Did all the humans in Star Wars have a common ancestor on a planet or did at least some of them develop independently?
In the Expanded Universe, it is explained that humans originated on Coruscant, and that the Rakatan Empire — which conquered almost the entire galaxy and used to enslave many species — frequently transported humans from one planet to another for slave labour. As a result, humans became widely dispersed, and by the time the Republic was founded, many planets were already inhabited by them. I personally find this explanation very compelling, because it makes a great deal of sense.