Could bacteria really survive a meteor-ride between Mars and Earth? Many scientists think so; frozen bacteria have been revived from deep under the Siberian tundra, why not in a meteor? True, the bacteria would have to survive for much longer to make a reasonable interplanetary trip, but no one has shown this is impossible.

So which way would the process go? At least a dozen Martian meteorites have been found on Earth (Antarctica is a good place to find them, because they lie on top of the ice pack). It's much harder for a meteor to go the other way. But when Earth's moon was created, by a major planetary impact >4 billion years ago, many ejecta meteors would have been created. Had life developed by then? The record isn't clear, but it's certainly possible that life developed on Earth and was then moved to Mars. Another possibility is that Mars was the original source of all life on Earth, although not the way it was portrayed in "Mission to Mars"...