In a move that will surely disappoint anyone hoping for 540-million-year-old worm-sized celebrities, scientists have determined that ancient microfossils from Brazil - once thought to be the earliest evidence of tiny animals slithering through primordial mud - are actually just the remains of bacterial and algal community gatherings. The findings, published in Gondwana Research, challenge previous assumptions about when small animals first appeared on Earth and suggest that oxygen levels in ancient oceans were still too low to support certain forms of animal life around 540 million years ago.
The research focused on fossils from Mato Grosso do Sul in Brazil. Earlier studies had optimistically 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 famous 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, first author of the study and a postdoctoral researcher at Harvard University (who did the work at the University of São Paulo and the Brazilian Center for Research in Energy and Materials, with support from FAPESP).
Becker-Kerber explains that if the marks had truly been left by animals, they would represent evidence of meiofauna - tiny invertebrates less than one millimeter long - during the Ediacaran. Finding them in rocks this old would have pushed back the fossil record for these organisms significantly, which would have been exciting. Instead, we get bacteria. Classic.
The project is part of the "Rio de la Plata Craton and Western Gondwana" study supported by FAPESP and coordinated by Miguel Angelo Stipp Basei of IGc-USP. Researchers reexamined fossils collected in Corumbá and analyzed newly studied material from Bonito in the Serra da Bodoquena region, both 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 split into what became South America and Africa.
To investigate the fossils in greater detail, the team used the MOGNO beamline at Sirius, CNPEM's particle accelerator facility in Campinas. This technology allowed researchers to study fossils ranging from a few micrometers to a few millimeters using microtomography and nanotomography. "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 - a polite way of saying they were working with worse tools.
Researchers also used Raman spectroscopy to examine the fossils' chemical makeup, identifying organic material within fossil cell walls and strengthening the interpretation that the structures were preserved microbial bodies. Some fossil samples contained pyrite (iron and sulfur), and based on shapes and chemistry, 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 don't 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 - features that 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 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.
So, the world before the Cambrian explosion was apparently full of bacteria throwing microscopic raves, not tiny animals on a stroll. The findings provide scientists with a clearer picture of environmental conditions that paved the way for complex animal life - and a cautionary tale about assuming every squiggle in ancient rock is a celebrity.