||p.13 top paragraph:"The core pathway of carbon and energy metabolism in anaerobic autotrophs that inhabit
such hydrothermal and deep crust environments [refs 22–24] is the acetyl-coenzyme A (CoA) pathway, the most ancient of the six pathways of CO2 fixation known and the only one present in archaea and bacteria [refs 2,25]. Spontaneous exergonic organic syntheses from H2 and CO2 occur today at hydrothermal vents, for example formate synthesis [refs 18,26] and methane synthesis [refs 19,20,27], that
are similar, if not homologous [ref 28], to core energy-releasing reactions of carbon and energy metabolism in methanogens and acetogens, which live by the reduction of CO2 by H2. Microbial
communities have been thriving in hydrothermal vents for over 3.3 billion years [primary source]."