News – Boulder, Colo., USA: For most of Earth’s history, life was confined to the microscopic realm, with bacteria taking over almost every possible space. Life is generally considered to have evolved in some of the most remote environments, such as deep-sea hydrothermal fins or hot springs that are still similar in Yellowstone. Much of what we know about the evolution of life comes from the rock plate, which holds rare fossils of germs billions of years ago. But there is controversy on that record, with every new discovery (rightly) criticized, questioned and analyzed from all sides. Even then, uncertainty about the authenticity of fossils can be a trace of life, and the field is plastered with “wrong things” of early life. To understand the evolution of our planet – and to find traces of life on others – scientists need to be able to tell the difference.
New experiments by geoscientists Julie Cosmidis, Christine Nims, and colleagues, published today in Geology, that may help resolve arguments about whether microfossils are signs of early life and not. They have shown that fossil fields and filaments – two common bacterial formations – made of organic carbon (usually related to life) can form mature (excluding living organisms) and may even easier to retain than bacteria.
“One major problem is that fossils have a very simple morphology, and there are a lot of non-biological processes that can reproduce them,” Cosmidis says. “If you find a skeleton full of a dinosaur, it’s a very complex structure that makes it impossible for a chemical process to reproduce.” It is much more difficult to get that proof with fossil microbes.
Their work was inspired by an accidental discovery a few years ago, in which Cosmidis and Nims were involved while working in the Alexis Templeton laboratory. When they mixed organic carbon and sulfide, they noticed that fields and filaments formed and assumed that they were the result of bacterial activity. But on closer inspection, Cosmidis quickly realized that they were maturely formed. “Very early on, we noticed that these things looked very much like bacteria, both chemically and morphologically,” she says.
“They just start to look like remnants at the bottom of the experimental vessel,” says researcher Christine Nims, “but under the microscope, you could see those beautiful microbial-looking structures . And they created in those really sterile conditions, so they were so amazing there were no features coming out of zero. It was an interesting job. “
“We were thinking, ‘What if they could create in a natural environment? What if they were preserved in rocks?'” Cosmidis says. “We had to try that, see if they can fossilize.”
Nims began running the new experiments, testing whether these mature structures, called biomorphs, could be fossilized, like a bacterium. By adding biomorphs to silica solution, they aimed to regenerate the formation of chert, a silica-filled rock that typically retains early microfossils. For weeks, she watched closely the progress of a small ‘fossil’ under a microscope. They discovered not only that they could be fossilized, but also that these mature shapes were much easier to preserve than bacterial remains. The abiotic fossil structures, made up of organic carbon and sulfur, were more stable and less prone to extinction than their biologically fragile counterparts.
“Microbes have no bones,” Cosmidis explains. “They have no skin or skeleton. They’re just squishy organic matter. So to preserve them, you need to have certain conditions” – such as low levels of photosynthesis and rapid sediment deposition – “so it’s very rare. that will happen. “
On one level, their discovery complicates matters: it is known that these shapes can be created without life and preserved more easily than bacteria generally suspect of our record of early life. But for a while, geoscientists have known better than to rely heavily on morphology to analyze potential microfossils. They will also include chemistry.
The Nims “organic envelopes” created in the laboratory were created in a sulfur-rich environment, reproducing early Earth conditions (and today’s hot springs). Pyrite, or “fool’s gold,” is an iron-sulfide mineral that would likely be formed in such a situation, so its presence could be used as a torch for microfossils that could be a problem. “If you look at old rocks that contain what we think are microfossils, they often contain pyrite as well,” Cosmidis says. “To me, that should be a red flag: ‘Let’ s be more careful here. ‘ It’s not like we’re sorry we can never tell what the real microfossils are. We need to get better at it. “
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