Can we correctly assume that intelligence would eventually evolve on any planet that has life for a sufficient length of time?

We often think of evolution as a ladder leading from the simplest single-cell organism up, rung by rung, to humans and thus to intelligent life. This does not square at all with the real path of evolution, however. In fact, while life has increased in complexity it has done so not in the form of a long ladder, but in the form of a grassy meadow with shrubs and trees scattered here and there, each having many branches.

In truth, we have something like 50 million species on earth today and only one of them in the 3.5 billion year history of life has evolved into a sentient, intelligent being. Isn’t it possible—even likely—that there may be many planets supporting teaming life of diverse forms but with none of those life forms having followed the unlikely path to intelligence. Large brains demand a great deal of food. Pound for pound, they use far more energy than any other part of our bodies. So natural selection may not always push life toward relatively expensive large brains.

Could it be that there is a flaw in the SETI project’s assumption that life elsewhere in the Universe will inherently lead to intelligent beings? What do you think the odds might be that life on any given planet, if allowed billions of years to evolve, would culminate in an intelligent life form capable of interstellar communication?