🔬 What were thought to be the oldest animals on Earth... are not animals

The Ediacaran, an era that ended about 540 million years ago, is considered the prelude to the Cambrian explosion, a period marked by a rapid diversification of species. Until now, some fossils from this era were attributed to small invertebrates, the meiofauna. But the latest analyses indicate that the oceans of this period were still too poor in oxygen to support such animals.


To understand this enigma, researchers used cutting-edge techniques such as microtomography and nanotomography, thanks to the MOGNO beamline of the Sirius synchrotron in Brazil. These methods revealed preserved cellular structures in the fossils, with walls and even organic materials, typical of microorganisms. Raman spectroscopy analysis confirmed the presence of organic compounds in the cell walls, reinforcing the hypothesis of microbial remains.

The examined fossils come from the Tamengo geological formation in Mato Grosso do Sul. Their diverse shapes – coiled filaments, concave or convex partitions – correspond to different species of bacteria and algae. Some specimens contained pyrite, an iron and sulfur mineral, indicating the presence of sulfur bacteria, capable of living in low-oxygen environments.


These results change the game for understanding the Cambrian explosion. If the oldest animal traces indeed date from the Cambrian, that means oxygen was not yet sufficient in the Ediacaran to support the meiofauna. Scientists must therefore reassess the conditions that allowed the emergence of animal life.

The researchers specify that these fossils are not mere traces of animal passage, but rather the remains of giant microbial communities. This discovery invites a reconsideration of the fossil evidence of the dawn of animal life and paves the way for new investigations into pre-Cambrian environmental conditions.

In a separate study, the same team identified what could be the oldest fossil of a lichen, also discovered in Mato Grosso do Sul.