It is perfectly possible to run a sophisticated metabolism using the energy available from an hydrogen rich geochemical proton gradient of the type still found in locations such as Lost City in the Atlantic. This provides an (almost) endless supply of electrons via FeS catalysts of sufficiently negative potential to reduce carbon dioxide to carbon monoxide. From here it is all down hill, energetically speaking, to acetate and metabolism. The process is completely dependent on geochemical conditions for the free-ride. These basic steps are still embedded in the carbon monoxide dehydrogenase/acetyl-CoA synthase complex discussed rather nicely in this 2018 paper (how did I miss it?):
Evolutionary history of carbon monoxide dehydrogenase/acetyl-CoA synthase, one of the oldest enzymatic complexes
Back in the Life series of posts I argued in favour of Koonin's concept that Na+ provided the primary electrochemical gradient which was used to drive a Na+ transporting rotary ATP synthase. It's all in here:
Evolutionary primacy of sodium bioenergetics
My own idea is that primordial Na+ energetics derived from a geochemical proton gradient which was converted to a Na+ gradient by a H+/Na+ antiporter. In order to detach from the geochemical proton gradient what is needed is a Na+ pump to replace this geochemically driven antiporter. Acetobacterium woodii has the most highly conserved version of such a Na+ coupled system and back in 2009 the Rnf complex was the primary suspect for being the site of Na+ pumping.
The ins and outs of Na+ bioenergetics in Acetobacterium woodii
By 2010 this was pretty well confirmed:
Bacterial Na+-translocating ferredoxin:NAD+ oxidoreductase
In its most autotrophic guise A woodii can still run its metabolism on molecular hydrogen. The process of electron bifurcation allows the generation of reduced ferredoxin from H2 and so can provide electrons of a potential to use NAD+ as their electron acceptor, with sufficient energy left over to pump a single Na+ ion, in imitation of the H+/Na+ geochemical driven antiporter used soon after the origin of life. This Na+ pumping allowed prokaryotes to leave hydrothermal vents, provided there was access to molecular hydrogen as food.
This core process which freed early life from the ties to geochemical alkaline hydrothermal vents is clearly the oxidation of reduced-ferredoxin, ie this is an oxidative reaction driving an electrochemical Na+ gradient to phosphorylate ADP to ATP using a Na+ driven rotary ATP synthase.
It is oxidative phosphorylation.
Oxidative-phosphorylation is as primordial as the exodus of prokaryotes from hydrothermal vents.