The big argument against intelligent life being common (and us not being unique) is the existence of stars. No civilization more advanced than us is going to let stupendous amounts of energy go to waste in the form of star light. If you assume von Neumann probes and Dyson spheres are possible, then star light is incompatible with the existence of advanced technology.
While life appeared very early in the history of the earth, technologically advanced life took billions of years to evolve. You require a planet with an extraordinarily stable climate to allow a technological society to evolve and I predict this is the filter.
Just one factor contributing to this stability is the need for a very large moon to keep the axis of rotation stable over billion of years. If the Earth didn't have its moon the Earth's axis would tip over creating massive climatic change. The more we look at the universe the more unlikely the stability of Earth's climate looks.
While life appeared very early in the history of the earth, technologically advanced life took billions of years to evolve. You require a planet with an extraordinarily stable climate to allow a technological society to evolve and I predict this is the filter.
Just one factor contributing to this stability is the need for a very large moon to keep the axis of rotation stable over billion of years. If the Earth didn't have its moon the Earth's axis would tip over creating massive climatic change. The more we look at the universe the more unlikely the stability of Earth's climate looks.