Onto the weird and wonderful world of aliens or something different being involved with our planet. While it's easy to write this things off, it would be incredibly difficult to prove conclusively that there is no other involvement. Most would believe that there isn't, but at the same time it would be incredibly difficult to know 100% for certain that there is not.
It's fair to say that one believes it's unlikely, but to say that we know for certain is overconfident. Avatar is a good fictional example of how it could happen. We should at least try to keep our minds open, even if it's just a little bit.
