Scientists studying ancient microfossils from Brazil have discovered that structures once believed to be traces left behind by tiny animals were actually formed by communities of microscopic bacteria and algae. The findings challenge previous ideas about when small animals first appeared on Earth and suggest oxygen levels in ancient oceans may still have been too low to support certain forms of animal life around 540 million years ago.
The research focused on fossils found in the Brazilian state of Mato Grosso do Sul and was published in the journal Gondwana Research. Earlier studies had interpreted the marks as evidence of wormlike creatures or other tiny marine animals moving through seafloor sediment during the Ediacaran period, which came just before the Cambrian explosion.
“Using microtomography and spectroscopy techniques, we observed that the microfossils have cellular structures — sometimes with preserved organic material — consistent with bacteria or algae that existed during that period. These aren’t traces of animals that may have passed through the area,” says Bruno Becker-Kerber, the first author of the study. He carried out the research during postdoctoral work at the Institute of Geosciences at the University of São Paulo (USP) and at the Brazilian Center for Research in Energy and Materials (CNPEM), with support from FAPESP.
Becker-Kerber, who is now conducting postdoctoral research at Harvard University, explains that if the marks had truly been left by animals, they would represent evidence of meiofauna during the Ediacaran period. Meiofauna are tiny invertebrates measuring less than one millimeter long. Finding them in rocks this old would have pushed back the fossil record for these organisms significantly.
Ancient Oceans Before the Cambrian Explosion
The Ediacaran period occurred before the Cambrian explosion, a major evolutionary turning point when rising oxygen levels helped complex organisms diversify rapidly across Earth’s oceans. Fossil evidence clearly shows meiofauna existed during the Cambrian, but the new findings suggest they may not have been present earlier in the way some scientists proposed.
The project forms part of the “Rio de la Plata Craton and Western Gondwana” study supported by FAPESP and coordinated by Miguel Angelo Stipp Basei, a professor at IGc-USP and coauthor of the paper.
Another coauthor, Lucas Warren of São Paulo State University (IGCE-UNESP) in Rio Claro, also received support from FAPESP.
Researchers reexamined fossils collected in Corumbá and also analyzed newly studied material from Bonito in the Serra da Bodoquena region. Both sites are located in Mato Grosso do Sul within the Tamengo geological formation.
These rocks formed in a shallow marine environment along a continental shelf during the final stages of Gondwana’s formation, before the supercontinent eventually split apart to form regions that became South America and Africa.
The same research group previously identified what may be the oldest known lichen fossil, also discovered in Mato Grosso do Sul and younger than the bacteria and algae described in the current study.
High Resolution Fossil Imaging Revealed Hidden Structures
To investigate the fossils in greater detail, the team used the MOGNO beamline at Sirius, CNPEM’s particle accelerator facility in Campinas. The technology allowed researchers to study fossils ranging from only a few micrometers to a few millimeters in size.
The scientists used both microtomography and nanotomography, techniques capable of generating images at extremely small scales, including micrometers (one-thousandth of a millimeter) and nanometers (one-billionth of a meter).
“When you have a large sample and want to image a structure inside it, the resolution obtained is often insufficient. The MOGNO beamline is one of the few in the world that performs so-called zoom tomography, in which we focus on something inside the sample and analyze it at the nanoscale without destroying the sample,” says Becker-Kerber.
He notes that the earlier study interpreting the structures as animal traces did not have access to this level of imaging technology.
Researchers also used Raman spectroscopy to examine the fossils’ chemical makeup. The technique identified organic material within fossil cell walls, strengthening the interpretation that the structures were preserved microbial bodies rather than marks left behind by passing animals.
Giant Ancient Bacteria and Algae
Some fossil samples contained pyrite, a mineral made of iron and sulfur. Based on the shapes and chemistry of the specimens, researchers believe some may represent sulfur-oxidizing bacteria, organisms that use sulfur in their metabolism.
“This group of bacteria is surprising. Some of the largest ever recorded belong precisely to this category. Unlike the common image we have of microscopic bacteria, certain species can reach diameters larger than a strand of hair and are visible to the naked eye,” says Becker-Kerber.
Although the fossils do not preserve enough detail to identify exact species, the researchers found preserved cells, divisions within cell walls, and traces of organic matter across multiple collection sites. According to the team, these features would not exist if the structures were simply disturbances created by moving animals.
The fossils also appear in three different size ranges, suggesting several species may have lived together in microbial communities. The largest forms resemble green or red algae, while the smaller fossils may represent algae, cyanobacteria, or sulfur-oxidizing bacteria.
“There are concave and convex partitions, coiled filaments, cells without sediment but containing organic matter. This evidence is much closer to bacteria or algae than to mere marks of disturbance caused by animals,” the researcher concludes.
The findings provide scientists with a clearer picture of the world before the Cambrian explosion and may help researchers better understand the environmental conditions that paved the way for the rise of complex animal life.