Fundamentally we're limited by two things- the boiling point of water, and the fact that biological macromolecules are very complex and fall apart or crumple up

Fundamentally we're limited by two things- the boiling point of water, and the fact that biological macromolecules are very complex and fall apart or crumple up at high temperatures. Deep sea creatures can bypass the first one due to the intense pressure in the deep ocean, but you're still going to have to deal with the fact that atoms bounce around and move more at higher temperatures, and that isn't good for the precision molecular machinery that life relies on. On the low end, we're pretty much limited by the same things, but in reverse- the freezing point of water, and the fact that chemistry slows down at low temperatures. So that puts us with a fundamental limit of 0-100 C for any system that relies liquid water at atmospheric pressure, and only a little bit beyond that for extremophiles. There is another fundamental limit beyond 100 for high temp when any organic molecule will start to burn or pyrolyze, but the boiling point is already a huge evolutionary hurdle on its own for most places on Earth.