So you did... my error. I am thinking about this in the same "plants/animals/similar" terms as yourself.
I suspect that once life becomes established in a resource-rich environment, progressive evolution is simply a numbers game. Selection drives adaptability, but there will inevitably be a long period of time before the first tipping point where one advantageous adaptation causes a leap forward in competitive strategies. This is because of the relative simplicity of the information structure and methods of transmission in primitive organisms.
I wouldn't call hydrothermal vents or salt lakes hospitable, but then I am not adapted to them . What we learn from ecosystems adapted to these environments is that given enough time, living systems can adapt to exploit resources even if the conditions where those resource are found are atypical in some extreme or another. That in itself is good evidence (although rather circumstantial, I have to admit) that once life arrives, it can adjust its complexity as it encounters new survival challenges. Such challenges might even be the specific environmental changes you mentioned earlier, but they do not have to occur on a planetary scale or across thousands of years - since a single population can evolve, and single populations are very vulnerable to changes in resource levels or abiotic conditions, they just have to happen, and further complexity results. Or, you know, extinction. Depends on the circumstances!