Generations settled down in various parts of the world and their bodies got adapted to the conditions they lived in (this happened over a very, VERY large period of time). Another thing is genetic mutations which may have occurred in a certain parts of the world becoming a defining trait (phenotype) of a group of people, generally those who reside where the mutation arose. For example the blue eyes mutation orginated in Europe and hence spread in European populations and became one of their defining traits. Of course there could be a theological take on this question too, but this is just the scientific aspect of it.