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Fossil microbes give sulphur insight on ancient Earth

Tiny structures found in 3.4bn-year-old sandstones in Western Australia represent some of the oldest, best preserved evidence of life on Earth.

Scientists say their analysis of the microfossils clearly shows the organisms were processing sulphur for energy and growth - not oxygen.

They report their discovery in the journal Nature Geoscience.

The team says the microbe remains offer a fascinating insight into conditions on the ancient Earth.

"At last we have good solid evidence for life over 3.4 billion years ago. It confirms there were bacteria at this time, living without oxygen," said co-researcher Professor Martin Brasier at Oxford University, UK.

"Such bacteria are still common today. Sulphur bacteria are found in smelly ditches, soil, hot springs, hydrothermal vents - anywhere where there's little free oxygen and they can live off organic matter," he explained.

The fossils were first identified in 2007 at Strelley Pool, a remote location of the Pilbara, a dry region about 60km west of Marble Bar.

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The fossils were found in sandstones found at the base of these prominent ridges

Their host sandstones were laid down in what would have been a shallow-water beach or estuary.

They measure just a few millionths of a metre (microns) across and have been subjected in recent years to a series of advanced analytical techniques to probe their origin.

The scientists say they are now confident that the spheroidal and ellipsoidal forms buried in the rock are the remains of bacterial cells, along with the protective tubes that once housed them.

Form and behaviour are good indicators. The shape and clustering are reminiscent of bacterial cells. But more than that, the fossils are associated with tiny crystals of "fool's gold" - the pyrite mineral composed of iron and sulphur.

The types of atoms, or isotopes, present in these crystals point to the pyrite being formed as a by-product of cellular metabolism based on compounds of sulphur.

"Life likes lighter isotopes, so if you have a light signature in these minerals then it looks biological," said lead author Dr David Wacey from the University of Western Australia.

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