A new study by an international team of scientists has found that the presence of non-microbial “false” microfossils in early Earth sediments may challenge our current understanding of the early evolution of life.
For most of Earth’s history, life was limited to the microscopic realm, with bacteria occupying nearly every possible niche. Life is generally thought to have evolved in some of the most extreme environments, like hydrothermal vents deep in the ocean or hot springs that still simmer in Yellowstone. Much of what we know about the evolution of life comes from the rock record, which preserves rare fossils of bacteria from billions of years ago. But there is still uncertainty in whether or not a purported fossil is indeed a trace of life, and the field is plagued by “false positives”.
Now in a study published in Geology, researchers have found evidence that could help settle these arguments over which microfossils are signs of early life and which are not. They have shown that biomorphs – fossilised spheres and filaments made of organic carbon and similar in shape to bacterial fossils— can form without life’s influence, and might even be easier to preserve in rocks than bacteria.
Biomorphs were accidentally discovered while mixing organic carbon and sulfides in the laboratory; spheres and filaments were forming in experiments and at first were assumed to be the result of bacterial activity. “Very early, we noticed that these things looked a lot like bacteria, both chemically and morphologically, but were in fact formed abiotically,” said lead investigator Julie Cosmidis of the University of Oxford. “We thought, ‘What if they could form in a natural environment? What if they could be preserved in rocks?’ We have to try that, to see if they can be fossilised” she added.
Lead author, Christine Nims, of the University of Michigan said “They start just looking like a residue at the bottom of the experimental vessel, but under the microscope, you could see these beautiful structures that looked like bacteria. And they formed in these very sterile conditions, so these stunning features essentially came out of nothing. It was really exciting work.”
New experiments were then run, testing to see if these abiotic “organic envelopes” could be fossilised in rocks, like a bacterium would be. By adding silica to the structures, they aimed to recreate the formation of chert, a silica-rich rock that commonly preserves early microfossils. The ‘fossilisation’ progress was constantly tracked in glass vessels. It was found that not only could the biomorphs be fossilised, but also that they were much easier to preserve than bacterial remains. These strong carbon-sulfur structures were more resilient and less likely to flatten out than their fragile biological counterparts.
“Microbes don’t have bones,” Cosmidis explained. “They don’t have hard skins or skeletons. They’re just squishy organic matter. So to preserve them, you need to have very specific conditions”—like low primary productivity and rapid sediment deposition—“so it’s kind of rare when that happens.”
On one level, their discovery complicates things: knowing that these shapes can be formed without life and preserved more easily than bacteria casts doubt, generally, on our record of early life. But for a while, geobiologists have known no better than to rely solely on morphology to analyse potential microfossils. They now bring in chemistry, too: the authors showed that these pseudo-microfossils may be chemically distinguished from bacteria based on their organic composition.
The organic objects that Nims created were formed in a high-sulfur environment, replicating conditions on early Earth (and some hot springs today). Pyrite, or “fool’s gold,” is an iron-sulfide mineral that would likely have formed in such conditions, so its presence could be used as a beacon for potentially problematic microfossils. “If you look at ancient rocks that contain what we think are microfossils, they very often also contain pyrite,” Cosmidis said. “For me, that should be a red flag: ‘Let’s be more careful here.’ It’s not like we are doomed to never be able to tell what the real microfossils are. We just have to get better at it.”
This work won’t only help scientists understand Earth; the same processes likely happened on Mars, too. “Life on Mars, if it ever existed—or exists now—was probably microbial, so we’ll have to learn to recognise microbial life in Martian rocks”, Cosmidis said.
The study “Organic biomorphs may be better preserved than microorganisms in early Earth sediments” has been published in Geology on January 27th, 2021. DOI: https://doi.org/10.1130/G48152.1
Article adapted from the Geology press release by Rebecca Dzombak. You can read the full version here.