It is often mooted that life on other planets, especially of our Solar system could be comprised of extremophile prokaryotes living in conditions generally believed to be too hostile for more regular life forms.
Comparisons are often drawn with the colonies of bacteria living near ocean floor fissures known as black smokers
Is it likely that this type of life could have developed in isolation and in absence of photosynthetic life closer to the surface?
Is it not more likely that the extremophiles evolved from life that developed in more generally habitable environments?
We know that the first organisms on Earth were not photosynthetic, although we don't know exactly when photosynthesis arose in the Archean. Also, current molecular evidence seems to imply (not without controversy) that the last universal common ancestor of all known organisms was a thermophile. Geochemical and kinetic considerations also suggest (again, not without controversy) that the first living organisms were thermophiles. So, even on this planet, what we call "extremophiles" may have come first.
It would seem logical that extremophiles are evolutionary genetic variants of a more generalised species and that they could not have evolved independently. This cast a long shadow over the theories about life having evolved on other planetary bodies or moons in our Solar System.
If life exists on Mars, and that is a very big IF it would by the very nature of the planet have to be some kind of extremophile. If the planet ever had a denser atmosphere and liquid water it was a very long time ago and that period was very short. It is of course possible that some form of life got started in the Mars equivalent of the Archean but it did not have very long to have evolved.
As for other Sol candiates for life there are similar constraints. I would be delighted of course if life does exist elsewhere in our Solar syatem but I am inclined to think the odds are very long indeed.
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