It would depend on whether or not you would want to imply that said humans (or humanoids; the population, if it is of long standing, would probably have diverged notably) had left Earth at some point in the past or not. If so, the seeding idea already mentioned would be good — one book I read (I think it was The Legend That Was Earth) also had as its premise that life with either a pre-prepared set of organisms that would adapt themselves to seeded planets, or else a predisposition for convergent evolution, had originated somewhere (I don't recall if it ever touched on where) in space, and been spread throughout the galaxy (or perhaps universe). It also had some odd notions about physics, too. With a fantasy, of course, you can do a great deal more with a great deal less justification (I imply all sorts of odd things about my own fantasy "humans" — I don't actually call them that in-setting, but rather just use the word in the internal language, since it's clear from the description they're meant to be fairly human-looking).
There are, incidentally, also the possibilities of convergent evolution (though this is quite wildly implausible outside a fantasy setting, or one that presupposes seeding with life predisposed to said convergent evolution), and of accidental transportation of humans and other Earth species through wormholes (this has already been mentioned, but it could be made also accidental; especially if the wormholes are sporadic, producing a sort of Telmarines-to-Narnia-world effect). One could also suppose that the "multiverse" theory actually simply has every possible universe in the same "instance", and that if you travelled far enough, you would start hitting alternate Earths or other sorts of alternative human populations; this, however, might, again, be a bit more "fantasy" than you would like.
On the note of seeding and wormholes, Atlantis, Mu, and Lemuria, and all their many fabulous ancient technological possibilities (or other ancient civilisations after the mode of Xenosaga, especially if they leave behind things for us to help find where the people who have gone offworld are now, or to become a galactic species even where they did not), are also, well, possibilities. Much will depend on the sort of story you have to tell. Such details might not even be relevant at all.