With all of the talk of water on Mars. . . .
Does anyone remember an ATM theory from years ago which proposed that if simple life evolved on Mars it's possible that a collision with an asteroid could have sent some of the bacteria to Earth and "seeded" our planet? Something to do with a hypothesis that Mars would have cooled faster than the Earth so simple life could have developed there before it developed on Earth.
It still seems like a long shot - and we would have to find the Martian life to prove it.