Threadlike filaments pressed in rock may be the remnants of archaea that burped methane near hydrothermal vents 3.42 billion years ago. ┬аIf so, these strands in rock excavated in South Africa around a decade ago, would provide the earliest direct evidence of a methane-based metabolism, researchers report July 14 in Science Advances.
Such ancient fossil filaments may contain clues about EarthтАЩs early inhabitants and hint at where to look for extraterrestrial life. ┬аScientists suspect that life on our planet could have arisen in such an environment (SN: 9/24/20).
Biologists have deduced that metabolisms based on munching or belching methane evolved early on, but donтАЩt know exactly when, says Barbara Cavalazzi, a geobiologist at University of Bologna in Italy. Previous research has found indirect evidence for methane-cycling microbes in the chemistry of fluid-filled pockets of ancient rocks from around 3.5 billion years ago. But that work didnтАЩt find the actual microbes. With this fossil analysis, тАЬwhat we find, basically, is evidence of about the same age. But this is a cellular remain тАФ itтАЩs the organism,тАЭ Cavalazzi says.
The newly identified fossil threads have a carbon-based shell. That shell is different structurally from the preserved interior, suggesting a cell envelope enclosing the cellsтАЩ insides, the authors write. And the team found relatively high nickel concentrations in the filaments. The concentrations were similar to levels found in modern methane-makers, suggesting the fossilsтАЩ metal may come from nickel-containing enzymes in the microbes.
тАЬThey can attribute a specific metabolic lifestyle to these early microorganisms,тАЭ says Dominic Papineau, a Precambrian biogeochemist at University College London, who was not part of the study and calls it тАЬbrilliant work.тАЭ
Yet the search for early life-forms has had its share of false signs (SN: 2/9/21) , and some researchers arenтАЩt convinced these fossils are the real deal. In silica-rich hydrothermal environments, the ingredients for structures that mimic cells mingle and can form life look-alikes through chemistry, says Julie Cosmidis, a geobiologist at the University of Oxford. тАЬThey fossilize better than actual cells, so I think it could very well be what those things are,тАЭ she says, pointing out that nickel, common in the early Earth, clings easily to organic matter, whether itтАЩs living or not. тАЬWe donтАЩt understand enough [about] the processes that can create false biosignatures,тАЭ says Cosmidis, whose lab studies such questions.
But Cavalazzi and her colleagues contend that the different lines of evidence together support the living origin of the microfossils. Papineau also notes that тАЬthe evidence is very good,тАЭ but adds that itтАЩs тАЬnot necessarily rock solid.тАЭ Other tests could strengthen the case for the earliest methane-using microbes, he says.
If the strands are ancient archaea, theyтАЩd become the earliest fossil evidence for this domain of life, predating specimens from less than 500 million years ago. And if such microbes evolved so quickly on Earth, within around 1 billion years of the planetтАЩs origin, methane-cyclers may be more common than realized on other planets where liquid water has been around for a while, Papineau says.
This fossil comes from a time when EarthтАЩs planetary ecosystem was likely very different than it is now, says Boris Sauterey, a paleoecologist at the University of Arizona in Tempe, who wasnтАЩt involved with this study. Back then, Earth most likely shared similarities with some of the extraterrestrial worlds that we today would consider potentially habitable, he says.
Researchers looking for signs of early life on Earth have explored sediments of surface waters more than hydrothermal systems, where these fossils were found, Cavalazzi says. The discovery suggests, she says, that here and on other planets, researchers should keep scratching below the surface.
