If you are really interested in this, read "Rare Earth", http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/d..., it goes through an incredible mass of astronomical, geological, and biological information. It concludes that, because so many things had to go just right, and so many others were completely random, primitive life (bacterial-level) is likely to be more common than previously thought, while advanced life (multicellular) is likely to be much less common. Because of the sheer size of the galaxy, it is probable that other intelligences evolved elsewhere, but they are probably going to be too far away for any sort of contact, even receipt of potentially meaningful radio. It is a well-written book, despite the amount of information it is very readable.