ET - Where Are You?

The second most famous idea in extraterrestrial biology is the Drake Equation.  The idea of the Drake Equation is to express the probability of finding ETs as a product of presumably independent factors (fraction of stars with planet, fraction of planets suitable for life, probability of life arising on a suitable planet, etc.)  When the equation was first written down, none of these factors were known.  We now know that planets are common - there are many billions in our galaxy and perhaps trillions.  We can't be sure yet, but it looks like something like a billion ought to be broadly suitable for life.

The fact that life arose nearly as soon as it possibly could have on Earth suggests but doesn't prove that life is likely to arise on suitable planets.  Experimental biophysics and biochemistry seem to be converging on an explanation of how life arose one Earth, but large - enormous - uncertainties remain.  Much of this uncertainty could be removed if we were to find other life in the Solar System, on Mars, say, or Enceladus, a moon of Saturn.  Detection of a telltale sign of life on an exoplanet - one outside the solar system - for example, an oxygen atmosphere, would also be decisive.

What about the evolution of life intelligent enough to produce complex technological civilization?  It happened on Earth, but it took hella long time, at least three and a half billion years from the origin of life.  We know a little about why it took so long.  It required several steps: the invention of photosynthesis, the production of an oxygen abundant atmosphere, the development of multicellularity, escape from the ocean to land, the development of something like hands and fingers, mastery of fire, and the development of big brains that could make detailed plans.  Once all those were achieved, technological civilization followed in a cosmic instant - about 50,000 years.

Could there be other paths?  Maybe, but the brains, something like hands, and fire seem crucial.

Some of final factors in the Drake Equation ask questions like how long can an advanced civilization last.  Here we have only one example, and indications are not especially promising.


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