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$f_i$ Once life emerges on a planet, what's the \textbf{probability that will evolve into intelligent life?}. Depending on the exact definition of intelligence, on Earth it has appeared probably at least a couple of times (\href{http://en.wikipedia.org/wiki/Cetacean_intelligence}{cetacean intelligence}). Also, given the evolutionary advantage of intelligence, it is plausible that it represents a common pathway of life\footnote{Note however that the fact that it took several billion years for "intelligent lifeforms" to evolve on Earth has been used as an argument supporting small values of $f_i$}. Frank Drake himself followed this line of thought and concluded that the factor  $f_i = 1$. To communicate with other intelligent life a civilization has to develope some sort of technology. If again we consider the Earth's case, this happened for humans but not, for example, for dolphins. The factor \textbf{$f_c$ measures the fraction of civilizations that develop communicative technology}. As intelligence and curiosity seem to go hand-in-hand, it seems conceivable that all intelligent lifeforms might evolve towards a technological form. $f_c = 1$