f_i = fraction of planets with life where intelligent civilizations arise = 3/4

This is a subject of great controversy (text uses 1/3).

With increasing biological diversity and numbers of organisms, there is the opportunity for more complex organisms to appear. If intelligence appears as a trait, it has huge survival value, and the species may thrive.

On the other hand, many species continue for hundreds of millions of years without developing intelligence (language, technology). Blue-green algae are so successful biologically that they are almost unchanged after billions of years. Recall also that evolution is punctuated by dramatic changes (for example, mass extinctions from meteoric impacts), so that the course of evolution may not be predictable.

Because we are the only known example of intelligence, it is very hard to estimate how commonplace the appearance of intelligence may be:
are we overly anthropocentric in thinking that human-like intelligence has appeared over and over again in the galaxy (Mayr - Harvard), or is it too improbable to imagine that we are unique, the only intelligent organism to arise (Drake)?