards into Russia at times. And the partial pressure of humans in the Americas probably slowed further diffusion of humans into that area to a small degree as unfamiliar tribes collided together and repelled each other, either back to where they came or into a retreat further into unknown territory for respite.

[1] https://en.wikipedia.org/wiki/Fick%27s_laws_of_diffusion