Miller–Urey experiment

In the early 1950s, chemist Stanley Miller, working with his doctoral adviser Harold Urey at the University of Chicago, set out to test whether organic compounds could be synthesized from simple starting materials thought to represent a possible early-Earth environment. In the apparatus, water was heated to produce vapour, which circulated through a gas mixture and past electrodes that generated electrical sparks as a model energy source. A condenser cooled the vapour, allowing liquid and reaction products to circulate through the closed apparatus. The original 1953 setup used water, methane, ammonia and hydrogen.

After the apparatus operated for about a week, the collected liquid contained organic compounds, including amino acids. Miller's initial analysis identified a smaller set than later analytical techniques could detect. Reanalysis of archived samples from the published experiment found additional amino acids and amines. Researchers also studied related, initially unpublished Miller experiments; samples from a volcanic-style apparatus were found to contain a broader variety of organic compounds. These results show how both experimental conditions and analytical methods influence what is detected.

The experiment became a landmark in prebiotic chemistry because it showed that some organic molecules relevant to life can arise from non-living starting materials through geochemical processes. It did not produce life, and it did not establish a complete pathway from simple chemicals to living organisms. The gas mixture in the original experiment was strongly reducing, and how closely it represents Earth's early atmosphere remains debated. Subsequent studies test different gas compositions, energy sources and environmental settings. The experiment is therefore best understood as a demonstration of chemical possibility under a specific model, not as a definitive reconstruction of early Earth.