re: How do you explain the origin of the various human races scientifically?

Gotta love evolution debates, people bringing crayons to a gun fight. The first mistake you've made is a very common one no both sides thinking there is such a thing as a superior species. People take the Survival of the Fittest phrase and run wild with it. Evolution IS NOT Linear Biological Advancement marching ever forward to some mythical standard until we're like Gods. Evolution IS life finding a way to fill/exploit new niches in the environment.

The goal/destination of evolution/speciation is biological diversity, not biological advancement.

Now that we have a proper frame of reference for what evolution does and doesn't lead to, why do you think that groups of men isolated for millennia lead to variation within the species? The Blacks and Asians pissed God off so he made the White Devil to get back at them?