It is pretty sure that other living forms (pluricellular forms) live somewere. Are they big, small, do they need oxygen? If whe consider todays life standar, you need water, or antoher liquid to survive. Mars had water, and has ice on both poles (or had) So technically mars could have been like earth, if it was closer to the sun. Of course, the constituion of the planet isnt the same, etc. If the big bang happened more than 100 000 000 years ago, then it is very possible that in that time another planet has existed.
Now, have aliens gone on earth? No, if you ask me. It is possible, and maybe the human is alien. Now that is up to your imagination to decide, because whe have no proof, and suppositions can be very easy to contradict, not like are there aliens somewere, wich you can begin a scientific war and the answer will probably be yes.