Thermoreduction, a hypothesis for the origin of prokaryotes

All thermophiles discovered so far are prokaryotes (Bacteria or Archaea). Furthermore, reconstructions of rRNA phylogenies suggest that the progenitor of all prokaryotes was a thermophile. These data are usually interpreted as supporting the hypothesis that all present day organisms, including eukaryotes, originated from hyperthermophiles. However, this scenario is difficult to reconcile with the RNA world theory, considering the instability of RNA at very high temperatures, and it is also contradicted by the finding of sophisticated devices for thermophilic adaptation in present day hyperthermophiles. Accordingly, I propose here 2 new hypotheses to explain the correlation between the procaryotic phenotype and thermophilic life without reference to a putative hot origin of life. Firstly, eukaryotes would be unable to live in thermophilic biotopes because of the susceptibility of their mRNA to degradation at high temperature. In prokaryotes, the absence of a nuclear membrane allows these organisms to bypass the problem of mRNA heat-induced hydrolysis by coupling transcription and translation. As a corollary of this first hypothesis, I also suggest that today prokaryotes might have originated from mesophilic ancestors via reductive evolution, in the process of adaptation to thermophily.