The biochemical theory for the origin of life

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There are many models for the origin of life, all based on an understanding of how the simplest living organisms today operate. The first 'modern' model for the origin of life was presented in the 1920s independently by two remarkable scientists, the Russian biochemist A. I. Oparin (1894-1980) and the British evolutionary biologist J. B. S. Haldane (1892-1964). Oparin and Haldane share the distinction of being independent co-founders of the so-called biochemical theory for the origin of life, as well as being known normally only by their initials.

According to the Oparin-Haldane model, life could have arisen through a series of organic chemical reactions that produced ever more complex biochemical structures. They proposed that common gases in the early Earth atmosphere combined to form simple organic chemicals, and that these in turn combined to form more complex molecules. Then, the complex molecules became separated from the surrounding medium, and acquired some of the characters of living organisms. They became able to absorb nutrients, to grow, to divide (reproduce), and so on. The Oparin-Haldane model was not tested until the 1950s.

In 1953, Stanley Miller (1920-2007), then a student of Harold Urey (1893-1981) at the University of Chicago, made a model of

5 the Precambrian atmosphere and ocean in a laboratory glass o t vessel. He exposed a mixture of water, nitrogen, carbon monoxide, o

| and nitrogen to electrical sparks, to mimic lightning, and found a

1= brownish sludge in the bottle after a few days. This contained sugars, amino acids, and nucleotides. So Miller had apparently recreated the first two steps in the Oparin-Haldane model, mixing the basic elements to produce simple organic compounds, and then combining these to produce the building blocks of proteins and nucleic acids.

It should be noted that critics have said that the mixture of gases that Miller used (with high percentage concentrations of hydrogen and methane) was rather different from the likely atmosphere of the early Earth. Atmospheric hydrogen is ultimately replenished from the mixture of gases released from the solid Earth; but the geochemistry of the subsurface means that the mixture generally should contain the oxidized form of hydrogen, namely water vapour, H2O, rather than the large proportion of free hydrogen gas in Miller's model atmosphere.

Further experiments in the 1950s and 1960s led to the production of polypeptides, polysaccharides, and other larger organic molecules, the next step in the hypothetical sequence. Sidney Fox at Florida State University even succeeded in creating cell-like structures, in which a soup of organic molecules became enclosed in a membrane. His 'protocells' seemed to feed and divide, but they did not survive for long, so they were not living, despite the hype made by the press at the time.

In a recent twist to the classic Oparin-Haldane biochemical model, Euan Nisbet (University of London) and Norman Sleep (Stanford University) proposed the hydrothermal model for the origin of life in 2001. In this model, the ancestor of all living things was a hyperthermophile, a simple organism that lived in unusually hot conditions. The transition from isolated amino acids to DNA may then have happened in a hot-water system associated with active volcanoes, rather than in some primeval soup at the ocean e surface. There are two main kinds of hot-water systems on Earth J today, 'black smokers' found in the deep oceans above mid-ocean f ridges where magma meets sea water, and hot pools and fumaroles e fed by rainwater that are found around active volcanoes.

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