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By about 3.5 billion years ago, Earth may already have hosted microbes using several different ways to obtain energy and process carbon. A report on Australian fossilized microbial material interpreted stable carbon isotope patterns as evidence for methane-producing microbes, methane-consuming microbes, and a proposed primitive form of photosynthesis that used hydrogen sulfide. These are metabolic interpretations—not identifications of modern species or a complete census of ancient life.
What kind of life may have existed 3.5 billion years ago?
The reported isotope evidence points to metabolic diversity: some microbes may have produced methane, others may have consumed it, and another proposed group may have used hydrogen sulfide in photosynthesis. The report, covered by Chemistry World, presents these as interpretations of stable carbon isotope measurements in Australian fossilized microbial material.
This does not mean researchers identified particular species, counted all the organisms in an ecosystem, or established precisely how common each metabolism was. The evidence supports possible kinds of microbial activity, not a full inventory of life at the time.
How can isotope measurements reveal what microbes did?
Carbon occurs in different stable isotopic forms. Biological processes can favor one form over another, leaving a characteristic balance in organic material. Researchers measure that balance and compare it with patterns expected from different processes. The result is indirect: an isotope pattern can be consistent with a metabolism and support an interpretation, but it does not directly identify the organism that produced the material.
That distinction matters especially in rocks billions of years old. Researchers must interpret chemical signals preserved in geological material, rather than observe the organisms or their activity directly. The reported carbon-isotope analysis was interpreted as supporting several metabolic pathways, but the accessible report does not establish a sample count, a headline isotope value, or detailed sample-by-sample results.
What the Australian fossil evidence does—and does not—show
Dresser Formation: supporting carbon-isotope context
The approximately 3.5-billion-year-old Dresser Formation in Western Australia contains stromatolites, layered structures associated with microbial communities. In a separate 2019 study, Baumgartner and colleagues analyzed exceptionally preserved Dresser Formation material taken from drill core below the weathered surface. They reported organic matter within nanoporous pyrite, nitrogen-bearing organic matter, and filament-like material interpreted as possible mineralized remains of biofilms.
For extracted organic matter, that study measured δ13C = −29.6‰ ± 0.3‰ VPDB. The authors interpreted the combined evidence as supporting a biological origin for some of the stromatolites. This is a result from the separate Dresser Formation study, not a reported value from the Chemistry World story about metabolic diversity; the available evidence does not establish that the two accounts concern the same samples or analysis. UCL repository record for Baumgartner et al. (2019).
Strelley Pool Formation: a separate sulfur-isotope study
A distinct study examined 3.45-billion-year-old stromatolites from the Strelley Pool Formation, also in Western Australia. It analyzed sulfur isotopes in preserved organic matter to investigate microbial metabolism. Sulfur-isotope work can complement questions about ancient microbial activity, but it is not the carbon-isotope result described in the Chemistry World report. 2012 study of sulfur isotopes in Strelley Pool stromatolites.
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Why the proposed photosynthesis is not proof of oxygen production
The reported interpretation includes a primitive form of photosynthesis using hydrogen sulfide. Photosynthesis does not necessarily produce oxygen: some microbes use other substances in the chemistry that captures light energy. A review of Archaean fossil evidence describes carbon-isotope patterns as consistent with photosynthesis by 3.5 billion years ago, while cautioning that oxygen-producing photosynthesis is not conclusively established by that evidence. Review of Archaean fossil evidence.
So the proposed hydrogen-sulfide-using metabolism should not be conflated with oxygen-producing photosynthesis. The isotope evidence supports a cautious case for early photosynthetic activity, but it does not, by itself, prove that these microbes released oxygen.
Quick Recap
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What can be concluded from the evidence?
- Carbon isotope patterns in very ancient Australian microbial material have been interpreted as evidence for more than one microbial metabolism, including methane production, methane consumption, and proposed hydrogen-sulfide-using photosynthesis.
- Those patterns are indirect evidence: they support metabolic interpretations but do not reveal exact species or provide a complete ecosystem census.
- Separate Dresser Formation and Strelley Pool Formation studies add geological and isotope context, but their measurements and samples must not be merged with the report about metabolic diversity.
- The evidence does not conclusively establish oxygen-producing photosynthesis at that time.
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