When it comes to finding fossils of very ancient microbial life, either on Earth or in other worlds, such as Mars, the odds are not in our favor.
New research finds that real microbial life forms are much less likely to fossilize safely in rocks compared to non-biological structures that mimic their shapes. The discovery suggests Earth’s first rocks may contain abundant small counterfeiters – tiny objects posing as fossilized evidence from early life – researchers report online Jan. 28 in Geology.
The discovery is "at least a warning tale," says study author Julie Cosmidis, a geomicrobiologist at Oxford University.
Tiny, often enigmatic structures, found in some of the oldest rocks on Earth, dating back more than 2.5 billion years, can offer tempting clues to the early lives of the planet. And the hunt for increasingly old signs of life on Earth has provoked intense debate; partly because the further back in time, the more difficult it is to interpret small scribbles, filaments, and spheres on the rock (SN: 1 / 3/20). One reason is that the movements of the Earth’s tectonic plates over time can squeeze and cook rocks, chemically deforming and altering small fossils, perhaps out of recognition.
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But an even more pernicious and controversial problem is that such small filaments or spheres may have no biological origin. Increasingly, scientists have discovered that non-biological chemical processes can create similar shapes, suggesting the possibility of “false positives” in the biological record.
One of these findings led to the new study, Cosmidis says. A few years ago, she and others tried to grow bacteria and make them produce sulfur. “We were mixing sulfides with organic matter and we started forming these objects,” he says. "We thought they were formed by bacteria, because they looked very biological. But then we found out that they were forming in laboratory tubes that had no bacteria."
That led her to wonder about such processes occurring in the rocks themselves. So she and others decided to examine what would happen if they tried to recreate the early stages of flint formation, a kind of compact, silica-rich rock common in early Earth. “Microfossils are often found in flint formations,” says study co-author Christine Nims, a geobiologist now at the University of Michigan in Ann Arbor. "Everything housed in (flint) will be well preserved."
Flint is formed from silica-rich water; the silica precipitates out of the water and accumulates, eventually hardening into rock. Cosmidis, Nims, and colleagues added sulfur-containing bacteria called Thiothrix to solidify silica to see what can happen during actual fossilization. To other flint samples, they added “biomorphs” containing sulfur, spheres, and filaments made of small crystals but shaped like bacteria.
At first, silica nanoparticles encrusted bacteria and biomorphs, Nims says. But after a week or so, the bacteria began to deform, their cells deflating from the cylinders into flattened, unrecognizable tapes as the sulfur inside the cells spread and reacted with the silica outside the cells, forming new minerals.
Biomorphs, on the other hand, “had this impressive resistance,” she says. Although they also lost sulfur with the surrounding solution, they maintained their silica bark. As a result, "they kept their shape and showed very little change over time." That resistance suggests that enigmatic structures found in early cave records are more likely to be pseudofossils, rather than actual fossils, the team says.
From above: C. Nims; R.J. Baumgartner et al / Geology 2019
Sean McMahon, an astrobiologist at the University of Edinburgh who did not participate in the new study, the idea that living creatures are already harder to conserve makes sense. “It’s not totally amazing,” he says. "We know that biomass tends to decompose pretty quickly."
In fact, scientists have known for centuries that certain chemical reactions can act as “gardens” that “grow” mineral objects in strange ways, twisting into tubes, or sprouting branches, or otherwise mimicking the weirdness of life. “There’s a complacency about it, a misconception that we know all of this and it’s already been addressed,” McMahon says.
Strategies to address this enigma have included the search for particular structures – such as mound-shaped stromatolites – or chemical compounds in a potential fossil that are believed to be uniquely formed or modified by the presence of life (SN: 17/10/18) . These criteria are the product of decades of field studies, through which scientists have accumulated a vast set of data on fossil structures, with which researchers can compare and evaluate any new findings.
“Anything we find, we can look through that lens,” McMahon says. But what is missing is an equally rich data set on how those structures could be formed in the absence of life. This study, he says, highlights that attempts "to define criteria for recognizing true fossils in very ancient rocks are premature, because we still don't know enough about how non-biological processes mimic true fossils."
It is an increasingly urgent issue with increasing participation, as NASA’s Perseverance explorer is about to settle on Mars to begin a new search for vestiges of life in ancient rocks (SN: 28/07/20), he adds. "Paleontologists and scientists exploring Mars should take (this study) very seriously."