Thermococci-to-Clostridia Pathway for the Evolution of the Bacteria Domain

With the identification of an archaeal Last Universal Common Ancestor phylogenetically related to the archaeon Methanopyrus , the origin of Bacteria becomes a choice between independent emergence versus descent from Archaea. The similarity bitscores between paralogous valyl-tRNA synthetase (VARS) and isoleucyl-tRNA synthetase (IARS) indicated that an Ancestral Bacteria Cluster centred at clostridial Mahella australiensis (Mau) and Thermincola potens (Tpo) were the oldest bacteria. Overall, the high-bitscore bacteria dominated by Clostridia included a number of hydrogen producers. A search for archaea capable of hydrogen production that might be ancestral to the Bacteria domain yielded candidate Archaeal Progenitors led by Thermococci which, like Clostridia , form hydrogen through dark fermentation. A two-domain VARS tree based on Mahella , Thermincola , a broad spectrum of archaea together with a range of well known as well as newly detected species of Thermococci and Euryarchaeota allocated the two Clostridia to a minor-Thermococcal division on the tree containing Thermococi and Euryarchaeota species isolated from high-biodiversity environments. The kinship between Thermoccoci and Clostridia suggested by this allocation was substantiated by highly conserved oligopeptide segments on their VARS sequences, leading to the proposal that a Thermococci -to- Clostridia evolutionary pathway mediated the emergence of the Bacteria domain under conditions of elevated biodiversity.