A question I've asked myself is that what if the beings we call aliens (visitors from other planets) are really not aliens at all, but inhabitants of our own planet. There are several ways for this to be true. The idea of ultraterrestrials and metaterrestrials is not new. However, there , is one more possibility that seems just as likely from a particular perspective.
There is a possibility that we are being visited by beings from the future. This is based on two assumptions. First, the human species will continue to evolve throughout time. Second, time travel is a technology that we will eventually obtain. If these two assumptions are true, then the most likely source of "alien" encounters is actually future evolved human-descended visitors that come back in time to observe their primitive past. They would come back to study us or to see us out of curiosity.
This would be that two major class of visitations would occur: site-seeing and scientific, or tourists and scientists. Almost on a lark to describe this possibility, I coined the term Future Evolved Terrestrial Tourists and Scientists (FETTS).
This would mean that some beings either come back to observe ancients times much in the same way we visit the pyramids or other ancients sites. In fact, there are stories that every president of the U.S. in modern times has seen U.F.O.'s at some point in their life. President Carter himself is even on the record as being a person who has seen one.
This also means that some other beings comes back to experiment on their primitive ancestors, much in the same way that we modern humans experiment on great apes, including climps.
This explanation is actually more likely than others if time travel proves to be easier than space travel. According to current understanding of the Universe, there is no reason as to why time travel shouldn't be allowed; though General Relativity does demonstrate that faster-than-light travel impossible. With this fact, assuming visitations from beyond are real, it seems most likely that these visitations are from our own world.