Getting it right is important for the search for life on Mars:
Found in 3.7-billion-year-old rocks in Greenland, the mounds strongly resemble cone-shaped microbial mats called stromatolites, researchers reported in 2016. But a new analysis of the shape, internal layers and chemistry of the structures suggests that the mounds weren’t shaped by microbes but by tectonic activity. The new work, led by astrobiologist Abigail Allwood of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., was published online October 17 in Nature.
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The debate highlights how important it will be to understand as much as possible about the geologic setting of a future Mars landing site, if scientists hope to spy evidence that there was once life there. It’s hard enough to collect follow-up samples to ascertain signs of ancient life in remote parts of Earth like Greenland, says Allwood, who was at a NASA workshop the week of October 15 to discuss potential landing sites for the Mars 2020 rover mission. “But you can’t arrive on Mars with no clue of what you’re looking at and then say, ‘Oh, we’ll work out the details when we get there,’” she says. “If you do that, you’re setting yourself up for failure.”Carolyn Gramling, “These ancient mounds may not be the earliest fossils on Earth after all” at Science News
Allwood’s team also found that the Australian Chert find (3.45-billion-year-old rock features) was not really fossil life. A term sometimes heard is “pseudo-fossils.”
Paper. (paywall)
In 2016, researchers found what they interpreted as stromatolites—layered formations made by sediments from microbes—in a 3.7-billion-year-old set of rocks known as the Isua Belt in Greenland. At the time, they were the oldest evidence of life on Earth by about 200 million years. But in a study published today (October 17) in Nature, another research team challenges the claim that the structures are microbe-made, proposing instead that the shapes in the rocks are due to deformations that occurred as the rocks aged.
“At face value, the results of this study provide a reminder that in geology, some observations are scale-dependent [and that] morphological analysis of putative ancient biological forms is also inherently subjective,” Aaron Cavosie, a geologist at Curtin University in Australia who was not involved in either study, writes in an email to The Scientist. “The new study presents intriguing results and earnest criticism of the earlier claims for biogenicity, but will likely not be the final word on the origin of [these] dome-like structures from Isua,” he adds. Abby Olena, “Signs of Ancient Microbial Life Questioned” at The Scientist
The original researchers are defending their work:
They said Allwood took samples from the far end of one of two sites and didn’t test the original specimens when offered.
“This is a classic comparing apples and oranges scenario, leading to the inevitable outcome that ours and their observations do not exactly match,” they said in the statement. Seth Borenstein, “Oldest fossils on Earth? New look finds might just be rocks” at Associated Press
Well, another day, another find.
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See also: Oldest fossils found in Greenland shrink time for origin of life
and
Science Mag on the ancient Greenland fossils (2016)
The original story: