We know that here on Earth, all the necessary building blocks were in place and that conditons were right for basic proteins to form, and from there we just mix it all together in the pot for a few million years and *bazinga*... life!
I get really frustrated whenever I read (fairy) stories about life originating on some extra-terrestrial body (comet, asteroid, planet) because even if (against the massively implausible odds) it originated elsewhere, it must have started there by the exact same mechanisms as postulated for life beginning on Earth. Occams razor says enough for me. Eveyone else is just looking for grant funding.
If you want to suggest that life originated somewhere else, you need to convince me that it couldn't have equally plausibly started here too.