This idea of aliens seeding life on Earth was raised in Star Trek Original Series' Return to Tomorrow, and later in Next Generation's The Chase. I found the latter quite entertaining, and at least at a sci-fi level, more plausible since it involved seeding DNA on planets that had not yet evolved life, and served as a nice excuse for all those humanoid races from different planets.
Something along those lines would seem necessary to me in order to posit any sort of relation of life on Earth with anything extra-Solar, since all life on Earth can be shown to have a common genetic base (so humans in particular could not be the only descendants of any purported seeding).
For each man, according to the measure of his intelligence, must speak what he can speak, and do what he can do. - Alfred, King of Wessex
Calm down, have some dip. -George Carlin