How did humans evolve to be so different from one another?
Actually, humans aren't very different from each other. The races differ in extremely superficial characteristics.
Lighter skinned individuals absorb more UV light to produce the necessary Vitamin D that equatorial people (such strong sunlight) get easily. Dark skinned individuals are more common where protection fron UV light is most needed.
The evolution of humans is no different - in any way - than the evolution of any other plant or animal. There is a niche; there is an advantage to a mutation in that niche; there are more people with that gene.
Sickle-cell anemia trait and disease is an example of this. Is it a racial difference?
Lactose tolerance after infancy is an example of a genetic mutation that swept Europe. Is it a racial difference?
I don't think so. I think these were mutations which helped the population in the niche. Like any other evolutionary "selection".
The human genome has been completely sequenced. The difference between races is almost infinitesimally small.