She certainly seems to have an affinity with Earth, doesn't she? I'm not sure I'd read that as her necessarily having been born there, but it's an interesting theory.

Mind you, I think I read somewhere that at this point they hadn't even decided the Doctor was going to be an alien - this episode definitely makes more of him talking about the future than about other worlds etc. Oh, though he does say something about them being cut off from their own race, I think.

Ah well. I guess they just hadn't decided at this point.