Ward and Brownlee, in their book Rare Earth, advance the idea that intelligent life is very unusual in the universe. However, since simple life (single-celled organisms) are widespread on Earth, and can survive in hostile environments (high-temperature, low pH etc), they also suggest that simple life may be comparatively common.
The problem with this suggestion is that it assumes that the widespread presence of simple life, and the fact that it copes with a wide range of environmental conditions, means that its initial appearance is more likely. However, life may still only have appeared once, and be incredibly improbable - just that having appeared, it became very adept at adapting to new environments. If this is the case, then intelligent life (which, according to this theory, flows from simple life) may be even less common.
Even more so than Privileged Planet, the Rare Earth hypothesis is limited in its usefulness by virtue of the fact that it has a sample size of 1.
