The article is nice, but in an attempt to summarize a long and complex history in a couple of sentences, it got some details wrong.
Especially this paragraph is very incorrect:
"The ancients were hydrogen sulfide metabolisers. H2S could be readily chowed down as an energy source — it fell apart within the cell at ordinary temperature, its hydrogens surrendered as easy fuel. But oxygen was harder to crack: it was locked in a molecular safe, water’s O–H bound 100 kJ/mol more strongly than S–H down a period. H2O required photon-driven electrical impulses and quantum trickery to unpick its bonds."
As it is written, this paragraph implies that H2S could be used without solar energy, while H2O requires solar energy. This is false, because most ways in which H2S can be used to reduce carbon dioxide into organic matter result in a negative energy balance, the same as when using water, so they require an additional source of energy, which is provided by capturing solar energy.
The true history has much more steps until the appearance of the blue-green algae that can produce free oxygen from water.
All autotrophic living beings require 3 resources in order to be able to make organic substances from carbon dioxide: energy, hydrogen and something that can be oxidized while the carbon from carbon dioxide is reduced. Being oxidized means losing electrons, which are transferred to that which is reduced, so "something that can be oxidized" is also known as an "electron donor".
For the first living beings that have ever existed, free hydrogen provided all 3 resources required for an autotrophic life: it provides hydrogen, it is an electron donor and the reaction of hydrogen with CO2 that results in acetic acid provides energy. Much later, some of the living beings, the so-called methanogens, have developed an alternative chemical reaction between H2 and CO2, which makes methane instead of acetic acid and which provides more energy than the reaction that makes acetic acid. Most humans have in their guts both acetogens and methanogens, which continue to have an autotrophic life style that does not require solar energy, in the same way as billions of years ago.
However, the requirement for free hydrogen caused a severe constraint on the places where the first living beings could live, as there were not many such places. For the colonization of the entire Earth surface it has been necessary to become able to also exploit other resources instead of free hydrogen.
Most other sources of hydrogen and of electron donors do not provide energy when reacting with CO2, so in order to use them an additional source of energy is needed.
That additional source of energy has been solar energy, which has been initially captured by using ion pumps that are powered by solar energy, which store the captured energy in ion gradients. These work pretty much similarly with solar cells, but instead of transporting electrons using the solar energy they transport either hydrogen ions or sodium ions.
Having a separate source of energy has simplified the problem of finding resources for reducing CO2 to satisfying only 2 requirements: providing hydrogen and providing an electron donor.
These 2 requirements are satisfied by H2S, so after becoming capable of using H2S, the phototrophic bacteria have become the most abundant living beings. Even today, after the oxygen-producing living beings the most abundant autotrophic living beings are the green sulfur bacteria and the purple sulfur bacteria, which use H2S instead of water.
So the autotrophic bacteria that could use H2S already needed solar light. The path from splitting H2S to splitting water was not direct, there have been other intermediate steps. When using H2S, it provides hydrogen and sulfur is the electron donor. Then some phototrophic bacteria have developed means to use other electron donors instead of sulfur.
The Fe(II) and Mn(II) iron and manganese ions are soluble in water, while the more oxidized ions Fe(III) and Mn(IV) are insoluble in water. Today, because of the oxygen in air, most iron and manganese are more oxidized, so the sea water has very little iron and manganese. Before the existence of free oxygen, most iron and manganese were less oxidized, so the sea water was rich in iron and manganese.
Because they were abundant in water, some phototrophic bacteria have developed means to use either Fe(II) or Mn(II) as the electron donors, instead of using the sulfur from H2S. In this case the source of hydrogen for making organic substances was the water, but this did not result in free oxygen, because the oxygen from water became bound to the more oxidized iron or manganese ions, instead of becoming free oxygen.
Iron is very difficult to oxidize more than at the Fe(III) level from rust, but manganese can be oxidized at levels higher than Mn(IV) (the level from black pyrolusite), even to levels as high as the Mn(VII) from permanganate, without great difficulties.
