In the bad lands, the cattle and people were often struck with a sickness, today identified as pernicious anemia. The anti-pernicious factor in humans is a compound of cobalt and in areas where the sickness occurred, cobalt was minimal in the soil. The presumption that there are a number of different races within the human species implies that there is a different genetic code for these separate races. However, within any certain race there is the same degree of difference between individuals. The development of varying skin pigmentation is not an influential enough of a difference to divide the human species into races.
Skin color is merely a reaction to a dangerous environment.
The same instances have been documented in immunity to diseases. It can also be argued that the geographical factors stated above are the same factors that have been responsible for the evolution of all species. Nature and its woes had an influence on the variations that were experienced by the people. But the amount of time that it takes for a new species to develop is immense. Human beings started to expand out into the world about 25,000 years ago.