In the recent past, scientists reportedly discovered a pair of harmless bacteria that could have possibly made the trip from Mars to Earth during the period when Mars could actually support life.

The bacteria -- Bacillus subtilis (wild) and Deinococcus radiodurans R1 -- are resistant to high speeds, extreme heat and radiation. Meteoroids, or large rocks that were blown from the surface of Mars by an incoming comet, would have been the source of transportation.

If microscopic life was present on Mars at the point of contact, anything that might have survived the heat of the blast and mnaged to leave Mars' gravitational pull could have been on it's way to Earth or any other planet.

If the bacteria was capable of surviving the heat from the blast, we can conject that it would be able to survive entrance into the Earth's atmosphere.