Source: Astrobiology.com / Chi Fru et al (2024)
We have always learned that all modern animal groups all originated in the first tens of millions of years of the Cambrian (Cambrian explosion, start of the Fanerozoic). Before then, some sponge- and jellyfish-like fossils were known (from 635 Ma), and the ‘failed experiment’ of the Ediacara at the end of the Proterozoic or Precambrian. But a recent study argues for a much older attempt to start an animal kingdom.
About 2.1 Billion years ago (Paleoproterozoic), two African cratons collided: the Congo and São Francisco. At the height of today’s Franceville Basin (Gabon), this created a shallow inland sea that became isolated from all other oceans. According to this research, this inland sea must have been very rich in phosphorus, resulting in a prolonged bloom of oxygen-producing cyanobacteria. This biotope would have been so nutrient- and oxygen-rich that, just like at the start of the Fanerozoic, there were no limiting factors left for an ecosystem to emerge with complex multicellular organisms. The start of plants and animals, in other words. These organisms disappeared back when the Franceville sea was no longer isolated, and made contact again with the nutrient-poor and oxygen-poor conditions on the rest of the planet.
The discoverer of the macrofossils of Franceville – geologist Abdarrazak El Albani of the University of Poitiers (FR) – is co-involved in this study. But not everyone is convinced that the fossils – called Gabonionta – are really from multicellular complex macroorganisms. They are fossils up to 17 cm in size with a circular or elliptical shape. Radial structures can be seen, indicating radial growth. Some structures are reminiscent of Ediacara. Others resemble slime moulds, but the latter are not aquatic animals (the Gabonionta are). Some structures resemble strings of pearls and bodies with chambers. Moreover, the fossils contain more zinc (Zn) than their surroundings, which is normal for eukaryotes.
But a lot of other authors consider these clues too weak, and certainly not conclusive evidence that the fossils do not have an abiotic origin. So-called concretions – geological cemented fills of gaps during a sedimentation process – can have similar shapes, for example. Certain fossil figures can also arise as artefacts from ‘diagenesis’. This is a process by which certain deposits are compressed and get somewhat deeper underground, but not deep and warm enough to undergo metamorphosis. On the other hand, it is then noteworthy that macrofossils of the Gabonionta do occur in high concentration in the basin. Even more remarkable: the oldest fossil evidence of eukaryotic cells is 400 million years younger than this Gabonionta….
For now, sufficient data are lacking to give the advantage to those for or against in this discussion.