Preserved cells, cell walls, and organic remnants—the signature of life itself
In the ancient rocks of Mato Grosso do Sul, Brazil, scientists have looked again at marks once believed to be the trails of tiny worms and found instead the preserved communities of bacteria and algae from 540 million years ago. The reinterpretation, made possible by nanoscale imaging at a particle accelerator, quietly removes what would have been the oldest known animal traces from the record. In doing so, it deepens our sense of how long the Earth remained a microbial world — and how much oxygen, patience, and time were required before complex life could take hold.
- Fossils long interpreted as evidence of ancient animal movement have been reidentified as microbial communities, erasing what would have been a landmark in the history of animal life.
- The stakes are significant: if the original interpretation had held, tiny invertebrates would have existed tens of millions of years earlier than current evidence supports.
- Advanced zoom tomography at Brazil's Sirius particle accelerator allowed researchers to peer inside the fossils at the nanoscale without destroying them — a capability simply unavailable to earlier investigators.
- The new analysis found cellular walls, coiled filaments, organic matter, and pyrite — signatures of bacteria and algae, not the disturbance patterns left by burrowing animals.
- The findings suggest Ediacaran oceans were too oxygen-poor to support animal evolution, placing the emergence of complex life more firmly in the Cambrian period that followed.
- The study is part of a broader investigation into Western Gondwana's fossil record, a project already yielding other surprises, including what may be the oldest known lichen ever found.
Scientists reexamining ancient rocks from the Brazilian state of Mato Grosso do Sul have overturned a previous interpretation that once pointed to the earliest known animal traces in the fossil record. What earlier researchers believed were the marks of tiny worms or small sea creatures moving through Ediacaran seafloor sediment have been reidentified as fossilized communities of bacteria and algae — organisms that lived roughly 540 million years ago, long before complex animal life took hold.
The reexamination, published in Gondwana Research, was led by Bruno Becker-Kerber of the University of São Paulo. Using the MOGNO beamline at Sirius, a particle accelerator in Campinas, his team performed zoom tomography — imaging the interior of fossils at the nanoscale without damaging them. What they found were cellular structures, preserved organic material, coiled filaments, and pyrite deposits: the hallmarks of microbial life, not animal activity. Three distinct size categories within the fossils suggest different species coexisting, possibly including green or red algae, cyanobacteria, and sulfur-oxidizing bacteria.
The fossils came from two sites in Mato Grosso do Sul — one near Corumbá, another in the Serra da Bodoquena region near Bonito — both preserving shallow marine environments from the final stages of Gondwana's assembly. Their small scale, ranging from micrometers to a few millimeters, made advanced imaging essential to the reinterpretation.
The broader implication is a quieter Ediacaran world than some had imagined — one still dominated by microbes, where oxygen levels remained too low to support the evolution of even the smallest invertebrates. That threshold would not be crossed until the Cambrian, when rising oxygen unleashed the explosion of animal diversity. The same research group has also identified what may be the oldest known lichen in the fossil record, also from Mato Grosso do Sul, adding another layer to an emerging portrait of life at the edge of complexity.
In the Brazilian state of Mato Grosso do Sul, scientists have taken another look at some very old rocks and found something unexpected: what earlier researchers had identified as traces of ancient worms or other small sea creatures are actually something far simpler—fossilized communities of bacteria and algae that lived roughly 540 million years ago, during the Ediacaran period.
The reexamination, published in Gondwana Research, carries implications that ripple backward through deep time. If these marks really were left by animals moving through the seafloor, it would mean that tiny invertebrates—creatures smaller than a millimeter—were already active and leaving traces during the Ediacaran. That would make them the oldest animals of their kind on record. But the new analysis suggests that interpretation was mistaken. Bruno Becker-Kerber, who led the study as a postdoctoral researcher at the University of São Paulo and the Brazilian Center for Research in Energy and Materials, used advanced imaging to look inside the fossils without damaging them. What he and his colleagues found were cellular structures, preserved organic material, and the architectural signatures of microorganisms—not the disturbance patterns you'd expect from animals burrowing or crawling through sediment.
