The archaea also use a unique form of electron transport in methanogenesis (Schafer 2004). The viral SF3 superfamily (Leitão 2015) helicase tree shows variants active on both RNA and SNA substrates, consistent with an origin in the RNA era (Caprari et al. Supporting the notion of subunits, a beta-chain of ATP synthase is homologous to a hepatic lipoprotein receptor (Martinez et al. Fig 1b2: Left: Rotary action of ATPsynthase, shown centre. Right: Evolutionary tree of viral RNAhelicase includes forms active in both RNA and single and double-stranded DNA viruses (Caprari et al. Respiratory electron transport occurs in both aerobic and anaerobic organisms and the terminal oxidases, iron-sulphur proteins and flavin-binding polypeptides all show evolutionary trees reaching back to the common ancestor of the three domains, implying terminal oxidases predate oxygenic photosynthesis.
The diversity of these, consistent with the existent tree of life, imply life must have originated 500 million years earlier or around 4 billion years ago. (2016) discovered putative stromatolites (f) colonies of photosdating to 3.7 billion years in the Isua formation, Greenland. (2011) have found clusters of putative sulphur-metabolizing cells (g) in 3.4-billion-year-old rocks of Western Australia. (2017) found carbon tube structures from fossil remains of ancient hydrothermal vents dated to 3.7-4.2 billion years.This would have then coupled to the larger subunit which could have assembled transfer RNAs coupled to amino acids via ribozymes, resulting in a simple genetic code, for example based on polar and non-polar amino acids.It is also possible to investigate aspects of the genetic landscape before LUCA.Fig 1c2: Left: Evolutionary tree of selenophosphate synthetase (Romero et al. Centre: SECIS hairpins of archaea (A), bacteria (B) and corresponding eukaryote variants (C, D) (Moldave ed 2006).Top right: Tertiary structure of SECIS showing highly conserved regions (hot) (Walczak et al. Lower right: SECIS acts as an RNA-enzyme to attach the selenocysteine t-RNA to the nascent protein (click to enlarge).In particular, in the evolution of the aminoacyl-t RNA synthetases (aa RS), coupling an amino acid to its respective t-RNA, analysis of genetic trees shows that there have been multiple horizontal transfer of such genes, including some from putative sister species of LUCA, in a similar manner to the introgression of Neanderthal DNA into One intriguing indication of the state of genetic translation in LUCA is the incorporation of selenocysteine into the genetic code.