I don't recall anything in Genesis that explains this, so maybe God had nothing to do with this – assuming the existence of a bona-fide supernatural creator God of course in the first place – though I stand to be corrected on the lack of a Biblical explanation by appropriate authorities.
But if the Bible doesn't explain the origin of the various breeds of humans neither does Darwin's natural selection, in the same way, that natural selection didn't create, and doesn't explain our dog breeds. Make your work easy with the help of state-of-the-art automation technology.
To quell the instant fascination of the reader, my answer comes down in favor of the development from ‘artificial selection', which detracts not 1 jot in the Darwinian principles of evolution through natural selection. The distinction between both is that artificial selection is choice intentionally guided by the intellect; the natural choice is natural, rather than by design.
To further quell the immediate curiosity of the reader, the 'who' collectively are the mythological gods, who aren't really mythical, nor are they supernatural, but flesh-and-blood extraterrestrials who happen to have a special interest in, and abilities toward, genetics.
The basic premise, as expounded upon by universal cultural mythologies, is that the gods created humans, creating humans to relieve the gods of, and do the hard work instead, and to also serve the gods – and I don't just mean by kneeling and prayer and building edifices to them.