Some ions or compounds are thought not to have become involved or important in the metabolism of living organisms until some time after certain mutations took place. For instance, early life is thought to selectively allow calcium ions through its membrane, but eventually also evolved the ability to selectively allow sodium ions, specifically through a mutation that lead to a change in the composition of a channel protein from glutamine to lysine.

Currently, iron is involved in oxidations involving molecular oxygen, such as in cytochromes and clearly holds a key role in modern life, despite that free iron or even ferric compounds are rarely accessible. From my understanding, iron most likely became incorporated into the metabolism of microbes after during/after aerobic organisms had developed, but this does not rule out the possibility that iron was involved earlier on. So, I am wondering if iron was involved in early life, and details on how would be appreciated.

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    $\begingroup$ "a mutation in the structure of amino acids to form lysine"? How exactly does a chemical compound mutate? I would stick to your question and avoid unscientific analogies and unprovable remarks about when different amino acids emerged. $\endgroup$
    – David
    Mar 25, 2018 at 23:41
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    $\begingroup$ @David since most biological compounds used in life are produced through a genetically control process, saying "X mutated" is a common acceptable shorthand for saying "the genes that controls the production of X mutated changing the structure of X". $\endgroup$
    – John
    Mar 26, 2018 at 3:57
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    $\begingroup$ @John I dont know in what circles is that acceptable, maybe creationists, but if my student wrote something like this on an exam, I would fail him on the spot - and that would be mercy in comparison what my master or phd advisors would do to him. Structure of a chemical compound cannot mutate, period. If you add or remove an atom from the structure of a chemical compound, it becomes a different compound. Also, the title is misleading. Asking IF iron was important for the first life on earth is not the same thing as asking WHEN did the iron become important. $\endgroup$
    – Maciej
    Mar 26, 2018 at 8:31
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    $\begingroup$ @david are you not aware cells will modify amino acids, this is how the majority of amino acids are produced after all. Post-translational modification of animo acids is fairly common. $\endgroup$
    – John
    Mar 26, 2018 at 13:41
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    $\begingroup$ I think it's inappropriate for you to make such a large change like that. Life relied on calcium, then selectively included sodium. I wanted to know if the same kind of thing happened with iron and how in order to track the timeline of the incorporation of different elements for microbial evolution, and my original question got me an answer. ncbi.nlm.nih.gov/pubmed/24517213 $\endgroup$
    – John Joe
    Mar 26, 2018 at 21:36

2 Answers 2


Cyanobacteria require iron for photosynthesis and can be found as fossil stromatolites dating back to 3.5 billion years ago. Stromatolites are layered structures made up of cyanobacteria and sediment.

enter image description here

Source: https://en.wikipedia.org/wiki/Stromatolite

Modern stromatolites can be found at Shark Bay in Australia, Chetumal Bay in Belize, and Laguna Bacalar in the Yucatan Peninsula.

Cyanobacteria are also believed to have evolved into the first microbes to produce oxygen by photosynthesis, which was a catalyst for the Great Oxygenation Event which occurred around 2.45 billion years ago.

  • $\begingroup$ So you're saying life nearly started out as cyanobacteria? This would have to be quite a recent development because I've seen multiple sources in the past say cyanobacteria didn't form until around 2.7 BYA. $\endgroup$
    – John Joe
    Mar 25, 2018 at 21:51
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    $\begingroup$ @John Joe: Sorry, I thought I had provided this source also. ucmp.berkeley.edu/bacteria/cyanofr.html $\endgroup$ Mar 25, 2018 at 22:33
  • $\begingroup$ So if that's true, why was it only much later that the atmosphere became oxygenated? $\endgroup$
    – John Joe
    Mar 26, 2018 at 2:17
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    $\begingroup$ While I largely agree with this answer, I'd point out that as far as I know we have of knowing how similar the metabolism of these ancient organisms was to extant organisms. There's evidence of morphological similarity but morphological similarity does not necessarily imply biochemical similarity. $\endgroup$ Mar 26, 2018 at 6:14
  • $\begingroup$ So if life already had the capacity to harness iron, why didn't life forms release that oxygen earlier? $\endgroup$
    – John Joe
    Mar 26, 2018 at 6:59


It has been proposed that iron was indeed important for the first life on earth, specifically in combination with sulphur. This is because conditions are thought to have existed in volcanic hydrothermal vents that would have allowed formation of iron–sulphur complexes which perform oxido-reduction reactions in modern proteins (as well as more sophisticated functions). Some system for oxido-reduction of chemical compounds is required for life to develop, and the inorganic simplicity of Fe–S clusters, together with their ability to interact with the backbone of proteins, argues for their early involvement, rather than the chemically more elaborate NAD and FAD.

