Oldest fossils or just rocks? Scientists at odds over 3.7 billion-year-old structures.

This photograph shows putative stromatolitic structures in an outcrop (arrows). A new study says some structures point down and some point up: Evidence that these are not relics of microbial mats growing up from a sea floor but are formed through geological processes, the authors say. (Abigail Allwood/NASA/Nature)

October 17 at 1:00 PM

It was an extraordinary claim: Scientists studying a rock formation in Greenland said they had discovered Earth’s oldest fossils, a series of small, cone-shaped structures left by microbial mats some 3.7 billion years ago. The announcement in 2016 in the pages of the journal Nature generated global media coverage (including on the front page of The Washington Post) and potentially carried cosmic significance. These alleged fossils suggested life appeared on Earth soon after the planet cooled enough to be habitable. The implication was that, given the right conditions, life is common, sparking into existence quickly anywhere in the universe.

A NASA astrobiologist, Abigail Allwood, hoped it was true — she is in a field that has a rooting interest in life in the cosmos — but she wanted to take a look for herself. In September 2016 she and her colleagues traveled to the Greenland site. On Wednesday they published their findings, once again in the journal Nature, a full-throated rebuttal of the previous study. Allwood and her colleagues say the Greenland structures do not have a biological origin. They are just rocks.

The “conical” structures previously identified as fossilized stromatolites are not truly conical, the new report states. The authors say they are the cross-section of what is a ridge, an elongated structure formed through natural tectonic forces.

“They’re not ice cream cones. They’re Toblerone bars,” said Allwood, who works at the NASA Jet Propulsion Laboratory in Pasadena, Calif.

The lead author of the 2016 paper, Allen Nutman, a geologist at the University of Wollongong in Australia, released a statement saying he and his co-authors are “mystified” by the Allwood report and stand by their earlier interpretation. Nutman said Allwood focused on only one part of the Isua, Greenland rock formation, a section he and his colleagues had avoided. He said there was another site, covered in snow, that had better examples of the fossil stromatolites.

“This is a classic comparing apples and oranges scenario, leading to the inevitable outcome that ours and their observations do not exactly match,” Nutman wrote in his statement, which he sent to The Washington Post.

The new report is the latest eruption of contentiousness in the field of paleobiology, which has long been marked by sharp disagreements over what’s a genuine fossil or signature of life and what’s just a bit of interesting geology. In Western Australia, fossilized stromatolites have been dated to nearly 3.5 billion years ago and are accepted in the scientific community as the oldest known relics of early life on the planet. The field is hampered by the scarcity of rocks that old because Earth’s surface has been eroded and reworked over the past several billion years.

The age of the Greenland site is not in dispute. It is possible there are relics of early life in the geological formation. But the burden of proof is on Nutman, not on the skeptical scientist who re-examined the site, said Roger Buick, a University of Washington geologist who two years ago expressed skepticism about Nutman’s original report.

Buick said Nutman’s extraordinary claim requires extraordinary evidence. According to Buick, “the evidence should be very strong indeed. This just ain’t, and so the Allwood et al. paper is a welcome corrective to something that should never have been published in the first place with such certainty.”

Allwood said she was disappointed the Greenland structures do not appear to be biological in origin. NASA is hoping to find signs of ancient life on Mars, and an early appearance of life on Earth would make that more likely. But she said the moment she arrived at the Greenland site she knew something was amiss.

“The instant we saw the outcrop, we thought, oh, gosh, those rocks have been stretched and folded to oblivion. There’s no way on Earth they’re not deformed,” she said.

She recognizes her paper will not end the debate over what the Greenland structures signify.

“The way to resolve this kind of thing in science is to have a lot of experts go out, relevant experts, respected experts, and go out and look at the rocks and see what they think. Otherwise it’s a ‘he said, she said’," she said.

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