REVIEW ARTICLE Hyperthermophily and the origin and earliest evolution of life

Abstract The possibility of a high-temperature origin of life has gained support based on indirect evidence of a hot, early Earth and on the basal position of hyper-thermophilic organisms in rRNA-based phylogenies. However, although the availability of more than 80 completely sequenced cellular genomes has led to the identification of hyperthermophilic-specific traits, such as a trend towards smaller genomes, reduced protein-encoding gene sizes, and glutamic-acid-rich simple se-quences, 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 C the half-lives of several organic compounds, including ribose, nucleo-bases, 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 hypertherm-ophily is not truly primordial, then heat-loving lifestyles may be relics of a secondary adaptation that evolved after the origin of life, and before or soon after separa-tion of the major lineages