Tiny structures found in a rock that is at least 3.75 billion years old may support the idea that microbial life on Earth originated around underwater hydrothermal vents – but the findings are controversial.
Exactly when and how life emerged on Earth is widely debated. All we know is that it occurred sometime after Earth formed 4.5 billion years ago and before the earliest confirmed microbial fossils appeared around 3.4 billion years ago.
Where on Earth life began is also in question. Some researchers believe microbes first sprang up around hydrothermal vents, where iron-rich water heated by magma passes through cracks in the sea floor. Others think life began in hot geothermal ponds found on land. And some believe life was transported to Earth through objects travelling from elsewhere in the universe, an idea known as panspermia.
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In 2017, at University College London and his colleagues analysed rocks collected from the coast of northern Quebec, Canada, in an area known as the Nuvvuagittuq Greenstone belt. By imaging sections of rock, the team found tiny tubes and filaments made of iron oxide – or rust – that resembled structures formed by bacteria that live in deep-sea hydrothermal vents today.
Now, the researchers have analysed a fist-sized rock from the same site by slicing it into pieces that are more than twice as thick as before – around 100 micrometres wide. This allowed them to get a bigger picture of the structures in the samples, which revealed a centimetre-long pattern of corkscrew-shaped iron filaments, arranged as a stem with parallel branches.
“The largest fossil in the rock is nearly a centimetre long and is organised in a structure that is tree-like. It’s very beautiful because the structures are red in colour,” says Papineau. “And those branching filaments are twisted like corkscrews, which has been thought of as a sign of life because [non-biological] processes are not known to make them.”
The tree-like pattern is surrounded by bubble-like ellipsoids that the team suggest could also be a result of microbial activity. But as these ellipsoids are known to sometimes result from certain types of non-biological chemical reaction, the team call them dubiofossils.
By analysing the mixture of chemicals found in the rock around the structures, the researchers argue that early microbes on Earth may have used iron, sulphur and potentially carbon dioxide and light to generate energy in a form of photosynthesis that didn’t require oxygen.
“The paper provides one of the most comprehensive analyses I’ve seen of [supposed] microfossils in 3.75-billion-year-old rock,” says at the Bigelow Laboratory for Ocean Sciences in Maine.
But further work is needed to establish whether non-biological reactions could have produced the structures. “The work challenges the field to come up with [non-living] mechanisms to produce similar structures under the likely conditions that would have existed on early Earth,” says Emerson.
Others are unconvinced. They point out that the ancient rocks have had a stressful existence since they formed billions of years ago, including being buried deep in the Earth’s crust for some time, where they were baked at temperatures of over 500°C and exposed to pressures that might have created the filamentous structures through non-organic processes.
“I think that this is a very sound study, although I do not agree that the filamentous structures or the ellipsoids are fossil bacteria,” says at the Centre for Molecular Biophysics in Orleans, France. “I am particularly concerned about the parallel [nature of the] filaments,” she says. This makes it look like they follow inorganic structures in the rock.
Westall says the filaments might relate to reactions that took place in the rock when it was exposed to high heat and pressure. “This is not a microbial feature,” she says.
at the University of New South Wales in Sydney is also sceptical. “I’m not convinced at all. The rocks are highly deformed and strongly affected by [high heat and pressure environments],” he says. The tree-like structure could simply result from a crack filled by iron that then recrystallised, he adds.
Another sticking point is that the diameters of the filaments are five to 10 times larger than the microbes that are involved in metal deposition today. But we don’t know very much about how iron-eating microbes fossilise, Emerson points out.
Nevertheless, Westall doesn’t rule out that life may have once lived in the ancient environment represented by the rock. “It is possible that the chemical sediments [in the rock]… did host traces of life associated with hydrothermal vents. Life forms using some kind of metabolism based on sulphur is plausible,” says Westall.
Papineau and his colleagues suggest that establishing whether the structures are truly microbial fossils could eventually help with efforts to identify signs of ancient life on Mars.
“If you just need water and volcanism, then, you know, there’s a good possibility that [microbial life] might have existed on Mars, and you can look for similar structures to work out if life was there,” says Papineau.
Science Advances
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