Archaea strike me as the kind of thing we'd find on Mars. I didn't realize it until doing some searching, but some archaea are methanogenic (they generate methane), which has been detected on Mars.
Look into Panspermia if you're not aware already. Hoyle and Wickramasinghe wrote a lot on this. I don't know how seriously it's regarded in the science world.
If life somehow hitched a ride from one planet to another, it seems far more likely that it would have gone from earth to Mars, than the other way around. ;-)
