The Rare Earth hypothesis claims that conditions for complex life in the universe are rare. It is one resolution to the Fermi Paradox—the apparent lack of extraterrestrial intelligence despite the existence of numerous extrasolar planets. The original advocates of this theory predict that simple life is fairly abundant in the universe, but that multicellular organisms are almost always prevented from evolving. There are many factors offered, ranging from the galactic level to the planetary, that could keep a biosphere primitive.
Perhaps the most convincing tenet of the Rare Earth hypothesis is the idea of a galactic habitable zone. The habitable zone is not a new concept in astronomy, but it has usually referred to the region around a star. Might there also be zones of a galaxy that are unsuitable for life? The center of the Milky Way is believed to be flooded with harmful radiation, while the outskirts lack the heavier elements—carbon, nitrogen, oxygen—that are found in living things. The spiral arms contain solar systems that frequently disturb each other with celestial impacts and supernovae; in fact, there may even be a correlation between Earth's mass extinctions and its passage through the Milky Way's spiral arms. It seems there must be entire galaxy types that are hostile to life. Though the boundaries of a galaxy's habitable zone may currently be less certain than a star's, it stands to reason that such a zone is meaningful.
Other factors of the Rare Earth hypothesis seem to be essentially conjecture. For instance, I don't quite understand the importance of plate tectonics in the development of complex life. Couldn't mere volcanism provide environmental variation and mineral recycling? It's also unclear to me whether a large moon is necessary to stabilize a planet's axial tilt. The tilt of a planet's axis is what causes seasons, which do seem to allow more biological variation—clearly helpful if complex life is to evolve. There may be some holes in the Rare Earth hypothesis, but they must be addressed scientifically. We must not let our excitement about the possibility of extraterrestrial life cripple our capacity to be objective.