In the 1920s, while working independently, Alexandr Oparin (1894–1980) and John Haldane (1892–1964) both proposed scenarios for the “prebiotic” conditions on Earth (the conditions that would have allowed organic life to evolve). Although they differed on details, both models described an early Earth with an atmosphere containing ammonia and water vapor. Both also surmised that the assemblage of organic molecules began in the atmosphere and then moved into the seas. The steps of the Oparin-Haldane model are:

1. Organic molecules, including amino acids and nucleotides, are synthesized abiotically (without living cells).
2. Organic building blocks in the prebiotic soup are assembled into polymers of proteins and nucleic acids.
3. Biological polymers are assembled into a self-replicating organism that fed on the existing organic molecules.