There are some sources that profess the idea of aliens already sharing the planet Earth with humans. A 'secret book of Aliens' was supposedly leaked from the KGB that includes dozens of alien races, factions and more. This source details where these particular races came from and partial exo-political implications of Earth governments and ETI relationships.
Additionally, a gentleman by the name of David Huggins claims to have been in contact with Extraterrestrials for most of his life and even says interstellar sexual relations have ensued as a result.
The actual question is, what do YOU believe?