Australasian Science: Australia's authority on science since 1938

Oldest Fossils Were Merely Minerals

New analysis of 3.46 billion-year-old rocks has found that structures once thought to be the Earth’s oldest microfossils instead have the character of peculiarly shaped minerals.

In 1993, US scientist Bill Schopf likened tiny carbon-rich filaments within the 3.46 billion-year-old Apex chert from the Pilbara region of Western Australia to certain forms of bacteria. These “Apex chert microfossils” soon became enshrined in textbooks as the earliest evidence for life on Earth, and were even used to test and help refute the case against the presence of microfossils in the Martian meteorite ALH 84001.

Even so, their curious colour and complexity gave rise to suspicions that these curious structures were not true microfossils but pseudofossils formed by the redistribution of carbon around mineral grains during hydrothermal events.

Now scientists at The University of Western Australia have demonstrated that the Apex chert microfossils comprise stacks of plate-like clay minerals arranged into branched and tapered worm-like chains. Carbon that was later absorbed onto the edges of these minerals during the circulation of hydrothermal fluids gave a false impression of carbon-rich cell-like walls.

Dr David Wacey and Prof Martin Saunders of UWA used transmission electron microscopy to examine ultrathin slices of microfossil candidates and create nanoscale maps of their size, shape, mineral chemistry and the distribution of carbon.

It soon became clear that the distribution of carbon was unlike anything seen in authentic microfossils. “A false appearance of cellular compartments is given by multiple plates of clay minerals having a chemistry entirely compatible with a high temperature hydrothermal setting,” Wacey said.

“We studied a range of authentic microfossils using the same transmission electron microscopy technique, and in all cases these reveal coherent, rounded envelopes of carbon having dimensions consistent with their origin from cell walls and sheaths. At high spatial resolution, the Apex ‘microfossils’ lack all evidence for coherent, rounded walls. Instead, they have a complex, incoherent spikey morphology, evidently formed by filaments of clay crystals coated with iron and carbon.”

The research was published in Proceedings of the National Academy of Sciences USA.