Thank you so much for taking the time, consideration and attention to detail to write the corrections and a provide fuller and more scientific story. It's a well delineated three pillar account of the requirements for biochemistry.

Oh yes! That paragraph is genuinely misleading, I should patch it immediately! I leant so hard in trying to set up a purple-green dichotomy and conveyed the chemical reactivity wrong.

I'm open to any further corrections. I have performed massive elisions --especially where my gathered knowledge was lacking-- to drive forward a horror narrative. Particularly the anthropomorphisation going from free and mineral Mn in some sort of direct way to a productive enzyme.

The further oxidation of Mn ions is an interesting and more compelling story, and probably should be expanded upon.

Those details about the locales where new metabolic pathways arose is fascinating, and might situate the evolution biology in-place far more than this 'ancient vs new life' single-issue portrayal.

I have also skipped over many details, because a reasonably complete history of what we already know would be book-length.

There are still a lot of details that are not known yet, and for many other details there are 2 or more possible alternatives, even when one of them seems much more plausible than the others.

Many details have been clarified during the last decade, so the older books have become partially obsolete.

I am not aware of any book that provides the full story of what is now known, so the state of the current knowledge is dispersed through a great number of published research articles. Frequently, not even the authors of those articles are aware of all the other published results, so some articles present advances in the knowledge in certain directions, while also repeating obsolete hypotheses about other things.

One of the most puzzling things is that the ancestors of the blue-green algae had in their cells the equivalent of 2 solar cells, which had evolved separately from the primordial form of a solar cell that was used by the first phototrophic bacteria. These 2 "solar cells" are named in the literature of this domain as type-I photosystem and type-II photosystem, which are connected in series from the point of view of the electron flow, in all oxygen-producing organisms.

The current evidence is consistent with the ancestor of all still-existing bacteria already being a phototrophic bacterium, but the capacity of producing free oxygen has appeared much later, only in the lineage that has led to the blue-green algae and their close relatives.

A possible scenario for the blue-green algae having 2 photosystems is if the original photosystem has been duplicated in one of their ancestors and then the 2 photosystems have evolved separately. However, this does not make much sense because there is no known reason why there would have been an advantage for the 2 photosystems to evolve in different directions. Only the final stage when they can be connected in series is useful, but the intermediate stages seem more harmful than useful.

The much more plausible alternative is that the 2 photosystems have evolved in different directions inside different bacteria, which lived in different environments. Then perhaps hundreds of millions or even billions years later, a hybridization event has reunited the 2 kinds of photosystems into the ancestor of the blue-green algae. This would be similar with the many other hybridization events that caused the most important events in the evolution of the living beings, which have created the eukaryotic living beings and the ancestors of the land plants and of various kinds of algae.

We already know that the ancestors of all still-existing bacteria have split billions of years ago into 2 distinct groups, which have evolved in different environments. One group has evolved in the oceans, while the other group has evolved first on the borders of the continents, later invading the fresh water and then the moist terrestrial habitats.

It seems that what is now called "type-I photosystem" is the photosystem that has evolved in the marine group of bacteria, while what is called now "type-II photosystem" has evolved in the continental group of bacteria ("Terrabacteria").

The most distant ancestors of the blue-green algae had a type-II photosystem, then after a hybridization event it acquired both photosystems.

The photosystem that oxidizes manganese so much that it splits water freeing oxygen from it is the type-II photosystem. The type-I photosystem saves energy in comparison with a bacterium that would have only a type-II photosystem, by producing in a more efficient way the reducing agents that are used for reducing the carbon from CO2.

While what I have written above is the most plausible supposition that is compatible with the current evidence, there is a lot of uncertainty about details, because the history is muddled by the fact that most still-existing bacteria have lost the photosystems of their ancestors, because once the more performant blue-green algae were producing tons of organic substances, it was futile to compete with them and a better strategy was to stop being autotrophic and switch to eating what the blue-green algae were producing.

Moreover, in several cases some bacteria that had lost far in the past their own photosystem have acquired again a photosystem from some unrelated bacterium, because the transfers of genes between unrelated bacteria are very frequent. So now it is quite difficult when examining a bacterium to distinguish which of its features come through inheritance from a distant ancestor and which might have been acquired recently from completely unrelated bacteria.

Of the many existing groups of phototrophic bacteria, the most likely hypothesis is that only 3 of them still have photosystems that have been transmitted through vertical inheritance (the blue-green algae, the green sulfur bacteria and the so-called Chloroflexota together with a few other related bacteria), while the many other groups of phototrophic bacteria have photosystems that have been altered by lateral gene transfers.

Your hypothesis that the unusual ground electronic state of the dioxygen molecule, which makes free oxygen paramagnetic instead of being diamagnetic like most substances where you expect that their electrons are paired in the ground state, is responsible for the high activation energy of the oxidation reactions between free oxygen and organic substances (and also for the oxidation reactions that affect other substances from our environment), seems original. I do not remember reading it elsewhere.

I do not know if this is actually true, but it seems quite plausible. There are many such exceptions in molecular or nuclear structures from the apparently general rules, like also in the properties of water, which are critical for determining our world to be how it is.