If there exists a species on this planet that did not originate here, and that species is one we have already discovered, it is likely the Tardigrade. Though they are just barely microscopic, they seem to share many characteristics with humans. This particular scientist discovered that these guys seem to be spread evenly across the Earth. Most importantly, however, it is the only known Earth-dwelling creature to be able to survive in the extreme conditions of space.
Have we been co-existing with an alien life-form we didn’t even realize was there? Maybe it’ll only take a little bit more prying to find out.