In 2010, Moroccan-French geologist Abderrazak El Albani of the University of Poitiers discovered the oldest fossils of complex, colony-forming organisms in 2.1-billion-year-old clay shales from Gabon. This groundbreaking discovery fundamentally changed our understanding of life’s evolution and pushed back the known emergence of macroscopic multicellularity by more than 1.5 billion years. The fossils, measuring up to 17 cm in size, feature a roughly spherical central body surrounded by a rim. In addition, elongated forms resembling flattened worms were also found. From a geochemical perspective, the timing of these organisms—referred to as ‘Gabonionta’—is no coincidence. They appeared shortly after one of Earth’s greatest environmental upheavals: the “Great Oxidation Event”. Between 2.4 and 2.3 billion years ago (e.g. during the Huronian glaciation), free oxygen first accumulated in the atmosphere, thereby enabling the emergence of larger life forms. Only with the availability of oxygen, new pathways for energy production could be established.
An environmental challenge for these unusual ‘Gabonionta’ was the presence of arsenic in seawater. Although arsenic concentrations were not higher than those in today’s oceans, arsenic entered the organisms’ bodies alongside the essential phosphate during metabolism and began to accumulate to toxic levels. Inside the body, arsenic can deactivate proteins and disrupt energy production. At elevated concentrations, arsenic is toxic to nearly all living organisms. Nevertheless, many species have evolved mechanisms to use arsenic as a trace element.
Even the human body contains - on average - about 7 milligrams of arsenic. At some point in evolutionary history, new physiological processes emerged that allowed organisms to selectively absorb arsenic and chemically neutralize its toxicity. But when exactly did this crucial evolutionary step occur?
A team of geochemists and geologists from the University of Poitiers and the Natural History Museum Vienna has now provided an answer to that question. “In the fossils from Gabon, we observed arsenic accumulations that were not randomly distributed within the organisms but concentrated in clearly defined areas,” explains study leader Abderrazak El Albani.
The toxic arsenic was chemically reduced within the organisms and thereby detoxified. Subsequently, the now harmless arsenic was transported to a specific region inside the body and stored. “This demonstrates that for the first time in Earth’s history, these organisms developed a strategy to neutralize arsenic and possibly even utilize it as a trace element,” says co-author Mathias Harzhauser from the Natural History Museum Vienna. After the organisms died, the arsenic re-bonded with other elements and became bound in the mineral pyrite. Thus, the chemical signature of the ‘Gabonionta’ has been preserved in the rock for 2.1 billion years!
Three of these remarkable fossils from Gabon are currently on loan from the University of Poitiers and can be viewed in Hall 6 of the Natural History Museum Vienna. The study was published in Nature Communications.
Publication:
El Khoury, A., Somogyi, A., Chi Frau, E., Saleh, F., Chraiki, I., Fontaine, C., Aubineau, J., Rollion-Bard, C., Harzhauser, M., El Albani, A. 2025. A battle against arsenic toxicity by Earth’s initially complex life forms. Nature Communications, 16, 4388. https://doi.org/10.1038/s41467-025-59760-9