The rocky roots of the acetyl-CoA pathway.

Geologists have suggested that life might have emerged at hydrothermal vents, chemists have shown that metal sulphides such as FeS and NiS can catalyse biochemical reactions in the absence of proteins, and biologists have suggested that the acetyl-coenzyme-A (CoA) pathway of CO2 fixation might be very ancient. New findings from the enzymes at the heart of the acetyl-CoA pathway, carbon monoxide dehydrogenase (CODH) and acetyl-CoA synthase (ACS), indicate that metals and metal sulphides do the biochemical work of CO2 fixation. Here we propose that biochemistry got started when the two volatiles that were thermodynamically furthest from equilibrium on the early Earth – namely, marine CO2 from volcanoes and hydrothermal H2 – met at a hydrothermal vent rich in metal sulphides. In this ‘hydrothermal reactor’ hypothesis, a primitive, inorganically catalysed analogue of the exergonic acetyl-CoA pathway, using H2 as the initial electron donor and CO2 as the initial acceptor, was instrumental in the synthesis of organic precursors to fuel primordial biochemical reactions. We suggest that primordial biochemistry was housed in an acetate-producing hydrothermal reactor that retained reduced carbon compounds produced within its naturally forming inorganic confines.