Candidate Filter Steps

Scientists have proposed several plausible Great Filter candidates. The origin of life may be extraordinarily improbable — despite decades of effort, no one has built life from scratch in a lab. The leap from prokaryotes to eukaryotes appears to have taken billions of years on Earth, hinting that it may be exceptionally hard. Multicellular life and, later, intelligence, language, and technology each add another hurdle. If even one step has odds of one in a billion, the galaxy could hold trillions of habitable planets and still produce only a handful of civilizations.