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Spontaneous formation of organic molecules.

Although Miller's experiments did not precisely reproduce the conditions of primitive Earth, they clearly demonstrated the plausibility of the spontaneous synthesis of organic molecules, providing the basic materials from which the first living organisms arose. The spontaneous formation of organic molecules was first demonstrated experimentally in the 1950s, when Stanley Miller (then a graduate student) showed that the discharge of electric sparks into a mixture of H 2, CH 4, and NH 3, in the presence of water, led to the formation of a variety of organic molecules, including several amino acids ( Figure 1.2). Such an atmosphere provides reducing conditions in which organic molecules, given a source of energy such as sunlight or electrical discharge, can form spontaneously. At the time life arose, the atmosphere of Earth is thought to have contained little or no free oxygen, instead consisting principally of CO 2 and N 2 in addition to smaller amounts of gases such as H 2, H 2S, and CO.

It was first suggested in the 1920s that simple organic molecules could form and spontaneously polymerize into macromolecules under the conditions thought to exist in primitive Earth's atmosphere. The scale indicates the approximate times at which some of the major events in the evolution of cells are thought to have occurred.