The looks of different races

I have always wondered why different races have different looks and how they got their unique look. For example in the novel, the Eskimos had broad shoulders, were short, had Asian coloured skin, and a square jaw. How did these Eskimos get to look like this. The African Americans have black coloured skin, how did they come to have this colour of skin. This has been a question that was lingering in the back of my mind for a long time, but this novel brought this question to the surface. I still don't have an answer to why different races have different looks.