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The next factor in the Drake equation measures the \textbf{probability of life evolving into "intelligent life".}Broadly speaking, on Earth intelligent life has appeared at least a couple of times (\href{http://en.wikipedia.org/wiki/Cetacean_intelligence}{cetacean intelligence}).  Intelligence gives a clear evolutionary advantage and therefore it might represents a common pathway of life. Broadly speaking on Earth intelligent life has appeared at least a couple of times (\href{http://en.wikipedia.org/wiki/Cetacean_intelligence}{cetacean intelligence}).  However we need to remember that transition from simple microbial life to complex animals was a very long process. On Earth it took more than 3 billion years for life to evolve from single-celled bacteria to Homo sapiens. Therefore the same reasoning used for estimating $f_l$ might argues for a small value of $f_i$. Overall this term is very uncertain; using Frank Drake optimistic guess one has $f_i = 1$.