In some aspects, the idea of life-forms beyond our planet is entirely plausible, for most of the universe is undiscovered. Since neither religious nor scientific evolution theories cannot entirely be proved or disproved, there is definitely a chance that another planet exists with life forms similar or dissimilar to ours. Keep in mind that we have adapted to our planet through Darwin's "Survival of the Fittest" ideology, and have changed in many ways since the beginning of Earth. Therefore, if life forms do exist outside of Earth, there is a large chance they do not look exactly like us.
Furthermore, keep in mind that many years went by with Native Americans on one side of our planet and Europeans on another, completely unaware of each others' presences. Anything is possible, really :)