Primordial soup

Go back far enough in time, and you eventually have to explain how the chemicals of life − especially proteins and nucleic acids − formed in Earth's primordial environment.

In 1929, biochemists John Haldane and Aleksander Oparin hypothesized independently that Earth's early atmosphere lacked free oxygen. In this harsh environment, they suggested, organic compounds could form from simple molecules if they were stimulated by a strong source of energy, either ultraviolet radiation or lightning. Haldane added that the oceans would have been a "primitive soup" of these organic compounds.

U.S. chemists Harold C. Urey and Stanley Miller set out to test the Oparin-Haldane hypothesis in 1953. They reproduced the early atmosphere of Earth by creating a carefully controlled, closed system. The ocean was a warmed flask of water. As water vapor rose from the water and collected in another chamber, Urey and Miller introduced hydrogen, methane and ammonia to simulate the oxygen-free atmosphere. Then they discharged sparks, representing lightning, into the mixture of gases. Finally, a condenser cooled the gases into a liquid they collected for analysis.

After a week, Urey and Miller had astonishing results: organic compounds were abundant in the cooled liquid. Most notably, Miller found several amino acids, including glycine, alanine and glutamic acid. Amino acids are the building blocks of proteins, which themselves are the key ingredients of both cellular structures and cellular enzymes responsible for important chemical reactions. Urey and Miller concluded that organic molecules could form in an oxygen-free atmosphere and that the simplest of living things might not be far behind.