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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, however life. However  we need to remember thatthe  transition from simple lifeforms (microbial life) microbial life  to complex  animals is was  a long, complex very long  process. If we follow the same reasoning used for estimating $f_l$, we find out that On Earth  it took more than 3 billion years for life to evolve from single-celled bacteria to Homo sapiens. This Therefore the same reasoning used for estimating $f_l$  might argue argues  for a small value of $f_i$. Overall this term is very uncertain; using Frank Drake optimistic guess one has $f_i = 1$.