Pre-oxygen (Image: David Wacey )

The oldest compelling fossil evidence for cellular life has been discovered on a 3.43-billion-year-old beach in western Australia. Grains of sand there provided a home for cells that dined on sulphur in a largely oxygen-free world.

The rounded, elongated and hollow tubular cells – probably bacteria – were found to have clumped together, formed chains and coated sand grains. Similar sulphur-processing bacteria are alive today, forming stagnant black layers beneath the surface of sandy beaches.

The remarkably well-preserved three-dimensional microbes will help resolve a fierce and long-running debate about what is the oldest known fossil – or at least add to it. In 1993, William Schopf of the University of California, Los Angeles, claimed he had discovered fossils that are 35 million years older than the present find in nearby deposits known as the Apex chert. He thought they were probably photosynthetic bacteria.


His arguments were widely accepted until 2002 when a reanalysis by a team led by Martin Brasier of the University of Oxford found that the bacteria-like shapes could have formed in a mineral process that had nothing to do with life. Scientists have been split between the camps ever since.

Brasier has now come back to the table with the new piece of the puzzle: slightly younger rocks that he claims do, this time, hold convincing evidence of cells.

Reasons to believe

He and his team offer multiple strands of evidence, including the physical structure of the putative microfossils and the geology and chemistry of the rocks, to prove that the forms they have discovered are biological structures. They appear to have cell walls that are consistent with bacterial life, and to have clustered and even split like modern bacteria.

The fossils were excavated from an ancient beach – now a sandstone formation near the Strelley pool in the Pilbara region – by Brasier’s colleague David Wacey from the University of Western Australia in Crawley.

“Nobody had looked at fossil beach deposits because it was thought oxygen had caused the decay of all traces of life there,” says Brasier. “In fact, there was minimal oxygen in the atmosphere at this time, meaning that the fossils could preserve well.”

Indeed, grains of unoxidised iron pyrite were found among the microbes, showing that there was no oxygen present at the time. Over time, the microbes became coated in silica, forming a glassy crust that was preserved in the sandstone.

Real old

The team’s findings have been well received so far. “The multiple lines of evidence have convinced me,” says Malcolm Walter of the University of New South Wales in Sydney, Australia, who thinks that the Apex chert fossils are real.

“These structures appear to be true microfossils,” says Emmanuelle Javaux from the University of Liège in Belgium, who remains to be convinced that the Apex chert fossils are biological. Neither fossil, however, is the oldest trace of life: indirect chemical evidence suggests life may go back to 3.8 billion years.

So is it just a matter of time before another, older group of cells is found? Not necessarily. Fossils older than 3.5 billion years are unlikely, as sedimentary rocks from that time are exceptionally rare and likely to have metamorphosed beyond recognition.

But the new find does open a new line of inquiry for astrobiologists looking for life on Mars. Sand and clay deposits on the Red Planet have been altered far less than equivalent Earth rocks. “This will be an interesting target in looking for ancient life,” says Javaux.

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1238