Compare this to the situation in the Traveller role-playing game, where humans are ubiquitous. In the game's present, scientists have identified at least forty different human subspecies, all but the Solomani of Earth deposited carelessly on dozens of different worlds by the Ancients three hundred thousand years ago before Grandfather began hunting down his children. Most of these human subspecies are classified as minor human races, never having developed into proper independent faster-than-light-capable civilizations. The three major human races--the Vilani, the Zhodani, and we Solomani--are overwhelmingly dominant, each of the major races having done its best to assimilate the dependent minor races littering the space near their empires.
The only problem with the Traveller scenario is that it can't work. Stargate's humans were separated from Earth tens of thousands of years ago at most, with a good deal of intercourse--literally and otherwise--between the different human populations into Earth-historical times. Going back thirty thousand years into human evolutionary history, we find the Neanderthal, a human subspecies that only became extinct at the end of the most recent Ice Age. Could Neanderthals have interbred with humans? Preliminary DNA tests suggest that they didn't leave a trace in the human gene pool. Traveller's human subspecies, most unlike those of Stargate, did not maintain a tenuous genetic intermingling before the dawn of the modern empires. Thus, there should be no Solomani/Vilani mixed populations.
Traveller was a game of the late 1970. This, and perhaps the McGuffin of genetic engineering of some kind by the Ancients, explains these anomalies. There's no excuse for 21st century types who take Traveller's assumptions at face value, though.