What caused the Cambrian explosion? The Economist

"For most of the Earth's history, life consisted of the simplest organisms; but then something happened that would give rise to staggering diversity, and, ultimately, life as complex as that which we see today. Scientists are still struggling to figure out just what that was."

This is relevant to the question of the development of extra-terrestrial life. There is a line of thought that says: "Simple life, probably microbial, one-cellular life may be common in the universe; but, more complex life, which could evolve upward to something that could build radio telescopes and maybe even starships, is probably rare."