I was reading an article the other day discussing the possibility of ever finding intelligent life elsewhere in the universe, and I was wondering about what conditions really do matter for intelligent, technologically advanced life. By technologically advanced, I mean basic Stone Age technology at the very least, not necessarily even modern technology. I've seen a lot of arguments that life might be completely different elsewhere and not even need such as liquid water or even oxygen in meaningful amounts. But that concept bugs me a bit. Looking at life here on Earth, it seems to me that all life has evolved on a rather ideal planet, at an ideal distance from an ideal star, within an ideal region of an ideal galaxy, located in an ideal galactic neighborhood in an ideal universe that had the perfect set of parameters to even allow for baryonic matter. That's an awful lot of perfect that has to fall into place to allow for not just life, but intelligent life to form. The reason the argument for very different types of intelligent life bugs me is the idea that water and oxygen might not be necessary for such life. Granted, there could be lifeforms that are silicon-based or what-have-you, but water is a rather special chemical. I guess my questions are: • Is there a chemical analogue for water? Is there anything that could substitute it biologically? And could any such analogue exist under different planetary conditions as a liquid? • Is there a possibility for fire without a significant amount of atmospheric oxygen? What else could be an "oxidizer"? I figure fire would be key for a civilization. • Is it really feasible for life to form if too close (higher radiation) or too far (less energy) to/from a star? Could the "Goldilocks Zone" have more to it than just enough heat for liquid water? • Does the type of water matter? If there were water on a planet that had a much higher concentration of Deuterium, would that biologically matter? • If the answer to all of the above were to point to an Earth-like type planet, does that still leave billions of possibilities, or does that really cut out the chance that intelligent life could have formed elsewhere? Could the Earth actually be special?