I'd probably go "THAT'S FUCKING SWEEEEET" and the ask "... why hasn't this become public yet, what kind of trade embargo is this, What agender does your culture have?"
Although, if they're exactly the same race inside, outside, and genetically, I'd question if they were aliens as opposed to "Humans who happen to be born/descended form a different planet than earth."
That's an intriguing question. I wonder the same thing sometimes... though I think it more likely that this could be true of some ancient cultures whose very mythologies say the same concept of their own origins or that of other cultures around them. And the world we live in now, today, which is so inebriated with references to the supernatural, could be a result of these different peoples' cultures and lifestyles converging and merging with humanity as we know it, as it was then to become what it is today. This is very fascinating to ponder... and there's a lot of evidence pointing to this possibility if you look at the history of humanity and the forming and development of society.