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The next factor in the Drake equation measures the \textbf{probability of life evolving into "intelligent life".} 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 consider that the 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, $f_i<<1$, but it is very hard to come up with a precise number. Note that this perspective is quite different from the very optimistic view of Frank Drake, who argued that life likely always evolves into intelligent life, that is $f_i \sim 1$.  To communicate with other intelligent life, a civilization has to develop some sort of technology. If again we consider 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 eventually evolve towards a technological, communicative form. $f_c = 1$  Putting these factors into the Drake equation we get   $N \approx \, \frac{1}{4}\, f_i \, L $.   This states that the number of communicative civilizations in our Galaxy is just the product of the chance of emergence of intelligent life times the average lifetime of a civilization's communicative phase. In the next and last post of this series I will show how the current search for radio signals from extraterrestrial intelligence has put some important constrains on the two factors $f_i$ and $L$, and how these informations can be used to make some predictions about the future of humankind.  %Note this is basically the same result that Frank Drake obtained in 1961! The difference is that this has been %obtained only making assumptions on the factors $f_i$,$f_l$ and $f_c$, since the other terms in the equation %are now established.  %\href{http://en.wikipedia.org/wiki/Fermi_paradox}{Fermi}  %\href{http://en.wikipedia.org/wiki/Rare_Earth_Hypothesis}{Rare Earth}  diff --git a/layout.md b/layout.md  index 08e3431..d4bc678 100644  --- a/layout.md  +++ b/layout.md 
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figures/dna_astro2/dna_astro.png  We_are_thinking_creatures_living__.tex  Astrobiology.tex  fc.tex  Derive L.tex  Last remarks.tex  figures/Lcritical/Lcritical.png