Ultraviolet radiation from the sun, together with lightning discharges, caused the gases in the primeval atmosphere to react, forming simple organic compounds. These dissolved in the primeval oceans and continued to react, forming what is sometimes referred to as the "primitive soup". The primitive soup contained amino acids, sugars, and nucleic acid bases among other randomly synthesized molecules (Fig. 20.01). Further reactions formed polymers and these associated, eventually forming globules. Ultimately, these evolved into the first primitive cells. This theory of the origin of life was put forward by the Russian biochemist Alexander Oparin in the 1920s. Charles Darwin himself had actually proposed that life might have started in a warm little pond provided with ammonia and other necessary chemicals. However, it was Oparin who outlined all the necessary steps and realized the critical point: life evolved before there was any oxygen in the air. Oxygen is highly reactive and would have reacted with the organic precursor molecules formed in the atmosphere, oxidizing them back to water and carbon dioxide.
primitive soup Mixture of random molecules, including amino acids, sugars, and nucleic acid bases, found in solution on the primeval earth
According to Oparin's theory of the origin of life, conditions on planet earth were sufficient for forming early biological molecules. The atmosphere at this point contained CO, CO2, CH4, N2, and NH3. When energy was supplied by electrical discharge from lightning, ultraviolet radiation from the sun, and/or b and g radiation from the earth, early organic compounds such as HCN and HCHO would form. These compounds would combine in the vapor phase and in the water to form amino acids. Dissolving in water protects the precursor molecules from being degraded again by the energy sources that triggered their formation.
Experiments to mimic reactions in the primeval atmosphere have generated many of the metabolites and monomers found in modern cells.
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