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Chemical Evolution

“Chemical evolution” names the history of molecules on the early Earth before there was life: the formation of ever more complex organic compounds from simple ones. It began as a stretch of the unfolding sense, a period in a longer history, and it is the home where selection was first found inside chemistry, so that the history of molecules hands over to descent with modification.

The soup

In the 1920s Alexander Oparin (1924) and J. B. S. Haldane (1929), independently, proposed that life arose from organic compounds formed in the early oceans under an atmosphere without oxygen, energised by lightning or ultraviolet light, which Haldane called a “hot dilute soup”. In 1953 Stanley Miller, working with Harold Urey, passed electric sparks through a mixture of gases standing in for that atmosphere and obtained amino acids, and the synthesis of life’s building blocks became an experimental programme.

Calvin’s four evolutions

The chemist Melvin Calvin gave the stretch its best-known formulation. In “Chemical Evolution and the Origin of Life” (1955) he divided cosmic time into four kinds of evolution: nuclear, chemical, biological and “psychosocial”. Chemical evolution was “the period after the formation of the earth and before biological evolution can be said to have begun”, a period, in the unfolding sense, of rising molecular complexity. Within it he found the other sense beginning. At first, he wrote, “there is no selection; it is a random process, so far”; but “some of the chemicals that are formed may themselves be catalysts for their own formation. As soon as you recognize this, then you have available the essence of the process of selection within the chemical system itself.” His book Chemical Evolution (1969) developed the argument.

Eigen’s molecular selection

Manfred Eigen’s “Selforganization of Matter and the Evolution of Biological Macromolecules” (1971) derived Darwinian selection mathematically from the kinetics of self-replicating molecules: where molecules copy themselves with errors and compete for resources, the variants that copy best take over. Selection needs no organism, only replication, variation and limited supply. Eigen found a limit in the same equations, the error threshold, which caps the length of sequence that imprecise copying can maintain, and proposed the hypercycle, a cycle of mutually supporting replicators developed with Peter Schuster, as the way past it.

Where the senses meet

Chemical evolution also meets physics. The far-from-equilibrium thermodynamics of Ilya Prigogine’s school, in which order arises through fluctuations in systems driven by flows of energy, was carried toward prebiotic chemistry in the 1970s, and Eigen drew on it in his theory of selection among replicators. Later programmes each place the hand-over in their own way: the RNA world puts it at replicating RNA, metabolism-first theories at self-sustaining networks of reactions such as Stuart Kauffman’s autocatalytic sets, and assembly theory, with Sara Walker and Lee Cronin, at the point where selection begins to produce objects too complex to arise by chance.

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See also: Evolution (the subject landing) · Cosmic evolution · The criterion · Assembly theory · Eigen · Haldane · Prigogine · Kauffman