They course of this thread got me thinking (l know.."oh, great...here we go again...")
Over the cycles of glaciation and ice sheet formation covering large expanses of this continent, with the concurrent ice bridges/significantly lowered sea levels, there may very well have been successive migrational events or periods.
So, just possibly, say the first time around, a species or population is pushed out of a region, as described by MIB, and crosses over to the new world. Either through continued movement, or expanding numbers, this group comes to occupy this new territory as it opens up with the receding ice sheets, absent the more assertive competitors that got them moving in the first place. Then, as the cycle repeats and the ice sheets return, this first wave of hominids get pushed south as the ice extends, depriving them of food sources and manageable weather.
But, just as these guys are moving south, to the north, the land bridge is reforming and the sea levels drop once again allowing a next wave of migration. However, this second wave is comprised of "a species" which, while initially may have been of the same stock as the first group, has developed faster, in that by the time of the second cycle they have had to compete and survive with those that drove out the first group. This results in a population more advanced, to some degree, than the first, moving through the same corridors, into the new territory.
As this cycle repeats, the original forms get pushed father and farther south by successive ice formations, while more developed(socially, physically, cognitively) forms cross over and expand into the regions emptied by the glaciers and opened by their receeding. Of course, these distinct groups or forms will inevitably come to cross paths, resulting in either domination of habitat, or zones of integration.
Meanwhile, those of the latest group to migrate who didn't get across, will once again face the selective pressures of the original habitat, and either evolve or perish. One would think that over repeated cycles the numbers of the stragglers populations would progressively diminish.
This paradigm would account for the seeming gradiation of form from the more ape like forms seem in the south east to the "I couldn't shoot cuz it looked so human" forms of the PNW.
Of course, the population dispersion of the various forms and integrations hasn't stopped, resulting in diverse populations, as reported by witnesses in recent times, with numerous types observed within the same region...
Just a thought.....
I realize this may have been proposed previously, but I'm now old enough not to remember that occurring, which also allows me to feel insightful, rather than grasping that I just formatted something I read in the past as original thinking on my part....