The Key Role of Plate Tectonics for Accelerating the Evolution of Complex Life: Quest for the Missing Extraterrestrial Civilizations

It is often assumed that quest for intelligent life is equivalent to quest for any life in general. In particular, the Drake Equation predicts that there should be many exoplanets in our galaxy hosting active, communicative civilizations (ACCs) but the Fermi Paradox notes that there is no evidence that any others exist. The paradox may be explained by recognizing that advanced complex life is only likely to develop on a very small fraction of planets hosting any life. Here, we suggest that ACCs can only develop on a convecting silicate body with life, prolonged plate tectonics and significant expanses of oceans and continents, and that meeting all three of these requirements may be extremely difficult to achieve.  Continents and oceans are required because early life evolution must happen in water but late evolution capable of creating technology must happen on slowly but continuously evolving land masses due to the critical influence of landscape and habitat diversity in space and time for accelerating the evolution of complex species. We further suggest that oceanic-continental plate tectonic environments of the modern Earth have formed only very recently at 1.0-0.5 Ga and accelerated the evolution of complex life in five ways: 1) increased nutrient supply; 2) increased free oxygen in the atmosphere and ocean; 3) climate amelioration; 4) accelerated habitat formation; and 5) moderate sustained environmental pressure.  The absence of comparable factors on the other possible types of global tectonic environments (such as squishy lid, stagnant lid, volcanic heat pipe) characteristic for silicate bodies (e.g., early Earth, Mars, Venus, Mercury, Moon, Io) makes the emergence of both complex life and ACCs on such bodies unlikely.  The Fermi Paradox may therefore be resolved if: 1) two additional terms are added to the Drake Equation: f_oc (the fraction of exoplanets with significant continents and oceans) and f_pt (the fraction of habitable planets that have had plate tectonics operating for at least 500 Ma; and 2) the product of f_oc and f_pt is very small.  The lack of evidence for extraterrestrial civilizations in our galaxy may reflect the scarcity of long-lived oceanic-continental plate tectonic environments in particular requiring goldilocks condition in term of the stable surface water volume allowing for long-term coexistence of both dry lands and oceans.