Level 1 — Absolute Beginner

Long ago, the Earth was very young. There were no people. There were no animals on land.

Scientists in Brazil found old marks on rocks. They thought the marks came from small sea animals.

But a new study shows the marks did not come from animals. They came from tiny algae and bacteria.

This means animals may have started later than people thought. Scientists are excited to learn more.

Earth

The planet that we live on.

rock

A hard piece of stone.

scientist

A person who studies the natural world.

Brazil

A large country in South America.

study

A piece of research about a topic.

animal

A living thing that is not a plant, like a worm or a bird.

algae

Small plant-like living things that often live in water.

bacteria

Tiny living things that you can only see with a microscope.

Level 2 — Elementary

About 540 million years ago, the Earth was a very strange place. There were no fish, no plants on land, and very little oxygen in the sea. This time is called the late Ediacaran period.

Scientists used to think that long, wavy marks found on rocks from this period were the tracks left by tiny worms and other early animals. The marks were found in the Brazilian state of Mato Grosso do Sul.

A new study, published this month in the journal Gondwana Research, tells a different story. Researchers used powerful X-rays and special light tools called spectroscopes to look inside the marks. They found cells, not animal bodies.

The marks were actually made by communities of microbes, including algae and bacteria, that lived stuck to the seafloor. This means the first true animals may have appeared a little later than scientists thought, and that the seas of that time may have had too little oxygen to support complex animal life.

Ediacaran period

A period in Earth's history about 635 to 539 million years ago, just before the Cambrian explosion.

oxygen

A gas in the air and water that animals need to breathe.

track

A mark or trail left behind by something moving.

Brazilian

Belonging to or coming from Brazil.

researcher

Someone who studies a topic in detail.

X-ray

A kind of light that can pass through some things and show what is inside.

cell

The smallest unit that makes up all living things.

community

A group of living things that share the same place.

Level 3 — Intermediate

A team of Brazilian and international paleontologists has overturned a decades-old reading of the fossil record. In a study published in the journal Gondwana Research on May 11, 2026, researchers report that long, sinuous structures preserved in 540-million-year-old siltstones from the Tamengo Formation in Mato Grosso do Sul are not, as previously believed, ichnofossil trace evidence of burrowing bilaterian animals. They are instead the cemented remains of microbial mats — vast communities of cyanobacteria and protistan algae living on the soft sediment of an Ediacaran sea floor.

The reinterpretation rests on synchrotron microtomography and Raman spectroscopy carried out at the Brazilian Synchrotron Light Laboratory in Campinas. These techniques revealed cellular architecture inside the impressions — filamentous sheaths, polysaccharide envelopes and preserved chlorophyll-derived molecules — that are inconsistent with the muscle-driven, peristaltic locomotion of an early animal. They are instead the unmistakable signatures of biofilm life.

The implications are significant. The Tamengo trails had been one of the cornerstone pieces of evidence for animal motility prior to the Cambrian explosion. Removing them from the bilaterian column suggests that complex animals capable of three-dimensional burrowing did not yet exist in the world's oceans late in the Ediacaran period. It also accords with isotope-based reconstructions that show severely depressed oxygen levels in marine shelf waters at that time, perhaps too low for the oxygen-hungry musculature of any but the simplest swimming animals.

The reinterpretation does not erase the Ediacaran biota. Iconic frond-like organisms such as Charnia, and the controversial disc-shaped Dickinsonia, remain part of the picture. But it does push the firm earliest date for true animal burrowing closer to the Cambrian boundary at 538.8 million years ago — sharpening, rather than blurring, what was already the most dramatic threshold in the history of life.

paleontologist

A scientist who studies fossils.

ichnofossil

A fossil trace such as a burrow or footprint, rather than the remains of the animal itself.

bilaterian

An animal with bilateral symmetry, meaning its body has a left and right side that mirror each other.

microbial mat

A thin layered community of microbes that lives on a surface.

cyanobacteria

A group of bacteria that get energy from sunlight through photosynthesis.

synchrotron

A particle accelerator that produces very bright X-rays for scientific imaging.

Raman spectroscopy

A technique that uses scattered light to identify the chemical makeup of a sample.

Cambrian explosion

A burst of evolutionary diversity around 538 million years ago when most major animal groups appeared.

Level 4 — Advanced

A team of paleontologists from the State University of Campinas, the Universidade Federal do Mato Grosso do Sul and the University of Cambridge has published a reinterpretation of the iconic Tamengo Formation Ediacaran traces in Gondwana Research, dated May 11, 2026. The paper argues that the long, sinuous impressions in the Mato Grosso do Sul siltstones, dated by U-Pb zircon geochronology to between 541.0 and 539.4 million years ago, do not record peristaltic locomotion by early bilaterian metazoans. They record, instead, the cemented topographies of mature microbial mats dominated by filamentous cyanobacteria and protistan green algae, mineralized in situ during early diagenesis.

The evidentiary basis is unusually rich. Using high-resolution synchrotron microtomography at the LNLS Sirius beamline and confocal Raman spectroscopy, the team resolved sub-micrometer cellular architecture: extracellular polysaccharide envelopes, filament sheaths of consistent diameter, organic remnants spectrally consistent with chlorophyll-derived porphyrin breakdown products, and the geochemical signature of microbially induced sedimentary structures (MISS). None of the impressions preserved cuticle, segmented body plans, or the asymmetric morphology expected from peristaltic muscle-driven burrowing. Statistically, the depth-to-width ratios and curvature distributions of the traces also map onto modern microbial-mat folds rather than known bilaterian burrow ichnotaxa.

The geochemical context bolsters the case. Iron-speciation data from interbedded shales recover a euxinic to ferruginous water column with shelf oxygen saturations below five percent of present atmospheric levels, broadly consistent with the Shuram-Wonoka negative carbon-isotope excursion seen at correlative sections worldwide. Such an environment would have been physiologically permissive only to microbial mats and the simplest non-motile metazoans — certainly not to the energetically expensive musculature required for active sediment burrowing. The findings therefore tighten, rather than loosen, the temporal constraint on the Cambrian explosion: bilaterian motility now appears narrowly confined to the post-538.8-Ma window.

The paper's authors are careful to specify what their reinterpretation does and does not change. The discrete-bodied Ediacaran macrofauna — Charniodiscus, Dickinsonia, Pteridinium, the Cloudina reef-builders — remain on the chart. The Tamengo's small shelly fossils, including Corumbella werneri, are likewise untouched. But the elastic-band timeline that allowed researchers to push trace-fossil-bearing bilaterians back into the upper Ediacaran has snapped. As one outside commentator at the University of Edinburgh put it, the Cambrian boundary is once again behaving like a wall, not a gradient — and that, in turn, sharpens the central evolutionary question: which physiological or environmental switch finally let the animals burrow?

U-Pb zircon geochronology

A radiometric dating technique that measures the radioactive decay of uranium to lead inside zircon crystals.