"Why We Haven't Met Any Aliens" →
Paleontology has shown that organic life evolved quickly after the Earth’s surface cooled and became life-hospitable. Given simple life forms, evolution shows progressive trends toward larger bodies, brains, and social complexity. Evolutionary psychology has revealed several credible paths from simpler social minds to human-level creative intelligence. So evolving intelligence seems likely, given a propitious habitat—and astronomers think such habitats are common. Moreover, at least 150 extrasolar planets have been identified in the last few years, suggesting that life-hospitable planets orbit most stars. Yet 40 years of intensive searching for extraterrestrial intelligence have yielded nothing: no radio signals, no credible spacecraft sightings, no close encounters of any kind.
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