For decades, a group of bacteria-shaped structures known as the Apex chert microfossils have been considered to be the oldest evidence of life on Earth. Dating all the way back to the early Archean epoch some 3.46 billion years ago, the structures appear to be carbon-rich filaments that resemble primitive cyanobacteria.
But recently, scientists have raised doubts over whether these structures, which were found in western Australia, are actually organic in origin. The result is the so-called Apex chert controversy, which has raged for the past decade, fueled by evidence that these filaments could have been formed by hydrothermal activity.
Now, a new study published in the Proceedings of the National Academy of Sciences could finally put this controversy to rest. As the skeptics suspected, the oldest fossils in the world are not fossils at all, but rather the signature of geological processes in Earth's primordial years.
"This research should, at long last, provide a closing chapter for the 'Apex microfossil' debate," said study co-author and paleontologist Martin Brasier in a statement. "It is hoped that textbooks and websites will now focus upon recent and more robust discoveries of microfossils of a similar age from western Australia, also examined by us in the same article."
Brasier and his colleagues reached this conclusion after examining the Apex chert structures in nanoscale-level detail, using a transmission electron microscope. The filaments varied in size from around 0.5 to 20 micrometers, and were characterized by their segmented appearance and carbon-heavy walls, which resemble bacterial cell membranes.
But when the researchers observed the structures through the electron microscope, they found a telltale anomaly: The distribution of the carbon in the filament walls was chaotic and uneven, which is not consistent with organic life.
The team concluded that the filaments were made of clay, but were regularly doused with carbon-rich hydrothermal floods. The surface clay absorbed this carbon, giving the structures the appearance of a biological membrane. As it turns out, Earth can whip up a pretty convincing microfossil imposter on a whim.
"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," said co-author David Wacey in a statement.
"At high spatial resolution, the Apex 'microfossils' lack all evidence for coherent, rounded walls," he continued. "Instead, they have a complex, incoherent spikey morphology, evidently formed by filaments of clay crystals coated with iron and carbon."
This study is likely to represent the end of the Apex chert microfossil theory, but that doesn't necessarily mean that life didn't exist on Earth 3.46 billion years ago. In fact, the same formation in western Australia that yielded these pseudofossils also contains more reputable microfossils dating back to 3.43 billion years, which have now claimed the hallowed title of world's oldest fossils.
So while it's always a bummer to find out a promising candidate for life was formed abiotically, the PNAS study demonstrates that new technologies can shed light on the origins of oldest structures on Earth—and beyond it. Exposing false positives like the Apex chert structures will help scientists recognize the genuine article in the future.