For over two decades, researchers have puzzled over what some strongly believe are the world’s earliest traces of life: fossilized cell walls within 3.46-billion-year-old rocks. But thanks to a new high-resolution analysis, it turns out that these “microfossils” weren’t left behind by living organisms. Rather, they’re the result of peculiarly shaped minerals, according to findings published in Proceedings of the National Academy of Sciences this week.
In a 1993 Science study, researchers described tiny carbon-rich filaments within 3.46-giga-year-old Apex chert (a fine-grained sedimentary rock) from the Pilbara region of Western Australia. These structures, which were between 0.5 and 20 micrometers wide, seemed to resemble photosynthetic cyanobacteria. The “Apex chert microfossils,” as they came to be called, became famous as the earliest evidence for life on the planet. They were even used to refute the case against microfossils in a Martian meteorite.
Despite being in textbooks and museum displays, the microfossils were still quite contentious. In 2002, a team led by the late Martin Brasier of Oxford revealed that the host rock wasn’t part of a simple sedimentary unit; rather, it came from a complex, high-temperature hydrothermal system that experienced multiple episodes of subsurface fluid flow over a long time. In this alternate hypothesis, the structures weren’t true microfossils: They were “pseudofossils” formed when carbon redistributed itself around mineral grains during hydrothermal events.
But without the technology to map them out at the sub-micrometer scale, researchers continued to debate their status. Now, University of Western Australia’s David Wacey and colleagues used transmission electron microscopy to examine ultrathin slices of these possible microfossils and build nanoscale maps of their size, shape, mineral chemistry, and distribution of carbon.
Earth’s oldest microfossils, it turns out, have the character of peculiarly shaped minerals. They’re comprised of stacks of plate-like clay minerals (pictured to the right, green) arranged into branched and tapered chains that appear worm-like. When carbon became absorbed onto the mineral edges during the circulation of hydrothermal fluids, this created the impression of carbon-rich walls—like the kind you’d find in living cells.
"It soon became clear that the distribution of carbon was unlike anything seen in authentic microfossils,” Wacey says in a news release. “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.”
Authentic microfossils contained rounded envelopes of carbon with dimensions that are consistent with a cell wall origin. “At high spatial resolution, the Apex 'microfossils' lack all evidence for coherent, rounded walls,” Wacey adds. “Instead, they have a complex, incoherent spiky morphology, evidently formed by filaments of clay crystals coated with iron and carbon." You can see these later generations of carbon (yellow) and iron (red) in the image on the right.
Just last year, 3.45-billion-year-old microbial activity was revealed to be not biological in origin as well. So for now, the record goes to much younger rocks, Science explains: The 3.43-billion-year-old Strelley Pool Formation (also from Western Australia) contains evidence of hollow, bag-shaped bodies arranged in chains or clusters.
Images: M.D. Brasier et al., PNAS 2015 via University of Bristol