Hyperthermophily and the origin and earliest evolution of life

The possibility of a hightemperature origin of life has gained support based on indirect evidence of a hot. early Earth and on the basal position of hyperthermophilic orizanisms in rRNAbased phylogenies. However, although the availability of more than 80 completely sequenced cellular genomes has led to the identification of hyperthermophilicspecific traits, such as a trend towards smaller genomes, reduced proteinencoding gene sizes, and glutamicacidrich simple sequences, none of these characteristics are in themselves an indication of primitiveness. There is no geological evidence for the physical setting in which life arose, but current models suggest that the Earth's surface cooled down rapidly. Moreover, at 100 degreesC the halflives of several organic compounds, including ribose, nucleobases, and amino acids, which are generally thought to have been essential for the emergence of the first living systems, are too short to allow for their accumulation in the prebiotic environment. Accordingly, if hyperthermophily is not truly primordial, then heatloving lifestyles may be relics of a secondary adaptation that evolved after the origin of life, and before or soon after separation of the major lineages