When the world was repopulated (by land and sea) after the Tower of Babel (following Noah’s Flood), according to the genealogical template provided in Genesis 10, the people moved off by families into isolated regions of the world, hence certain genetic traits became dominant in certain regions, such as dark skin for living in the low latitudes (and light skin for living in more extreme latitudes), so that is the extent of the “human races,” really only one, of one syngameon, one biblical kind of creature.
