Actually, I'm only here because I had the urge to post that emoticon.
But since this is Crossfire, with Crossfire rules, I'd better say something.
To debate this properly, we really need some good definitions of what we mean by aliens. I'm going to go with the simple one of "life from somewhere other than this planet".
With that in mind, I'd say that almost certainly Yes, there is plenty of alien life elsewhere in the universe. It would be extremely arrogant of us to think we are so unique that it could only happen here (given how much "Universe" there is, besides planet Earth).
But has it been here? I very much doubt that intelligent life has been here, & for the same reasons that so many previous posters have already stated.
One thing which I think is entirely possible though, is that small, simple organisms could have arrived here from elsewhere, perhaps entombed & protected within a meteorite. This could, in particular, include viruses.
So if these life forms can be described as aliens, then Yes, we have quite possibly had them here at some point in the Planet's history. Who knows, they could have even "seeded" the start of life here.
In which case, are we all descended from aliens?!!
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