Did Christ travel to other planets and die to save ETs. from their original sin? I do not know. But if the ETs where not in the garden of eden, they did not eat the forbidden fruit so no original sin was committed, if one takes that story literal.
Are we the only ones who ate the apple? If you take the Garden of Eden story as literal then I would say yes.
Where we once all together in the garden of Eden? No mention in the text of anyone other than Adam and Eve so I'm going to say no. However some midrash puts another woman named Lilith there prior to Eve. Never made the cut though. Hope you like the pearls.