Evidence of the discovery of the oldest fossils on Earth is the subject of close scrutiny

Tiny mounds vaunted as the oldest fossilized evidence of life on Earth could be twisted rocks.

Found in rocks 3.7 billion years old in Greenland, the mounds are very similar to cone-shaped microbial mats called stromatolites, researchers reported in 2016. But a new analysis of the shape, layers Internal and structural chemistry suggests that montains are not formed by microbes but by tectonic activity. The new work, led by astrobiologist Abigail Allwood of NASA's Jet Propulsion Laboratory in Pasadena, California, was published online Oct. 17. Nature.

The more you go back in time, the more difficult it is to identify the signs of life. Allwood herself is aware of the skepticism surrounding the assertion of such an assertion: in 2006, she and her colleagues suggested that the 3.45 billion-year-old rock formations found in Australian film Strelley Pool Chert were stromatolites. Although this claim initially sparked skepticism, a growing body of research finally supported it.

Then, in a 2016 article in Nature, geologist Allen Nutman of Wollongong University in Australia and his colleagues reported finding a series of reddish mounds in a group of ancient Greenland rocks known as the Isua supracrustal belt. Most belts have been twisted over time by tectonic forces. But Nutman and his colleagues discovered the mounds in a part of the belt that seems relatively unchanged, and the team presented extensive evidence suggesting that the structures were actually stromatolites (SN: 01/10/16, p. 7). If true, the discovery would postpone the date of the oldest fossilized proofs of life of some 250 million years.

But this study also generated skepticism, including from Allwood. "The evidence presented was solid," she says. "But some structures seemed strange about structures." On the one hand, stromatolites all grew from the seabed, forming cones that point in one direction. But one of the cone-shaped structures of the 2016 study was curiously looking down.

Allwood took a closer look at the situation by visiting Greenland in September 2016 to study outcrops on the structures. With permission from Greenland, the Allwood team cut a piece of rock at the end of one of the two sites where the structures had been found. "We took a slightly larger sample than Nutman," she says. "It was a good thing because it provided the context that gave us the answer."

This sample gave a more three-dimensional image of one of the structures. Although cone-shaped, seen from one side, the structure was flatter, more like an edge than a cone. That's important, Allwood says. Cones "are difficult to produce other than by biology. If it is an elongated ridge, it is no longer part of this special category. "

His team analyzed several other sources of data that may indicate stromatolite, including the inner layers, the chemical composition, and the environment in which the mounds probably formed. Stromatolites are carpets that develop slowly in shallow marine environments as marine algae secrete one layer after another of carbonate-rich sediments.

But the new structural and chemical analyzes of the thin layers within Greenland's structures suggest that these layers are actually fronts of chemical weathering. The Allwood team explains that the fronts were produced by the gradual reaction of carbon-rich fluids circulating in the silicate-rich rock, part of the long history of tectonic alteration in this belt. "It's like taking a piece of sponge cake, dropping it in alcohol and watching it change from the outside," says Allwood.

In addition, the rocks surrounding the structures show signs of metamorphic deformation, changes in the rock such as minor folds and cleavage caused by heat and pressure. Allwood's team finally concluded that the structures were probably formed tectonically rather than biologically.

Nutman and his colleagues stick to their interpretation. In an email, Nutman suggested that the sample collected by Allwood 's team was not representative of the other structures: The sample appeared at the edge of the panel. one of two outcrops described in the initial study – a site more tectonically altered than the other parts of the site. "It's a classic scenario that compares the apple and orange scenarios," wrote Nutman.

Mark van Zuilen, a geomicrobiologist at the Paris Institute of Earth Physics, who studied Isua's supracrustal belt, said that at the time of publication of the 2016 paper, he was excited. But he also asked about the deformation characteristics in the rocks. The new study reveals "obvious signs of deformation," says van Zuilen, who wrote a commentary accompanying the new study. "The big problem is that these rocks are so old and metamorphosed. They have been completely recrystallized and deformed under high pressure and high temperature, "he says. "That means we're still figuring out what features of the rock are primary and what characteristics have been caused at a later stage."

The discovery of Nutman and his colleagues is so recent, van Zuilen notes, that few scientists have had the chance to see the structures. Future research could help clarify the structure of the structures, he says.

The debate underscores how important it will be to understand as much as possible the geological setting of a future landing site on Mars, if scientists are spying evidence that there was a life before. It is already quite difficult to collect follow-up samples to identify signs of ancient life in isolated areas of the Earth, such as Greenland, said Allwood, who was attending a NASA workshop the week of October 15 to discuss potential landing sites for the March 2020 rover mission. "But you can not get to Mars without having a clue what you're looking at and then say," Oh, we'll fix the details when we get there, "says "If you do that, you are preparing for failure."