Life on Mars
Both Mars and Earth have been hit by many meteors big enough to eject rocks from the planet. As there was a period when both Mars and Earth had water and Earth had life it is highly probable that at least some rocks ejected from one planet would have carried bacteria to the other planet infecting it with life.
What is not known and might be very difficult to determine is - did life start on Earth and infect Mars or did it start on Mars and infect Earth ?
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Biting the hand that feeds IT
