Isn't it a bit arrogant to think that we are totally alone in the universe? Isn't it also a bit arrogant to believe that the only other living creatures among us are little green men?
Consider for a moment the episode of Doctor Who entitled "The Stolen Earth". Now, pretty much every episode contains some sort of creature that has been living on Earth and the human race has had no knowledge of them. However, this episode contains one single line that really stuck with me. In the episode, 27 planets have gone missing (including Earth). The Doctor and Donna Noble go to the Shadow Proclamation (basically, the Galactic police) to try and figure out what has happened to all the planets. At one point, the Doctor asks Donna if anything strange has been going on on Earth that would be a sort of sign that something was amiss. She says something along the lines of, "Well, the bees are disappearing." At first glance, it's a daft statement, and even the Doctor seems to just shove it off. But then, he remembers. Some, not all, of the bees on Earth are from the planet Melissa Majoria. When they noticed the disturbance in the Tandocca Scale caused by the Dalek invasion, they fled back to Melissa Majoria causing the Earth bee population to decrease.
It really makes you wonder if there are things on Earth that we've had our entire lives that are actually so totally alien in every sense. What if some of the bees are actually from a distant planet. It is something to think about.
