There are differences in humans just as there are variations within other species on the planet. Different environments, climates and living conditions result in changes to the organism.
They say everyone who isn't African is descended from the same group of East Africans who left that continent. Lots of physical changes, such as changes in skin color, happened in order to adapt to a different climate. Social changes would occur independently in different human groups which did not have interaction or even contact with one another.
The interesting thing is that after millenia of physical separation, modern travel and communication technology has broken those barriers. Whether we handle that by evolving or destroying one another will determine the future of our species.