Long ago, people lived in Africa. There was a very large periode of drought. There wasn't any food anymore. So the people went to Europe and ASIA. The circumstances in Europe and in Asia were different than they were in Africa. That's called a niche. A niche is a collection of abiotical factors which affect on life (like temperature, preasure, etc). So the people who lived in Asia, evoluded to the Asians we see today, the same for the Europeans. You may now ask, but how do you explain that there're still Africans? Well, because some of those people stayed in africa and evolved also, influenced by there own niche.
Simple, isn't it?
