[Will_Parker]

> most obviously, that the continent was much colder, and therefore supportive of much smaller populations,

Okay, but we're assuming a population capable of migrating as far as San Diego, and capable of hunting mammoths. You'd need enough population to sustain the many hundreds of years of generations it would take to get that far, and just exactly the sort of population balance where they wouldn't leave any obvious traces? And given they made it halfway down the continent, and through the most inhospitable places, why didn't they go further south where they could have immediately flourished in lush lands full of megafauna who hadn't evolved to fear us? Humanity's record seems full enough of such population explosion incidents after all, including the Clovis-people themselves.