Previously, analyses of ionizing-radiation-sensitive mutants of Deinococcus radiodurans and investigation into the genomic expression profile of the wild-type organism indicated a positive correlation between radiation resistance and desiccation tolerance phenotypes. Mainly because a terrestrial selective force for acquiring resistance to ionizing radiation was unfound, it has been assumed that the radiation resistance of D. radiodurans is an incidental phenotype due to its anhydrobiosis defense abilities. Here, following the stratification of the D. radiodurans genome, we discuss these issues from evolutionary, genomic, and experimental perspectives. Through the data collected we propose a reconciliatory model wherein radiation resistance is a unique molecular reflection of the early Earth resilience and desiccation tolerance is a mark of cells that colonized land during the Archaean epoch.
Although the RNA world hypothesis is important as a model of the emergence of life-like systems, this hypothesis has some drawbacks. This hypothesis was already evaluated from the hydrothermal origin of life hypothesis in our recent paper. On the other hand, a unified theory for the origin and evolution of life has been attempted to develop on the basis of our recent investigations concerning the subjectivity of life and the molecular machinery of assigning genotype and phenotype. In the present paper, the RNA world hypothesis is further evaluated on the basis of the unified theory as well as the experimental evidences concerning the chemical evolution under hydrothermal environments.
There have been several studies on biopolymer synthesis under hydrothermal conditions. Although dimer and short oligomers were obtained from a monomer unit, few of longer oligomers were obtained. These studies that were applied with various quenching methods suggested the importance of quenching speed from hydrothermal conditions. We hypothesized a rapid quenching could avoid hydrolysis of the oligomers that had already been synthesized under hydrothermal conditions. We designed a novel hydrothermal flow reactor with adiabatic expansion cooling, which was thought to be one of the most rapid quenching methods. This system simulates geysers, fumaroles, hot springs and volcanic eruptions. After aqueous solutions of monomers were treated at high temperature and pressure, they were released into the atmosphere through an orifice to be depressurized and cooled down simultaneously with the Joule-Thomson effect. Using the flow reactor, we have demonstrated oligomerization of glycine up to decamer (Gly10), which had never been yielded with any other quenching methods. This suggests that rapid quenching methods under non-equilibrium conditions such as adiabatic expansion cooling is an efficient way to produce long oligomers connected by covalent bonds via dehydration condensation.