I think it has to do with natural resources and outside influences. Tribes in Africa tend to be small due to food, disease, and other dangers of jungle living, leading to an isolationist lifestyle. This way of life tends to stagnate.
People that moved to temperate zones had more resources as the land is easier to work with, less disease and predators, and because moving around is easier they had more outside influences.
That makes sense. Too busy dealing with food and shelter demands on a daily basis.
I've often thought there would be no civilization as we know it if not for nagging women.
Cave woman: I'm sick to death of trying to keep this fvcking cave clean!!! Can't we figure out a better way? :twisted:
Cave man: Oh Fvck you, it's good enough! :awe: