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On this photo from a Greenland research site, the arrows indicate the structures that a team of paleontologists from 2016 suggested to have been created by microbial life. Note that while most structures are oriented in one direction, the red arrow indicates that direction in another direction. This is a clue, say other scientists, that these structures have not all developed from the bottom of the sea.
Courtesy of Abigail Allwood
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Courtesy of Abigail Allwood
On this photo from a Greenland research site, the arrows indicate the structures that a team of paleontologists from 2016 suggested to have been created by microbial life. Note that while most structures are oriented in one direction, the red arrow indicates that direction in another direction. This is a clue, say other scientists, that these structures have not all developed from the bottom of the sea.
Courtesy of Abigail Allwood
The earliest evidence of life on Earth is probably not found in rocks 3.7 billion years old found in Greenland, despite what a group of scientists said a few years ago. .
It's according to a new analysis, published Wednesday in the newspaper Nature by another team of experts.
This second group examined the rock structures that, in 2016, would have been produced by communities of unicellular microbes, born from the bottom of a shallow and salty sea. A three-dimensional look at these structures shows that instead of having a shape of ice cream cone upside down – the type produced by microorganisms – they have the shape of a Toblerone chocolate bar.
"These are extensive crests that extend deep into the rock," said Joel Hurowitz, geochemist at Stony Brook University in New York and author of the journal Mercredi. "This form is difficult to explain as a biological structure and much easier to explain as a result of compaction and deformation of rocks under tectonic pressures."
When asked what were the chances of these structures being created by ancient microbes, astrobiologist Abigail Allwood – of NASA's Jet Propulsion Laboratory and lead author of this second analysis – says "I do not think there is much luck at all."
The oldest widely accepted evidence of life on Earth is found in Western Australia, where Allwood and other researchers have revealed cone-shaped structures created by microbes in 3.5 billion-year-old rocks. ; years. A decade later, in 2016, she and others were surprised when scientists reported seeing similar structures in Greenland, in rocks 200 million years old, that had suffered from much more of heat and deformation under pressure, during the first days of the Earth.
"Preserving those in such deformed rocks seemed unbelievable," says Allwood. "And in fact, that was it, it was not credible."
That was his conclusion after visiting the Greenland site to collect rock samples. She immediately noticed that some of the structures were upside down, in a configuration that would not have been possible if they had been created by communities of microbial life growing from the bottom of the sea.
Allwood brought rock samples back to the lab to map the chemical composition of the structures with a prototype instrument called PIXL. This is a tool that NASA will send on Mars in 2020 to look for evidence of past life on the red planet. She and her colleagues discovered that the so-called fossils do not have the same kind of stratification as when microbes accumulate minerals like a stack of pancakes. Instead, says Allwood, it appears that the Toblerone rock structure was formed and other minerals infiltrated thereafter.
All of this is vigorously contested by researchers who originally claimed that Greenland the rocks contained the oldest fossils in the world. They stick to this statement and say that Allwood and his colleagues based their work on a simple, one – day, summary visit to the site.
"It's a classic scenario of" comparing apples and oranges, "which results in an inevitable result that our observations and observations do not match," Allen Nutman notes. 39, University of Wollongong, Australia, in a written statement.
Vickie Bennett, from the Australian National University, added that she found the new study "disappointing" and "unfortunate" in the sense that it "simply blurs" previous research that she and her colleagues had done. on these ancient rocks.
"Basically, they did not bend over the same rocks – and the details matter," Bennett told NPR in an email. In his opinion, the rocks of this study are "a poor cousin equivalent to those in our original study" and the new analysis "was not conducted with caution".
But Allwood rejects these claims, noting that she sampled rocks within one meter of the original sampling site.
She also points out that Nutman recently claimed that Allwood 's group had sampled the well – preserved material – as it had complained that too much of the "key outcrop" had been taken by the company. other researchers and wasted. Allwood says his team has carefully collected their samples, with the permission and guidance of the Ministry of Mineral Resources of Greenland. She hopes to come back next year with other colleagues.
"I find it hard to imagine that anyone who will see these rocks will argue that structural deformation is not at least a strong possibility for which these rocks look like this," Hurowitz said. "I hope people will look at this and say," Well, that's the science that does what it's supposed to do. " "We are supposed to test the assumptions of people and see if they resist closely."
Mark van Zuilen, at the Institute of Earth Physics in Paris, France, wrote a comment on this new work. He was not a member of any of the two research teams and points out that the rocky outcrop in Greenland has not been discovered for a long time, and few researchers have studied this rock in relation with its geological environment ".
Van Zuilen believes that more research could lead to a better understanding of all the forces that shaped it.
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