Then how do explain the starving idiots that live in a desert? They and their ancesters were too stupid to move their tent 10 feet every few hundred years?
Some would remain, but humans like animals radiate toward suitable environments, the places we lived 1,000 years ago may not be as suitable as they once were.
The deserts of SE California were covered with lakes and had a temperate climate 4,000 - 5,000 years ago. This changed and the most natives moved on, those remaining had to adopt different strategies.
OBTW: I don't think that this migration is a cognitive decision.