But a new analysis of the microscopic structures has returned a surprising result - that they are not biological material at all.
In a new study published in Nature Geoscience, the US scientists suggest they may in fact simply be a series of rock fractures filled with crystals.
A new study claims the so-called fossils found in rock in the Pilbara region of Western Australia do not even contain carbon
The finding suggests that astrobiologists examining distant planets such as Mars must be careful labelling alien objects 'life', when researchers cannot be certain even with the evidence in front of them on Earth.
The Apex Chert formation in Western Australia's Pilbara region had been interpreted as cyanobacteria, once known as blue-green algae.
But the claim that the structures were carbon-based lifeforms has always been controversial, and so geologists from the University of Kansas decided to take another look.
Like previous teams, they cut slices of rock just 300 microns thick - roughly three times the diameter of a human hair - to study under the microscope, but this time added 30 micron sections to allow more light into the sample.
This revealed fractures within the rock that were filled with two unknown materials. Further analysis using a laser identified the mystery minerals were in fact haematite and quartz, neither of which are biological materials.
'It's one of those funny moments in science when you go out to do one thing and it completely flips 180 on you,' co-author Craig Marshall said. 'We discovered things were a little more complex than we thought they would be.'
The scientist who originally interpreted the structures as fossils, J. William Schopf, has prepared a response to the study but it has not yet been published. ( dailymail.co.uk )
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