I've always had a belief about us, the human race, being an alien race from another planet because
a.) no other creature on Earth comes close to being EXACTLY like us (i.e. no different species, just races),
b.) all animals rely on pure base instinct, while we rely on our intelligence to get through the day,
c.) we tend to harm the environment by merely settling on it, and
d.) all animals (even the ones that can rip us to shreds) FEAR us, for no apparent reason other than we are not food, thus, we are scary. If all the animals would fight against the humans (ala Rise of the Planet of the Apes-style), the human race in general would be screwed.
There is also a lot of talk about how the humans in the B.C. era had relations or contact with aliens, how they helped humans develop their civilization and structures that would be difficult/impossible to accomplish in that era, even by today's standards. How do we know that we were merely speaking with our 'relations' in order to better inhabit this world?
If we go along this train of thought, essentially the main questions come down to:
1.) What happened in the past that has caused us to break ties with our 'alien brethren?'
2.) Why would the past interactions with these aliens not recorded or mentioned? Was the 'colonized' alien/human race so arrogant that they decided to take all the credit for their accomplishments on Earth and leave their alien help out of the picture?
3.) Are there any attempts to re-establish the lines of communication?
4.) Is there something on Earth that has been long forgotten that is preventing the aliens from coming to/landing on Earth? And what is it?
I could ramble on, but I think this is a good place to stop.