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

My theory:

Homo sapiens started in Africa and began migrating out to different environments. Natural selection began to take place over many years, leading to different expression of traits that were most likely to succeed.

In colder environments, humans had to find shelter and form larger social networks different from clans. This led to skill differentiation and primitive agriculture.

So my theory is that migration and the encountering of different environments forced natural selection in humans...