Being retired I have watched many documentaries on the subject. Number 1 is the fact that about 120,000 years ago all but 600 people were killed outright in the world. Thus limiting out DNA. Mainly ones in Africa were left. As rain-forest to desert to rain-forest cycles progressed over centuries man lost his hair/fur. This enable him to run long distances and even leave the continent - hence spreading around the world. Well it is not quite that cut and dried .. but close.