Thus some of the phototrophic bacteria that used manganese as the electron donor became capable to oxidize manganese to levels higher than Mn(IV) and these very oxidized manganese ions were able in turn to oxidize the oxygen from water into free oxygen, which was released into the environment.
These are the steps through which the autotrophic bacteria have evolved from the original use of free hydrogen to the use of solar energy coupled with the use of hydrogen from water and with the use of oxygen from water as an electron donor, which results in the release of free oxygen into the atmosphere.
An interesting fact is that the development of the ability of using water for providing both hydrogen and an electron donor, which enabled the living beings to colonize the entire surface of the Earth, regardless of the availability of sulfur, iron or manganese, has not happened in the oceans, presumably because in the oceans there was no need for this, as in the oceans there were sufficient amounts of sulfur, iron or manganese.
Instead of in the oceans, the ability to use water and release free oxygen appeared somewhere on the borders of the continents, and once it was available the ancestors of the blue-green algae have colonized the fresh water on the continents, an environment that was previously lifeless. Only much later the blue-green algae have colonized again the oceans. Actually it is likely that the oceanic blue-green algae have appeared much more recently than the invasion of the oceans by red algae, followed by the invasion of the oceans by green algae, which both happened more than a billion years ago.
The common ancestor of red algae and of green algae has also appeared not in the oceans, but it has appeared somewhere on the borders of continents, through the symbiosis between a blue-green alga and an eukaryotic organism. Later the descendants of this hybrid have split into the ancestors of red algae and of green algae, which have then colonized the oceans separately.
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.
Some people write/speak in sentences, others in paragraphs, some fewer number can spin off a cohearant chapter, and the very occasional creature will discource at book length.
Which is my way of saying, thanks!
The article is nice, but in an attempt to summarize a long and complex history in a couple of sentences, it got some details wrong.
Especially this paragraph is very incorrect:
"The ancients were hydrogen sulfide metabolisers. H2S could be readily chowed down as an energy source — it fell apart within the cell at ordinary temperature, its hydrogens surrendered as easy fuel. But oxygen was harder to crack: it was locked in a molecular safe, water’s O–H bound 100 kJ/mol more strongly than S–H down a period. H2O required photon-driven electrical impulses and quantum trickery to unpick its bonds."
As it is written, this paragraph implies that H2S could be used without solar energy, while H2O requires solar energy. This is false, because most ways in which H2S can be used to reduce carbon dioxide into organic matter result in a negative energy balance, the same as when using water, so they require an additional source of energy, which is provided by capturing solar energy.
The true history has much more steps until the appearance of the blue-green algae that can produce free oxygen from water.
All autotrophic living beings require 3 resources in order to be able to make organic substances from carbon dioxide: energy, hydrogen and something that can be oxidized while the carbon from carbon dioxide is reduced. Being oxidized means losing electrons, which are transferred to that which is reduced, so "something that can be oxidized" is also known as an "electron donor".
For the first living beings that have ever existed, free hydrogen provided all 3 resources required for an autotrophic life: it provides hydrogen, it is an electron donor and the reaction of hydrogen with CO2 that results in acetic acid provides energy. Much later, some of the living beings, the so-called methanogens, have developed an alternative chemical reaction between H2 and CO2, which makes methane instead of acetic acid and which provides more energy than the reaction that makes acetic acid. Most humans have in their guts both acetogens and methanogens, which continue to have an autotrophic life style that does not require solar energy, in the same way as billions of years ago.
However, the requirement for free hydrogen caused a severe constraint on the places where the first living beings could live, as there were not many such places. For the colonization of the entire Earth surface it has been necessary to become able to also exploit other resources instead of free hydrogen.
Most other sources of hydrogen and of electron donors do not provide energy when reacting with CO2, so in order to use them an additional source of energy is needed.
