Your Ancestors May Be Martian

In this article they consider the scenario that life may have originally started on Mars, and was transferred onto the surface of Earth. The scenario was that due to Mars' once positioned within the habitable region from the Sun, it is likely that life had developed, especially since it once had an abundance of water on its surface. They then believe that the microbial life that started would burrow deep into rocks, and on impact by asteroids, these rocks were plunged into space and that some of these rocks had made it to the surface of Earth. Where by then, Earth had become the planet within the habitable zone of the Sun, hence making the planet a perfect environment for harboring life. They then state that if this theory was proposed several years ago, it would have been dismissed just as fast as it was presented.
http://www.space.com/scienceastronomy/astronomy/rocks_fromspace_991108.html