Why does the Drake equation differentiate between percentage of intelligent life, and percentage of intelligent life that develops communication ability?  I understand they are steps in evolution but they seem like arbitrarily drawn lines. Lines could easily be drawn at the development of language, writing, discovery of electricity, harnessing of nuclear power —all of which will help to facilitate our transition to another planet if we ever do.  You could also presume that intelligent life has the ability to communicate as a prerequisite.  I wouldn’t be as concerned about extraneous variables, except that this is an equation with multiplication implications.,(reply>), Yep. That's a hard line to draw