Interesting Astrobiology Magazine article: at the deepest level beneath the earth's surface, the only living thing is a single bacteria species. What interests me is that a stable environment that's existed for millions of years hasn't created a diverse ecosystem.
I think it's likely that life is common in other solar systems (I'm less certain about other parts of our own solar system). I also think intelligent life is likely, based on the idea that as biological systems become more diverse, one of those branches of diversity that will be explored in many places would be through species becoming more intelligent.
I don't really expect intelligent life to arise anywhere like miles beneath the surface where multicellular organisms can't exist, but I had expected something like a rule that biological systems tend towards diversity, or at least a certain level of diversity. This example says the rule isn't always true.