London: The 2.1billion-year-old, thumb-sized fossils discovered in Gabon, West Africa, represent ancient signs of multicultural life, according to palaeontologists.
Fossils of putative multicellular organisms, found in India, were nearly half a billion years younger. And not until the Cambrian period, which began about 542 million years ago, were large, complex organisms commonplace.
"We have these macrofossils turning up in a world that was purely microbial. That’s a big deal because when you finally get big organisms, it changes the way the biosphere works, as they interact with microbes and each other," Nature quoted Stefan Bengtson, a palaeozoologist at the Swedish Museum of Natural History in Stockholm and a co-author on the report, as saying.
Palaeontologists believe a rise in global atmospheric oxygen some 750 million years ago made the ``explosion`` of multicellular animals in the Cambrian period possible. Similarly, the so-called Great Oxidation Event 2.4 billion years ago might have allowed the Gabon organisms to survive, according to Bengtson.
Philip Donoghue of the University of Bristol, UK, said: "It’s just remarkable how large the fossils they’ve found are. Normally, to find fossils from this time period you need to dissolve rocks and look under a microscope."
The 21-person team analysed structure of the fossils and their chemical content using micro-computed tomography and mass spectrometry, and concluded that the specimens were not rock formations but the remains of living organisms.
They had flexible, fringed flaps surrounding their 7-120-millimetre-long bodies, which led the researchers to surmise that the organisms were multi-cellular.
The study group believes the organisms might have been colonial bacteria that signalled to one another and so were able to form complicated structures. Mats of bacteria bound with sediment existed for millions of years before the Gabon fossils. Yet the Gabon specimens look different.
Donoghue said: "These aren’t aggregations of bacteria binding sediment. They’re three-dimensional, which suggests coordinated multicellularity at a time just after the Great Oxidation Event."
Classifying the Gabon fossils is a difficult matter because they don``t resemble any fossil or living organism.
Bengtson said: "The only modern analog might be microbial colonies, but these tend to be quite small and flimsy, while these are large and thick and resilient.
"It’s quite possible they represent eukaryotes, which tend to make more resilient and larger structures."
Eukaryotes, such as animals and algae, have cells with membrane-bound nuclei. But seeing those cells is impossible here.
The fossils contain traces of sterol compounds usually found in the cell walls of eukaryotes. Coupled with their size and complexity, this evidence supports a eukaryotic affinity, said Abderrazak El Albani of the University of Poitiers in France, who is also the lead author of the Nature report.
But sterols and other organic soluble molecules can migrate into older sediment from organisms buried at a later date. And if these were eukaryotes, they would be very much older than any known members, pointed out Donoghue.
He said: "Why go out on a limb and argue that [sterol] is indicative of eukaryotes? We should take the most pessimistic view."
Bengtson anticipates debate about the report.
He said: "Early macrofossils are always a contentious subject."