How Did Life Originate?

Abstract

Although Charles Darwin was reluctant to address the problem of the origin of life, soon after the publication of the Origin of Species in 1859, many naturalists adopted the idea that living organisms were the historical outcome of gradual transformation of lifeless matter. These views soon merged with the developments of biochemistry and cell biology and led to the proposals by Oparin and Haldane of a heterotrophic theory of the origin of life, which assumed that the first life forms had resulted from a process of evolution that started with the abiotic synthesis of organic compounds. Our current understanding of the basic molecular processes of living organisms has challenged many original assumptions of the heterotrophic theory. However, laboratory experiments have shown how easy it is to produce a variety of organic compounds, including biochemically important monomers, under plausible cosmic and geochemical conditions. The remarkable coincidence between the monomeric constituents of living organisms and those synthesized in Miller-type experiments appears to be too striking to be fortuitous. However, how the ubiquitous nucleic acid-based genetic system of extant life may have originated from such a mixture is one of the major unsolved problems in contemporary biology. The discovery of catalytically active RNA molecules provided considerable credibility to prior suggestions that the first living entities were largely based on ribozymes, in an early stage called the RNA world . There is convincing evidence suggesting that the genetic code and protein synthesis first evolved in such an RNA world, but at the time being, the hiatus between the primitive soup and the RNA world is discouragingly enormous. Bioinformatics and comparative genomics provide important insights into some very early stages of biological evolution, but it is difficult to see how their applicability can be extended beyond a threshold that corresponds to a period in which protein biosynthesis was already in operation, i.e., the RNA/protein world.