Francevillian Biota: Gabon's 2.1-Billion-Year-Old Fossils

The Francevillian biota is a group of centimetre-scale, fossil-like structures preserved in black shales about 2.1 billion years old in the Franceville Basin of south-eastern Gabon. First reported in 2010, they have been interpreted by their discoverers as the remains of early large, possibly multicellular organisms, which would make them far older than any widely accepted complex life. Their nature remains one of the most debated questions in early palaeontology.

Discovery in Gabon

The Franceville Basin, near the town of Franceville in Gabon’s Haut-Ogooué Province, contains a thick sequence of little-altered sedimentary rocks known as the Francevillian Series. Because these rocks have not been strongly heated or deformed, they preserve fine details of ancient sea-floor environments unusually well.

Around 2008, a team led by the geologist Abderrazak El Albani of the University of Poitiers collected unusual flattened structures from the FB2 unit, a layer of black shale deposited in a shallow marine setting. In 2010 the team published its findings in Nature, describing more than 250 specimens. Later collecting raised the total to several hundred.

Dating of volcanic material and minerals in the basin places the fossil-bearing layer at roughly 2.1 billion years old, with published estimates ranging from about 2.14 to 2.08 billion years. Some specimens have since been shown in museum exhibitions, including at the Natural History Museum in Vienna, where they were presented under the informal name “Gabonionta”.

What the structures look like

The specimens are flat, lobed or disc-shaped objects up to about 12 centimetres across. Many have a scalloped or radially folded edge and a denser central area. They are made largely of pyrite, an iron sulphide mineral that replaced the original material soon after burial.

The researchers used X-ray micro-tomography, a technique that builds three-dimensional images from many X-ray scans, to study their internal structure without breaking them. They argued that the regular, coordinated shapes and the chemistry of the surrounding sediment pointed to living organisms that grew in colonies.

A 2014 study in PLOS ONE described a wider range of forms, and a 2019 paper in the Proceedings of the National Academy of Sciences reported string-like, tubular structures that the team interpreted as possible traces left by organisms moving through soft mud. If correct, that would be the oldest known evidence of motility, or self-propelled movement, by a relatively large organism.

The debate

The central question is whether the structures are biological at all. Pyrite can grow into concretions with complex, rounded shapes, and some geologists have suggested that at least some specimens are mineral growths or the remains of microbial mats rather than true organisms.

Even researchers who accept a biological origin disagree about what the organisms were. Possibilities include colonies of bacteria, early eukaryotes (complex cells with a nucleus) or an independent and short-lived experiment in multicellularity that left no descendants. None of the specimens has been linked to any living group.

The discoverers note that the shales were deposited shortly after the Great Oxidation Event, when oxygen first accumulated in the atmosphere and shallow seas. They suggest that a local rise in oxygen and nutrients may have allowed larger organisms to appear briefly. Critics point out that such an early burst of large life would be difficult to reconcile with the rest of the fossil record.

Context in the history of life

Widely accepted microfossils of eukaryotic cells appear in rocks around 1.6 to 1.8 billion years old, and complex multicellular organisms become common only with the Ediacaran biota, roughly 575 to 539 million years ago. The Francevillian structures, if biological and multicellular, would push the story back by well over a billion years.

The Franceville Basin is also famous for a completely different reason. Its uranium deposits at Oklo host natural nuclear reactors, where fission chain reactions took place spontaneously about 2 billion years ago. French scientists identified them in 1972 after noticing an unusual shortage of uranium-235 in ore from the mine.

Timeline

• c. 2.14–2.08 billion years ago: Black muds of the FB2 unit are deposited in a shallow sea in the Franceville Basin.
• c. 2 billion years ago: Natural nuclear fission reactions occur in uranium deposits at Oklo in the same basin.
• 1972: French scientists identify the Oklo natural reactors.
• c. 2008: El Albani’s team begins collecting the unusual structures from the FB2 shales.
• 2010: The discovery of more than 250 specimens is published in Nature.
• 2014: A follow-up study in PLOS ONE describes a broader diversity of forms.
• 2019: Possible traces of movement are reported in the Proceedings of the National Academy of Sciences.

Why it matters

The Francevillian biota challenges the standard picture that large life appeared only in the last 600 million years or so. Whether the structures turn out to be early organisms, microbial colonies or mineral growths, the debate sharpens how palaeontologists decide what counts as evidence of ancient life, and it focuses attention on how the origin of eukaryotes and rising oxygen shaped what was possible on the early Earth.