Prebiotic chemical evolution and the origin of life on Earth

Although there is no geological evidence of the environmental conditions on the Earth at the time of the origin of life, astronomical observations and laboratory simulations of the prebiotic environment support the possibility of the synthesis and accumulation of organic monomers of biochemical significance in the primitive terrestrial environment. It is likely that no single mechanism can account for the wide range of organic compounds that may have accumulated on the primitive Earth, and that the prebiotic soup was formed by contributions from endogenous syntheses in a reducing atmosphere, metal sulfide-mediated synthesis in deep-sea vents, and exogenous sources such as comets, meteorites and interplanetary dust. The robustness of this type of chemistry is supported by the occurrence of most of these biochemical compounds in the Murchison and and other meteorites, which makes it plausible, but does not prove, that similar synthesis took place on the primitive Earth. The discovery of ribozymes has given considerable support to the hypothesis of the so called-RNA world, from which the nucleic acid-based genetic system of extant life is believed to have originated. However, the chemical lability of RNA and its components, combined with the high number of possible random combinations of derivatives of nucleobases, sugars, and phosphate, suggest that RNA was not a direct outcome of prebiotic evolution, but may have been the evolutionary outcome of even older, more primitive living systems now referred to as pre-RNA worlds.