&gt; And, IIRC, we have strong reason to believe that life that could live without O2 predates life that needs O2, and, in fact, it is life that made free O2 available, not free O2 that made life possible.<p>Yes, early life on Earth was all anaerobic. That started around 4.1 Gya (billion years ago)[1]. The Great Oxygenation Event[2] happened around 2.3 Gya - that wasn&#x27;t the first appearance of photosynthesis, but it was the point at which it changed the atmosphere enough to wipe out most of the other life on Earth at the time. Obligate aerobes - organisms that <i>need</i> oxygen to survive - were later still.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Timeline_of_the_evolutionary_history_of_life" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Timeline_of_the_evolutionary_h...</a><p>[2] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Great_Oxygenation_Event" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Great_Oxygenation_Event</a>