I'd go with an Earthly origin, just because the conditions in space are less than conducive to life. Could early life have come here from Mars? Possible, but 1) it would need to survive the trip, which would be no easy matter even for today's highly-evolved lifeforms with 5 billion years of survival adaptations, and 2) there's no reason to think Mars would be more likely to develop life than Earth.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort