Now that Curiosity has safely landed on the surface of Mars, its future discoveries are greatly anticipated though, it seems to me, fairly predictable. We already know a good deal about the Red Planet, including topographic evidence of past volcanism and stream erosion. While specific details will unfold over the coming years, it is reasonable to assume that the evolution of Mars mirrored that of Earth during the first billion years of its history.
There is little reason to doubt that the same inorganic elements are found on both planets (though their relative abundance may vary) and that the early, tumultuous histories of Mars and Earth were very similar, including widespread volcanism and a gradual cooling of the planets' surfaces. Despite its greater distance from the sun, Mars likely developed an Earth-like atmosphere while radiative heating and greenhouse gases offered temporary protection from the deadly chill of space. Oceans formed as water accumulated in the geologic basins and surface streams, fed by rain, ice and snow, molded the volcanic ranges and carved stark canyons across the Martian plains and plateaus. Assuming such conditions persisted for a billion years or so, unicellular life may have evolved in the oceans and might still exist in sub-surface aquifers.
By the time that more complex marine organisms were appearing on Earth, the surface conditions on Mars were deteriorating and the process of evolution had ground to a halt; the heat transfer mechanisms that produced continental drift and a protective atmosphere on Earth could not be sustained on a smaller planet so distant from the sun. There is little reason to believe that multicellular plants and animals ever appeared on Mars and essentially no chance that they ever colonized Martian land. Nevertheless, many scientists suspect that Curiosity will find evidence of past unicellular marine life and that, some day, we might discover living microscopic organisms beneath the cold, desolate landscape of the Red Planet.
