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This photo shows putative stromatolithic structures in an outcrop (arrows). A new study indicates that some structures are oriented downwards and a little higher: it is proven that they are not remnants of microbial mats growing on the bottom of the sea, but resulting from geological processes, explain the authors. (Abigail Allwood / NASA / Nature)
It was an extraordinary statement: scientists studying a rock formation in Greenland claimed discovered the Earth's oldest fossils, a series of small cone-shaped structures left by microbial mats about 3.7 billion years ago. The announcement made in 2016 in the pages of the journal Nature has generated global media coverage (including on the front page of the Washington Post) and potential for cosmic significance. These supposed fossils suggest that life appeared on Earth soon after the planet cooled enough to be habitable. The implication was that, under good conditions, life is common and begins to exist quickly, anywhere in the universe.
NASA astrobiologist Abigail Allwood hoped that was true – she is in a field that is close to life in the cosmos – but she wanted to get an idea. In September 2016, she and her colleagues visited the Greenland site. On Wednesday, they published their findings, once again in the journal Nature, rebuttal aloud from the previous study. Allwood and his colleagues say that Greenland's structures do not have a biological origin. These are just rocks.
The new "conical" structures previously identified as fossilized stromatolites are not really conical, says the new report. The authors say that they are the cross section of what is an edge, an elongated structure formed by natural tectonic forces.
"They are not ice cream cones. These are Toblerone bars, "said Allwood, who works at NASA's Jet Propulsion Laboratory in Pasadena, California.
The lead author of the 2016 paper, Allen Nutman, a geologist at Wollongong University in Australia, issued a statement claiming that he and his co-perpetrators were "mystified" by the Allwood report, and that he was not the only one. in their previous interpretation. Nutman said Allwood focused on only one part of the Isua rock formation in Greenland, a section that he and his colleagues had avoided. He said that there was another site, covered with snow, which contained better examples of fossil stromatolites.
"It's a classic comparison scenario of apples and oranges, resulting in the inevitable result that our observations and observations do not match, "wrote Nutman in his statement, which he sent to the Washington Post.
The new report is the latest litigation eruption in the field of paleobiology, which has long been marked by deep disagreements over what is a true fossil or life signature and what is only an interesting geological element . In Western Australia, fossilized stromatolites date back nearly 3.5 billion years and are accepted by the scientific community as the oldest known remains of the ancient era of the planet. The land suffers from the scarcity of ancient rocks, as the surface of the Earth has been eroded and reworked over the last billion years.
The age of the Greenland site is not in dispute. It is possible that there are remnants of life early in the geological formation. But the burden of proof lies with Nutman, not the skeptical scientist who has re-examined the site, said Roger Buick, a geologist at the University of Washington, who expressed skepticism about Nutman's original report.
Buick said Nutman's extraordinary demand requires extraordinary evidence. According to Buick, "the evidence should be very solid. This is simply not the case, so that Allwood et al. paper is a welcome fix for something that should never have been published with such certainty. "
Allwood is said to be disappointed that Greenland's structures do not appear to be of biological origin. NASA hopes to find signs of ancient life on Mars, and an early appearance of life on Earth would make this more likely. But she said that as soon as she arrived on the Greenland site, she knew something was wrong.
"From the moment we saw the outcrop, we thought, oh, that's it, these rocks were stretched and folded up to the point of forgetfulness. There is no way on Earth to be deformed, "she said.
She acknowledges that her article will not end the debate about the meaning of Greenland's structures.
"To solve this kind of problem in the scientific field, we need to call on many experts, competent experts, respected experts, and look closely at the rocks and see what they think. If not, it's a "he said, she said," she said.
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