Sort of pushes one outside the box a bit. Those sulphide-chomping critters who don't rely on photosynthesis are certainly outside the box, too.

They don't seem to have directly evolved beyond rather simple organisms even though they could well have had billions of years to do so.

On the other hand, their habitat is limited, and the world beyond those vents wouldn't support them. So, their evolution may have been into creatures which could take advantage of the many opportunities available to an oxygenated, CO2-loving creature.

Would be interesting to see what happened in an entire world which supported their particular metabolism. In fact, it would be interesting to know what would happen in a world which supported any other metabolism than ours. Would "intelligent" life appear, as it did here, only to disappear when it entered the voting booth. (The debbil again.)

Arf