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All of the anaerobic environments I can think of are that way because a layer of aerobic life above them separates them from oxygen. If the aerobic life were removed, the anaerobic compartment would equilibriate with the partial pressures of the atmosphere, however slowly, and it would remain at equilibrium until competition between aerobic cells once again pushed the oxygen boundary to the top.

After the great oxygenation event, there was oxygen in the atmosphere but no layers of aerobic life to stop it from diffusing to the bottom of the ocean or deep underground. If there are anaerobic environments that exist independently of aerobic life, our anaerobic ancestors could have survived there until aerobic respiration or tolerance evolved. Otherwise aerobic tolerance would have had to have been present before the great oxygenation event for anything to survive at all.

Which is closer to the truth? (An environment that takes extremely long to equilibriate with the atmosphere, long enough for evolution to be fast in comparison, would count as isolated for the purposes of this question since it would serve as a reservoir of life from which aerobic respiration could emerge.)

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  • $\begingroup$ well since most of the oxygen is made by aerobic life, anywhere they want. $\endgroup$
    – John
    Commented Mar 23, 2022 at 21:17
  • $\begingroup$ A lot of chemicals would have buffered the oxidation event. Especially Fe2+ in water oxidising to Fe3+. $\endgroup$
    – Polypipe Wrangler
    Commented Mar 24, 2022 at 0:10

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In the question body it says:

After the great oxygenation event, there was oxygen in the atmosphere but no layers of aerobic life to stop it from diffusing to the bottom of the ocean or deep underground.

This statement isn't justified, given that the Great Oxidation "Event" developed over one to two billion years. It took half a billion years for the atmospheric oxygen concentration to go from nil to around 3%. This is a large amount of time for evolution to select organisms that can tolerate the presence of free oxygen at various levels. There would be a wide range of environments with differing levels of oxygen, depending on many factors including distance from the oxygen emitters (cyanobacteria) and the nature of the intervening materials. The atmospheric oxygen concentration remained at fairly stable levels of 2% to 4% for nearly a billion more years, giving more time for evolution of oxygen-tolerant and -consuming organisms to evolve. Thus, to the extent that anaerobic organisms depend on aerobic organisms to shield them from oxygen diffusion, there would have apparently been plenty of evolutionary time for that to occur.

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Deep sea hydrothermal vents release reducing chemicals like sulfide ($S_2^-$) and ferrous iron ($Fe^{2+}$). These chemicals establish anaerobic conditions abiotically[1].

[1] "M. jannaschii uses sulfide (S2-) for growth and energy, which is good because sulfide is present in high levels in the vent fluid. It reacts with oxygen to establish anaerobic growth conditions for the archaea."

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