It is unclear whether stromatolite rock structures like this one in Western Australia are always produced by living organisms Auscape/Universal Images Group via Getty Images

We have finally uncovered hard evidence that 3.5 billion-year-old rocks in Australia really do contain fossils of the oldest known microorganisms. The findings put to bed a debate that has raged for years and may even enlighten us as to how some of Earth’s earliest life forms functioned.

Raphael Baumgartner at the University of New South Wales in Australia and his colleagues looked at rocks in the Pilbara region of Western Australia. This area contains some of the oldest preserved rocks on Earth. Of the three most important sites, the Dresser Formation is the oldest, with rocks that are 3.48 billion years old.

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The Dresser Formation appears to contain layered structures called stromatolites. These are known to form when microbes grow into thin layers, which then become covered in sediment, only for another layer of microbes to form on top, and so forth.


However, many researchers aren’t convinced that these rock structures really are stromatolites, arguing that they could have formed without life being present.

Baumgartner and his colleagues drilled into the rocks to get the best-preserved samples. They found many layers that looked like stromatolites. These contained “exceptionally preserved organic matter”, says Baumgartner, including strands of the sort seen when microbes form slimy layers called biofilms. Multiple chemical analyses indicate that the organic matter came from living organisms.

“We have found smoking gun evidence for some of the earliest life on Earth,” says Baumgartner. “There are no convincing organic matter or microbial remains older than ours.”

There are plenty of claims of older fossils, or of chemical traces of life, some dating to over 4 billion years ago. But none have found widespread acceptance.

The organic matter that Baumgartner and his colleagues found was mostly trapped inside a mineral called pyrite or fool’s gold, which is based on iron and sulphur.

Fool’s gold

“The pyrite is extraordinary,” says Baumgartner. Because the microbes are so well-preserved, it must have formed quickly – perhaps even while they were alive. If that is true, it could reveal their survival strategy. Some modern microbes live off sulphur and produce pyrite as a waste product. The Dresser Formation microbes may have done the same, says Baumgartner.

Pyrite may even have played a role in the origin of life, if the first life used sulphur as an energy source. This is the basis of the iron-sulphur World hypothesis, which was proposed by Günter Wächtershäuser in the late 1980s.

Baumgartner says it isn’t clear what sort of environment his stromatolites were originally formed in. In 2017, his colleague Tara Djokic and her team showed that parts of the Dresser Formation preserve hot springs from on land, but other regions seem to have been shallow seas. Baumgartner suspects the Dresser Formation preserves a coastal region. “From this, we can speculate that the origin of life might have been near,” he says.

“They’ve done a good job,” says Lindsay Hays, deputy program scientist for the NASA astrobiology initiative in Washington, DC. “I can’t say this is definitely true or definitely not true,” she says, as closer examination of the rocks may reveal alternative explanations.

However, she says Baumgartner’s evidence is based on multiple techniques, which makes it more reliable: “That has become the standard in this type of work, not to just say ‘we looked at this one line of evidence and it showed what we hoped it would show’, but ‘we looked at multiple lines of evidence and they all line up together’.”

Journal reference: Geology, DOI: 10.1130/G46365.1