Abstract
With the identification of an archaeal Last Universal Common Ancestor (LUCA) related to the archaeon Methanopyrus, the origin of Bacteria became a choice between autonomic development versus descent from Archaea. The similarity bitscores between paralogous valyl-tRNA synthetase (VARS) and isoleucyl-tRNA synthetase (IARS) suggest that the five oldest bacteria were Mahella australiensis, Thermincola potens, Halobacteroides halobius, Desulfosporosinus orientis and Caldicellulosiruptor lactoaceticus, which were all Clostridia species and hydrogen producers. A search for archaea that could be a candidate Progenitor of Bacteria endowed with an Emden-Myerhof-Parnas type glycolytic pathway and a clostridial-like dark-fermentation mechanism for generating hydrogen pointed to such a role for the anaerobic chemoorganotroph Thermococci, which were known to engage in rapid evolutionary changes at high-biodiversity sites abundant in sugars and other small molecular substrates, and constitute together with Clostridia the two most powerful microbial generators of hydrogen. Moreover, two-domain maximum-likelihood and maximum-parsimony phylogenetic trees for VARS showed that Thermococci and Clostridia formed sister clades on both trees, and close similarity was evident between their VARS sequences, which were consistent with kinship between them. On this basis, it was proposed that the Bacteria domain emerged from a thermocococal or thermococcal-like Progenitor of Bacteria possibly at some high-biodiversity site, through the formation of a Thermincola-proximal Last Bacterial Common Ancestor (LBCA).