It has been said that humans spread like a weed throughout the galaxy, infesting every planet they came across, to the point that they were the majority, and the minority, at the same time. Im guessing that the humans discovered space travel first, but lost contact with whatever home world they had, and eventually created different sub-cultures, different from whatever they had, and eventually created all the different cultures we see today. There is also the 'seeding' theory, but lets not get into that.