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Chapter 1 : Extremophiles and the Origin of Life
During the past 2 decades, the description of a diverse assortment of prokaryotic species that thrive under extreme environments that used to be considered inhospitable has broadened our understanding of the range of conditions under which life can persist. With the exception of heat-loving prokaryotes, however, the phylogenetic distribution of other extremophiles in molecular cladograms does not provide clues to their possible antiquity. Furthermore, given the huge gap existing in current descriptions of the evolutionary transition between the prebiotic synthesis of biochemical compounds and the last common ancestor (LCA) of all extant living beings, it is probably naïve to attempt to describe the origin of life and the nature of the first living systems from molecular phylogenies. It is unlikely that data on how life originated will be provided by the geological record. The remarkable coincidence between the monomeric constituents of living organisms and those synthesized in laboratory simulations of the prebiotic environment is too striking to be fortuitous, but at the same time the hiatus between the primitive soup and the RNA world, i.e., the evolutionary stage prior to the development of proteins and DNA genomes during which early life forms largely based on ribozymes may have existed, is discouragingly enormous. The diversity of environmental conditions under which prokaryotes can thrive should be understood as evidence of their adaptability and not as evidence that the origin of life took place under extreme conditions.