Geomorphic Evidence for Life on Mars
There are three geomorphic forms on Mars I have identified: the filling of old, deeply eroded craters with uneroded domes, some only partly burying eroded original true resurgent domes; channels hundreds of km long filled with similar dome-like material that are arched in crossection, the arches rising two and more km above their margins; and vast areas of Mars between latitudes 25 to 60 degrees covered with mat-like material approximately one km thick in many places which buries completely Noachian age impact craters 5 km in diameter and smaller.
The organic material composing these geomorphic forms is almost certainly cyanobacteria combined with calcium carbonate. Stromatolites on Earth, growing today and going back 3.4 Gyr but on a comparatively miniscule scale, are my model.
I believe that the growth of these gigantic stromatolitic forms gradually decreased the carbon dioxide in the atmosphere of Mars leading to progressive cooling and finally freezing conditions that ended the growth of the three geomorphic forms and the mostly warm, wet Noachian era. The following cold, dry Hesperian and Amazonian eras for the next 3 plus Gyr to the present time have none of these geomorphic forms, proving they needed water for their formation, but water with some other essential materials to cause their growth.
As several types of meteorites from Mars have been identified here on Earth, it is likely that Mars has received meteorites from the bombardment of Earth as well. The question is, which one seeded the other? We probably are not going to find the prokaryotic progenitors of the eukaryotic cyanobacteria here on Earth, but we have a good chance of finding the progenitors on Mars if they ever existed there, thus telling us which planet seeded the other.