How did the first human races arise?
By isolation and inbreeding and some selection. A small group of people leaves the original population or is driven away. The original wealth of genes that is present within the parent population is no longer present within that small group. When for example there are no genes in the fouders that take care of curly hair, or brown eyes, than the progeny will never get it either. The progeny will globally only have those characteristics that the founders carried along them. When the primitive men Adam and Eve were maximally heterogeneous (read: different) for their neutral genes, than within their progeny the original heterogeneousity will decrease more and more by isolation, inbreeding and selection, because more genes will become homogeneous (read: the same). Mutations then can terminate genes that are still on, or in a less way can cause diverse variants (like blood types for example) especially among the neutral genes. There will hardly have been selection at the time of the arising
