Professor Andrew Watson of the University of East Anglia has recently published a paper in the February issue of Astrobiology entitled Implications of an anthropic model of evolution for emergence of complex life and intelligence. In this article he argues that a number of limitations must be overcome in order for evolution to progress to the point to leading to intelligent live.
Watson postulates that for intelligent observers to evolve, a small number (n) of very difficult evolutionary steps must be passed. Once passed, evolution occurs quickly until the next stage is reached. Complex and intelligent life evolved quite late on Earth and Watson suggests that this may be because of the difficulty in passing these stages. He suggests that n is less than 10 and most likely equal to 4. These stages include the emergence of single-celled bacteria, bacteria with complex cells, cells allowing complex life forms, and intelligent life.
Professor Watson uses the Earth's fossil records to establish upper bounds on the probability for each state.
The work supports the Rare Earth hypothesis which postulates that the emergence of complex multicellular life (metazoa) on Earth required an improbable combination of astrophysical and geological events and circumstances.