Unfortunately this is one of those questions for which there is little data (at the moment) to support whatever your viewpoint is. If this was a test question at school your teacher wouldn't care about the answer so much as your thinking leading up to that answer. I guess that will have to be the criteria for this thread. Life came about rather quickly here on Earth (single cells) but as I recall another couple of billion years passed before multi-cellular life came about and it's only been in the last 500 million years (out of 4.5 billion years) that life as we know has flourished. It would be very helpful if we could find evidence of any low level life in the rocks on Mars (or elsewhere within our Solar System). It would be helpful if we could create any kind of life in the laboratory. I'm guessing that, given the large number of planets in our Universe that simple life is not too rare. That's the biggest hurdle to overcome though. If that's the case there it's just a matter of time and numbers. There are many things that can end life or prevent it from developing further and we also may be rather early in our Universe for life to develop as widely as it may develop over time. I'm not so sure about highly intelligent/evolved/adaptable life (similar to man) being anything other than very rare. As far as finding an answer to this question, there's SETI-like projects. Even though intelligent life is probably very rare, it should be the easiest to detect but I don't personally have a lot of faith in SETI-like approaches. I also don't particularly think we should be trying to contact such life. It might be like trying to make friends with a wild lion...not a good idea. The best shot of getting more questions answered is probably from within our own Solar System. As technology improves, I think it will be possible to examine the atmosphere's of planets in other star systems to look for gases (particular isotopes) that don't usually occur without some form of life generating it. I guess I'm expecting low level life (multi-cellular) to be somewhat common and highly evolved life to be pretty rare for many reasons such as lack of need, natural extinction events, etc. What are your thoughts?