FIG. 2.

Major evolutionary events of life on Earth represented together with the possible trajectory of a hypothetical martian biosphere. The origin of life could have occurred on Earth, on Mars, or on both planets, and then transferred from one to the other. On Earth, life gained a foothold early on and started to diversify (represented by the fat cone), driven by genetic interchange through promiscuous horizontal gene transfer (represented by the lines in the cone), and to transform the planet. The possible biological history of Mars is totally unknown (represented by the hypothetical thinner and blurred cone), maybe including scarce horizontal gene transfer events, resulting in smaller phylogenetic groups. The yellow arrows represent possible natural transfer of microbes from Earth to Mars through meteorites, a common event throughout the entire history of the Solar System. Today, the concern seems to be the possible presence of hitchhikers onboard our spacecrafts. LHB = Late Heavy Bombardment.