I think we have races because of something I learned in bio called genetic drift. When the first humans re-emerged from Africa they must've split up into pretty small nomadic groups from tribal conflicts or whatever. In small groups, genetic variations happen to drift more and more. So the same species, humans, must have come out of Africa and once they splitted into different groups, they start to drift apart genetically. This is probably how races developed. Before the races could become their own species, however, our intellectual and technological development on globalization grew too fast. Today, the genetic drift has pretty much stopped due to large populations in most races. You can see how modern-day humans have different genetic adaptations, especially in Africa. Africans have more resistance to solar radiation and have heterozygote superiority to sickle-cell anemia. Also, the faces of Native Americans almost became indistinguishable with that of the Asians, indicating that a small group (maybe hundreds or several thousands) migrated across the Bering Strait 12,000 years ago, but still you can see the similarity.