The fossils came from two locations in Mato Grosso do Sul: some from the municipality of Corumbá, others from a rock outcrop near Bonito in the Serra da Bodoquena region. Both sites preserve rocks that formed in a shallow marine environment during the final stages of Gondwana's assembly, before that ancient supercontinent split into Africa and South America. The specimens themselves are tiny—ranging from a few micrometers to a few millimeters—which is why the technology mattered so much. The MOGNO beamline at Sirius, a particle accelerator in Campinas, could perform what researchers call zoom tomography: focusing on structures deep inside a sample and imaging them at the nanoscale without destroying anything. Earlier studies lacked access to this capability.
When Becker-Kerber and his team examined the fossils more closely, they found evidence of three distinct size categories, suggesting different species living together in what may have been a microbial consortium. Some of the larger structures resembled green or red algae. The smaller ones could be algae, cyanobacteria, or possibly sulfur-oxidizing bacteria—a remarkable group that uses sulfur as fuel and can grow to sizes visible to the naked eye, far larger than the typical microscopic bacteria most people imagine. Several samples contained pyrite, an iron-sulfur mineral that hints at the presence of sulfur-metabolizing organisms. Raman spectroscopy revealed the organic composition of cell walls. The fossils showed concave and convex partitions, coiled filaments, cells filled with organic matter but no sediment. All of this pointed toward living things, not merely the scars left behind by something passing through.
The reinterpretation shifts our understanding of the Ediacaran world. If meiofauna—those tiny invertebrates—were not yet present, it suggests that oxygen levels in the oceans may have been too low to support their evolution. That changed dramatically during the Cambrian period, when rising oxygen enabled the evolution of complex organisms and the explosion of animal diversity that followed. The Ediacaran, in this view, was still a realm of microbes: bacteria and algae dominating a world not yet ready for animals. The study is part of a larger project examining the Rio de la Plata Craton and Western Gondwana, coordinated by Miguel Angelo Stipp Basei at USP, with contributions from Lucas Warren at São Paulo State University. In related work, the same group has identified the oldest known lichen in the fossil record, also from Mato Grosso do Sul, though younger than these bacterial and algal remains. Together, these findings refine the picture of life in the deep past and the conditions that eventually transformed Earth.
Notable Quotes
These aren't traces of animals that may have passed through the area. The microfossils have cellular structures—sometimes with preserved organic material—consistent with bacteria or algae that existed during that period.— Bruno Becker-Kerber, lead researcher
This evidence is much closer to bacteria or algae than to mere marks of disturbance caused by animals.— Bruno Becker-Kerber
The Hearth Conversation Another angle on the story
So these fossils were misidentified for how long?
The earlier interpretation had been accepted, but we don't know exactly when it was first proposed. What matters is that the technology to see inside them clearly didn't exist before. You need the right tools to see what's actually there.
Why does it matter whether these are animal traces or bacteria?
Because if animals were already burrowing around 540 million years ago, it would mean the ocean had enough oxygen to support them. But if it's just microbes, it tells us oxygen was still scarce. That changes the whole timeline of when complex life could emerge.
Could they be both—bacteria living in burrows made by animals?
The distribution and structure don't support that. You'd see different patterns if animals had disturbed the sediment. These fossils show preserved cells, cell walls, organic material in specific arrangements. That's what living organisms leave behind, not what a passing creature would create.
What's surprising about sulfur-oxidizing bacteria?
Most people think bacteria are invisible specks. But some sulfur-oxidizing bacteria can grow to the size of a human hair. They're still bacteria, but they operate at a completely different scale than we usually imagine.
Does this change when the Cambrian explosion happened?
No, the Cambrian explosion still happened when it did. But this suggests the Ediacaran ocean wasn't ready for animals yet. The explosion wasn't just about time passing—it was about oxygen finally rising enough to make complex life possible.
What would prove you wrong?
If someone found preserved reproductive structures or other features that could only belong to animals. But these fossils don't have that. They have what microorganisms leave behind.