That additional source of energy has been solar energy, which has been initially captured by using ion pumps that are powered by solar energy, which store the captured energy in ion gradients. These work pretty much similarly with solar cells, but instead of transporting electrons using the solar energy they transport either hydrogen ions or sodium ions.
Having a separate source of energy has simplified the problem of finding resources for reducing CO2 to satisfying only 2 requirements: providing hydrogen and providing an electron donor.
These 2 requirements are satisfied by H2S, so after becoming capable of using H2S, the phototrophic bacteria have become the most abundant living beings. Even today, after the oxygen-producing living beings the most abundant autotrophic living beings are the green sulfur bacteria and the purple sulfur bacteria, which use H2S instead of water.
So the autotrophic bacteria that could use H2S already needed solar light. The path from splitting H2S to splitting water was not direct, there have been other intermediate steps. When using H2S, it provides hydrogen and sulfur is the electron donor. Then some phototrophic bacteria have developed means to use other electron donors instead of sulfur.
The Fe(II) and Mn(II) iron and manganese ions are soluble in water, while the more oxidized ions Fe(III) and Mn(IV) are insoluble in water. Today, because of the oxygen in air, most iron and manganese are more oxidized, so the sea water has very little iron and manganese. Before the existence of free oxygen, most iron and manganese were less oxidized, so the sea water was rich in iron and manganese.
Because they were abundant in water, some phototrophic bacteria have developed means to use either Fe(II) or Mn(II) as the electron donors, instead of using the sulfur from H2S. In this case the source of hydrogen for making organic substances was the water, but this did not result in free oxygen, because the oxygen from water became bound to the more oxidized iron or manganese ions, instead of becoming free oxygen.
Iron is very difficult to oxidize more than at the Fe(III) level from rust, but manganese can be oxidized at levels higher than Mn(IV) (the level from black pyrolusite), even to levels as high as the Mn(VII) from permanganate, without great difficulties.
Thus some of the phototrophic bacteria that used manganese as the electron donor became capable to oxidize manganese to levels higher than Mn(IV) and these very oxidized manganese ions were able in turn to oxidize the oxygen from water into free oxygen, which was released into the environment.
These are the steps through which the autotrophic bacteria have evolved from the original use of free hydrogen to the use of solar energy coupled with the use of hydrogen from water and with the use of oxygen from water as an electron donor, which results in the release of free oxygen into the atmosphere.
An interesting fact is that the development of the ability of using water for providing both hydrogen and an electron donor, which enabled the living beings to colonize the entire surface of the Earth, regardless of the availability of sulfur, iron or manganese, has not happened in the oceans, presumably because in the oceans there was no need for this, as in the oceans there were sufficient amounts of sulfur, iron or manganese.
Instead of in the oceans, the ability to use water and release free oxygen appeared somewhere on the borders of the continents, and once it was available the ancestors of the blue-green algae have colonized the fresh water on the continents, an environment that was previously lifeless. Only much later the blue-green algae have colonized again the oceans. Actually it is likely that the oceanic blue-green algae have appeared much more recently than the invasion of the oceans by red algae, followed by the invasion of the oceans by green algae, which both happened more than a billion years ago.
The common ancestor of red algae and of green algae has also appeared not in the oceans, but it has appeared somewhere on the borders of continents, through the symbiosis between a blue-green alga and an eukaryotic organism. Later the descendants of this hybrid have split into the ancestors of red algae and of green algae, which have then colonized the oceans separately.
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.
Some people write/speak in sentences, others in paragraphs, some fewer number can spin off a cohearant chapter, and the very occasional creature will discource at book length. Which is my way of saying, thanks!
Really enjoyed your post, Keiran
Thank you! I got the idea after thinking on the vast quantities of triplet O2 in the atmosphere and just kept following - HOW? Where did it come from?
Then when I learnt the biogeological consequences I wanted to try writing it up as a planetary horror story