Approximately 2.1-billion-year-old fossilized tracks discovered in Gabon suggest the existence of a cluster of single cells that came together to form a slug-like multicellular organism that moved through the mud in search of a more favorable environment.
Reconstructions of string-shaped structures — named Gabonionta — from the site Francevillian in Gabon; white and yellow arrows point to string-shaped specimens and microbial mats, respectively: (A) volume rendering showing the external surface of straight structures; inset (B) shows enlargement of string ending with a pyrite crystal (black arrows); (C) external surface volume rendering showing weakly sinuous string; (D) external surface volume rendering, frame denotes the position of subvertical tubes; (E) external volume transparencies of the same specimen as in D, lateral view showing the string-shaped specimens inside the host rock; frame denotes the position of subvertical tubes; (F and G) external volume transparencies of the same sample as in D and E at different heights in the sample; (H) twinned contorted strings; box denotes portion (cross-section) figured in I; (I) virtual cross-section of contorted strings, black arrow points to the precompactional deformation of silty shale laminae. Scale bars – 1 cm. Image credit: El Albani et al, doi: 10.1073/pnas.1815721116.
“The preservation of fossilized tracks, or trace fossils, suggests that multicellular organisms that could move around to reach food resources may already have existed 2.1 billion years ago, more than 1.5 billion years older than previously thought,” said University of Alberta’s Professor Kurt Konhauser, co-author of the study.
The trace fossils were found at the site of Franceville in the Haut-Ogooué Province of Gabon, Africa.
Preserved as tubular structures, they range from 6 mm across and 1.7 cm in length through the sediment layers and are likely the result of ancient mucus trails, left by multicellular life such as modern amoeboid cells in the search for food.
“A detailed 3D analysis using a non-destructive X-ray imagining technique, alongside geometrical and chemical dating, revealed that the fossils belong to an organism that likely spent most of its time in oxygenated waters, and was therefore oxygen-dependent,” the paleontologists said.
“Located next to these tubular structures were fossilized microbial biofilms which acted as grazing grounds for these organisms.”
Volume rendering showing continuity between sheet and string morphologies in a single specimen. Scale bars – 1 cm. Image credit: El Albani et al, doi: 10.1073/pnas.1815721116.
“It is plausible that the organisms behind this phenomenon moved in search of nutrients and oxygen that were produced by bacteria mats on the seafloor-water interface,” said co-author Dr. Ernest Chi Fru, a researcher at Cardiff University.
“This sea floor environment was calm and shallow at the time, and rich in oxygen following the first wave of the Great Oxygenation Event, which began approximately 300 million years earlier,” said first author Dr. Abderrazak El Albani, from CNRS and the University of Poitiers.
“The rise in ocean oxygen levels at this time could have played an important role by providing the energy needed for both the metabolism and development of these multicellular organisms, as well as for their movement.”
“The question arising from this research then is why do we go 1.5 billion years before we see similar features in the rock record. We don’t see anything like this again until 585 million years ago,” Professor Konhauser said.
Some scientists speculate that this early emergence of complex life went extinct due to some environmental factor.
Others suggest that similar fossilized traces may have existed but were not preserved, or simply gone unnoticed elsewhere.
“The broader community is right to be skeptical about the interpretation,” Professor Konhauser said.
“However, one of the current paradigms relating to the evolution of multicellular organisms is oxygen availability, and 2.1 billion years ago there was no shortage of oxygen in shallow marine waters.”
The research appears in the Proceedings of the National Academy of Sciences.
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Abderrazak El Albani et al. Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago. PNAS, published online February 11, 2019; doi: 10.1073/pnas.1815721116