The Archaean Environment

One theory suggests that life originated volcanic hydrothermal conditions in which the oxidation of iron minerals by hydrogen sulphide would have been possible. It has even been suggested that life originated on the surface of iron sulphides (the ‘Iron–sulphur world hypothesis’), although this answer is more modest in its scope.

Iron–Sulphur Metalloclusters

Modern proteins contain a range of iron-sulphur clusters of varying complexity, but it has been argued in a review in Science by Rees and Howard that the simplest of these — the 2Fe:2S cluster in, e.g. ferredoxin — could have evolved in the conditions mentioned above.

FeS clusters

[Different FeS clusters. Note that the external yellow sulphur atoms are from cysteine side-chains of the protein. (Adapted from a review by Johnson et al. (2005))]

Interaction of Iron–Sulphur Clusters with Proteins

In some iron-containing proteins, like cytochrome c, the iron is held in place by a sophisticated organic molecule (haem) and interacts with amino acid side-chains. However in simple iron–sulphur clusters interaction with the NH groups of the protein backbone (in special conformations — ‘nests’ and ‘crowns’) can occur. Thus initially they could have been acquired by simpler proteins, which some believe to have preceded the development of those with the side-chains to provide more sophisticated interaction.

FeS interaction with the protein backbone

[Interaction of FeS clusters with protein backbone NH groups (blue): A. ‘nest’, B. ‘crown’ with the N atoms in space-filling mode.]

Other Contemporary Redox Cofactors

The intellectual appeal of Fe–S cofactors as being evolutionarily most ancient can be seen if one compares them with the structures of other contemporary redox cofactors (images taken from Berg et al.):


It has been argued that the adenine and ribose moieties of these cofactors attest to an ancient origin — in the supposed RNA world — an idea that I, personally, find appealing. Nevertheless, their emergence would require a mechanism for the synthesis of their functional organic components — nicotinamide and flavin, respectively. In contrast, Fe–S clusters come ‘ready made’.

More Complex Iron–Sulphur Clusters

Iron sulphur clusters are also found in the nitrogenase protein, which converts nitrogen gas to ammonia. This must be a very ancient protein, but its greater complexity (the cluster includes molybdenum) suggests it may have arisen later. Another elaboration is proteins, such as aconitase, that have Fe–S clusters linked by more complex organic groups.

More complex Fe proteins

[Iron-containing proteins of greater complexity. Haem, the non-sulphur iron prosthetic group of cytochromes etc. is also shown. Adapted from Berg et al.]

Iron in Photosynthesis

As mentioned by @Kurt, iron is a component of photosystem II in contemporary cyanobacteria. However photosystem II is extremely complex and the precise chemical basis of the non-haem iron requirement is currently unclear. Although perhaps necessary if cyanobacterial photosynthesis were responsible for the great oxygenation, this complexity would suggest a relatively late emergence in the pre-aerobic era.

  • $\begingroup$ I have expanded my answer, which is now essentially complete, unless anyone requires details of the metabolic pathways aluded to, which I will be happy to provide. $\endgroup$
    – David
    Mar 26, 2018 at 20:07
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    $\begingroup$ I think if this was hybridized with the first answer it would have made both better, because the first answer showed that the same premise of cellular respiration took place much earlier than thought with direct physical evidence which explains iron and its role but without much theory, whereas you mainly rely on a conjecture of a specific compound and not direct evidence so much. They "can" interact with background proteins, but did that actually make them necessary? Was there actually a mutation back then that lead microbes to incorporate a modern organelle or metabolite from that compound? $\endgroup$
    – John Joe
    Mar 26, 2018 at 21:54
  • $\begingroup$ Any statement about metabolism in early evolution depends on deduction rather than physical evidence. The argument of @Kurt could be restated as: contemporary cyanobacteria require iron (for PSII); they are thought to have caused the great oxygenation; hence iron must have been used in metabolism before aerobic life. The argument assumes that photosynthesis in ancient cyanobacteria was the same as today. His photo is only evidence for the existence of cyanobacteria even earlier, but if they had the current PSII they would still be far more elaborate than anything in “the first life on earth”. $\endgroup$
    – David
    Mar 27, 2018 at 9:55
  • $\begingroup$ Like I said if there was a way to combine both answers I would be open to it, but it's not mere speculation if we can actually see the imprints of the structure of microorganisms. It does assume the same photosynthesis, but it assumes that because there is consistent material evidence for it, whereas your argument is "well, this compound could have been involved because it could have interacted..." and that's why it's called a "hypothesis." $\endgroup$
    – John Joe
    Mar 27, 2018 at 19:56

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