Miller-Urey experiment

The Miller-Urey experiment (1952), conducted by Stanley Miller and Harold Urey at the University of Chicago, demonstrated that amino acids and other organic compounds form spontaneously when a mixture of reduced gases — methane, ammonia, hydrogen, and water vapor — is subjected to electrical sparks simulating lightning. The experiment was initially interpreted as a simulation of Earth's primordial atmosphere and a demonstration of abiotic prebiotic chemistry. Its deeper significance is the proof that the building blocks of biochemistry are thermodynamically favorable products of common planetary energy gradients, not rare accidents requiring living intervention.

The original experiment used a closed apparatus with a water cycle, gas mixture, and spark electrodes. Within days, the flask contained glycine, alanine, and other amino acids, plus hydroxy acids and urea. Subsequent replications with varied atmospheric compositions — including more neutral or oxidizing mixtures — have produced more limited yields, revealing that the specific chemistry matters but the fundamental principle does not: energy + simple molecules + catalytic surfaces can produce complex organics without biology. The Miller-Urey experiment is therefore not merely a historical curiosity but a persistent constraint on any theory that treats biochemistry as a unique invention of life.

The Miller-Urey experiment did not simulate the origin of life; it simulated the origin of the origin of life — the demonstration that chemistry was already doing biochemistry before biochemistry had a name.