In before Rich!
As much as I want to believe, I haven't seen the evidence yet.
And there lies the problem.
Would we know it? if we saw it? The evidence i'm meaning.
We as Humans, ASSUME..that if there is life out there somewhere...that it will in some way resemble us, or at least be recognizable.
According to most science minded people, Everything on this planet..came from outer space.... Therefore, Everything you see..is in fact.."Alien".
It's entirely possible there's life out there based on silicon instead of carbon, and metabolises chemicals that would be toxic to us and might appear nonliving. Crystal based life forms, perhaps. But something that truly came from another planet resembling something here so exactly that no one knows about it, that's much harder to believe. I'm talking complex life, not bacteria and virii.
I know about the panspermia theory, it's quite interesting. Some think bacteria came from Mars, or that an asteroid from outside the solar system crashed and kickstarted abiogenisis. Until we find confirmed bacteria fossils on Mars, we won't know for sure.
Live in New Mexico all your life, You believe!
I'm sure some people thought the stealth bomber was an alien ship before it was announced.
Regardless of how much they kept it secret, that still does not explain how something from another planet, evolved with different dna, would pass as human here. Unless the argument is that greys themselves are working for the government and not making an attempt to cover up their appearance. Going by the vast array of critters that have lived and died since abiogenisis began the 4.something billion years ago, and of course omitting what never actually fossilized (which is probably a lot more than we can imagine), it is unlikely that alien dna would manage to construct something that looks human.
Or you could make an argument for a seeded galaxy, in which case the dna would be shared and it is possible that the human body plan is more common, but that's yet another whole kettle of fish and no current evidence.
I'm not saying it's impossible, just very unlikely, given our current lack of evidence.