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\section*{Option 1: Intelligent life is rare}  On Earth it took more than 3 billion years for life to evolve from single-celled bacteria to Homo Sapiens. It's a long time, an argument used to state conclude  that $f_i << \ll  1$. Moreover P. Ward and D. Brownlee in their book \href{http://bit.ly/1GGQt5B}{"Rare Earth"} claim that "Intelligent life on Earth relied on so many unlikely accidents that we are probably alone in the universe".