I think I am a little late to this thread, but I'm surprised no one has discussed the Rare Earth Theory in this thread. Basically, this theory attempts to add in new developments in our understanding of astrobiolgy, astrophysics, planetary formation, and cosmology to the original Drake Equation in order to come up with an improved, more modern, and more realistic formula for the number of advanced civilizations in our galaxy. They add in a bunch of factors not considered in the Drake equation, such as the benefits of having an extremely large moon to stabilize a planet's tilt along its axis, the presence of a large Jupiter to guard the rocky inner planets from excessive bombardment by wayward asteroids and comets as well as the frequency and timing of the impacts that DO occur, beneficial locations within galactic spiral arms (eg galactic habitable zones in addition to the already considered planetary habitable zones), metallicity of planets, types of planetary orbits, and numerous other factors. The Rare Earth Theory takes a pretty realistic approach to evaluating new factors that Drake never did. Depending on your viewpoint on the new factors they introduce, the number of advanced alien civilizations in our galaxy other than us may be as low as 0. They do, however, believe that primitive life is common. It is certainly an interesting theory that deserves some consideration.