Pick up a current issue of Discover magazine. There is an article in it about mitochondrial DNA and how it is used by scientists to trace man's ancestry. There is less genetic variation in the entire human population than you would find in a large troop of chimpanzees. This indicates, the article states, that all humans are descended from a core population of people of approximately 2,000 people who lived in central Africa about 100,000 years ago.
All Europeans, Asians, and Native Americans are descended from a small group of Africans, estimated at between 150 and 200 individuals, who migrated out of Africa during the last Ice Age because of climatic changes. This event was covered very well by an excellent program broadcast, on the Discovery Channel I believe, that was entitled The Real Eve. (There may be more to the title. It seems like there as another part to it, but I cannot remember for certain.)
Regards, Shuckins