White skin is a genetic mutation. It doesn't mean that it has only happened once in human evolution. But when this mutation happened it proved advantageous to those who ventured into less sunny climates by enabling them to maintain vitamin D.
Mutations happen all the time with most of them dying out because they present no advantage to which ever species it occurred in.
Choosing to refuse to believe something doesn't make it untrue.
As Darwin said, "ignorance more frequently begets confidence than does knowledge".
