Oops, the oldest fossils ever found could be just rocks

"In their defense," she adds, "fieldwork in Greenland is very difficult to do. But for our defense, that's why the good weather and the good exposure allowed us to see everything clearly. "

Back in their lab, she and her colleagues analyzed the chemical composition of the Greenland Rock and found more evidence to support their interpretation. The true stromatolites, like those of Australia, should have internal layers, but not those of Greenland. Instead, they are made of almost pure silicon inside, with edges of dolomite minerals separating them from the overlying rock. These structures are not the work of microbes, says Allwood. Instead, they were created when liquids containing dolomite minerals infiltrated into pieces of silicon and crystallized, "like chocolate soaked in a vanilla sponge".

Nutman states that the inner layers, though less well preserved than younger Australian stromatolites, are found in other samples from Greenland. And he points to the distinct chemistry of cones. They have lower titanium and potassium concentrations than surrounding rocks, as well as unusual levels of yttrium and other rare elements that indicate seawater. These signatures suggest that cones are not just random fragments of rock that have been folded into stromatolite form, but the work of marine microbes removing fragments of minerals from the ocean.

Not so, said Allwood. Titanium and potassium mean nothing; they are also exhausted in other parts of the outcrop outside the cones. For yttrium and other rare elements, a more detailed analysis shows that they are concentrated in microscopic particles of mica and quartz, minerals that may form in the rocks at a later stage of their existence. . "It has nothing to do with biology," says Allwood. (Nutman does not find this plausible and notes that other rocks in the same area do not have the same signature. "We maintain our interpretation," he says.)

Phoebe Cohen, a paleontologist at Williams College who did not participate in either study, thinks Allwood's interpretation is more likely. "Exceptional requests require exceptional data to back them up, and while the [team] did a good job in collecting evidence of their application, it was not quite convincing, "she says." This follow-up study is exactly what I would have hoped. "I'm sure it will only be not the last time an article on the "oldest proofs of life" will be refuted by other research, but I hope to be convinced one day! "

Bennett says that the controversy has not changed much, since there is other evidence showing the existence of life more than 3.6 billion years ago. "It has become difficult to challenge the presence of old life from the very beginning of the rock record on Earth," she says. And again, Allwood does not agree. Beyond the Australian stromatolites, less than 3.5 billion years old, "we have no other unequivocal proof," she says. "That does not mean that there is was not microbes around, but we can not stick to that fact. "