and I don't really know the answer, but I think its a combination of the two. Since all of the extremophile metazoa have evolutionary cousins that live in more ordinary environments, it is reasonable to assume they adapted to those extreme habitats gradually. All multicellular phyla arose when the world was much like it is now, with an oxygen atmosphere. Most of them simply eat other critters that are already there, so they exploit those extreme habitats and live on their fringes.
On the other hand, those primitive single-celled creatures that actually can do things like metabolize dissolved SO2 in a superheated brine solution may very well be descended from earth's earliest life forms, the last survivors from an anaerobic past. Perhaps a biologist here can share his thoughts on this.
Space/Science » in reply to About extremophiles
A very good question,
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