This interesting website gives you an interactive timeline on the globe in order to show how and where people arose and spread. This is based on the current state of knowledge of mitochondrial DNA and other genetic studies. All non-African people are descended from a single group which crossed from Africa into the Arabian peninsula circa 85,000 years ago. Nearly 74,000 years ago there was a dramatic decrease in population; it is estimated that the number of human adults worldwide fell to around 10,000, believed to be due to the explosion of a super-volcano. It is striking that the human population has suffered multiple times from too cold an Earth, but never yet from too warm an Earth. "The final collapse of the ice age heralded the dawn of agriculture." Makes you think.