In the 1920s, Aleksandr Oparin, a Russian biochemist, proposed and developed the idea that life originated in the warm, watery environment of early Earth's surface, under an atmosphere mostly composed of methane. The early seas were believed by Oparin to be rich in simple organic molecules, which reacted to form more complex molecules, eventually leading to proteins and life. Then, almost 30 years after Oparin published his ideas, Stanley Miller demonstrated that amino acids, the building blocks of the proteins necessary for life, could form under conditions thought to prevail on early Earth. Miller's experiment was elegant. He passed electric discharges through a mixture of methane, hydrogen, ammonia, and steam, and when he analyzed the results, found that he had made amino acids. The discharges were a proxy for lightning, the gas mixture an educated guess about what the early atmosphere may have been like. Amino acids cannot replicate themselves, and are not themselves alive. Nevertheless, this experiment has long been recognized as a landmark for understanding a process that must have been one of the important steps in the evolution of life on Earth, the natural synthesis of amino acids. However, it now seems likely that Miller's experiments may not be directly applicable to the events of the early Archean (that is, early in the geologic eon that lasted from Earth's formation until about 2.5 billion years ago).
