More than 50 years ago a group of schoolchildren discovered a fronded fossil (Charnia) in the Precambrian rocks of Charnwood Forest in the English Midlands. Since then it has been clear that multicellular life originated before the Cambrian Period, when the first tangible life had previously been considered to have emerged. Discovery of the rich Ediacaran fauna of quilted, baglike and disc-like animals in 635 Ma old Neoproterozoic sediments in South Australia, and many other occurrences re-established the start of the ‘carnival of animals’ in the Ediacaran Period (635 to 541 Ma). It happened to follow the climatic and environmental turmoil of at least two Snowball Earth episodes during the preceding Cryogenian Period (850 to 635 Ma), which has led to a flurry of suggestions for the transition from protozoan to metazoan life. Yet, applying a ‘molecular-clock’ approach to the genetic differences between living metazoan organisms seems to suggest a considerable earlier evolutionary event that started ‘life as we know it’. That may have been confirmed by a discovery in much older sediments in Gabon, West Africa.
A sequence of shallow-marine sediments in the Francevillian Series in Gabon was laid down at a time of fluctuating sea level around 2100 Ma ago, when the upper oceans had become oxygenated. In them are black shales that preserve an abundance of intricate sedimentary features. Among them are curious stringy structures rich in crystalline pyrite (Fe2S). They are infilled wiggly tubes that lie in the shale bedding. CT scans reveal that the bedding has been flattened around the tubules as it became lithified. So the tubes formed while the sediment was wet and soft (El Albani, A. and 22 others 2019. Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago. Proceedings of the National Academy of Sciences, online preprint; DOI: 10.1073/pnas.1815721116). They look very like burrows. Up to 5 mm across, they can be considered large by comparison with almost all organisms known from that time. The exception comes from the same stratigraphic Series in Gabon. In 2010, El Albani and colleagues published an account of fossils preserved by pyrite that look like fried eggs, 1 to 2 cm across, with scalloped edges. Internal structures revealed by CT scanning include radial slits in the ‘whites’ and folding within the central ‘yolk’. That paper reported the geochemical presence in the host shales of steranes, which are breakdown products of steroids that are unique to eukaryotes. Could these organisms and the wiggly tube-like trace fossils indicate the presence of the earliest metazoans in the Francevillian Series?
Until the discoveries in Gabon, the oldest organic structure that had been suggested to be a metazoan was the rare Grypania, a spiral, strap-like fossil found in a variety of strata ranging in age from 1870 to 650 Ma. Being made of a structureless ribbon of graphite, Grypania seems most likely to have been made by colonial bacteria. The two Gabon life forms cannot be disposed of quite so easily. The discoids have organised structures rivalling those in Ediacaran animals, while the wiggly tubes clearly seem to indicate something capable of movement. In both cases preservation is by iron sulfide, which suggests the presence at some stage of chemo-autotrophic bacteria that reduce sulfate ions to sulfide. Could these not have formed mats taking up irregular discs and plates? The burrows may have been formed by unicellular eukaryotes, one type of which – the slime moulds – is capable of aggregating together to form multi-celled reproductive structures as well as living freely as single amoeba. Some form slug-like masses that are capable of movement; not metazoans, but perhaps their precursors.