greg, what happened was geographic isolation in different climes - humans started out black (like Africans and Aborigines, not like American blacks) and, when they'd migrated to different climates, lost some of their melanin, leaving people who are dusky brown like some Asians, yellowish-brown like other Asians or Eskimo, reddish-brown like native Americans, and pale as all getout for Northern Europe. But before enough millions of years had passed for us to speciate, we all got mixed back up, which is why there aren't enough biological bases for the human term Race. Although of course there are biological differences among the races, notably the amount of melanin produced.
Hybrid vigor is a wonderful thing.
If everyone were of mixed ancestry, there would still be some (white-looking people) and some (black-looking people) and some (asian-looking people) and so on, but they would be rare; the rarest would be the white-lookers as their traits seem most recessive.