Paleontological Tests: Human-Like Intelligence Is Not a Convergent Feature of Evolution

I taught a course called “Are We Alone?” at the University of New South Wales for a few years. The most popular lecture was “The Great Drake Equation Debate” — half a dozen “experts” would sit at the front of the crowded lecture theater defending their estimates for the various terms in the Drake Equation (an equation created by Frank Drake to estimate the number of civilizations in the Milky Way with whom we might communicate via radio telescopes). The first terms of the equation are astronomical. How many stars are in our galaxy? — most experts agreed — about 300 billion. What fraction of those stars are orbited by “Earth-like” planets? — estimates ranged from ∼100% to ∼ 0.1% depending roughly proportionally on how specific “Earth-like” was interpreted to be. Then came the more contentious biological terms: What fraction of these Earth-like planets would harbor life? I defended a relatively high probability (∼10%) based on how rapidly biogenesis occurred on Earth (Lineweaver and Davis, 2002). We argued back and forth about how probable or improbable the steps of molecular evolution were, that led to life on Earth — and whether there were places on Earth where life could still be emerging. We all learned a lot about biochemistry, autocatalytic cycles and hydrothermal vents. However, the most contentious term was: Once there is life of any kind, what is the probability that it will evolve into a human-like intelligence that can build and operate radio telescopes? (We define intelligence this way not out of some geeky technophilic perversity but because posed this way, we have the ability to answer the question by searching for other telescopes with our telescopes. So far, no signals from intelligent aliens have been identified, Tarter, 2001.)