Author: Carolyn Gramling / Source: Science News
Tiny mounds touted as the earliest fossilized evidence of life on Earth may just be twisted rock.
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.
The further back in time you go, the more devilishly difficult it is to identify signs of life. Allwood herself is familiar with the skepticism that comes with making such a big claim: In 2006, she and colleagues suggested that 3.45-billion-year-old rock features found in Australia’s Strelley Pool Chert were stromatolites. Though that claim was met with skepticism initially, a growing body of research has ultimately supported it.
Then, in a 2016 paper in Nature, geologist Allen Nutman of the University of Wollongong in Australia and colleagues reported finding a series of reddish mounds within a group of ancient Greenland rocks known as the Isua supracrustal belt. Most of the belt has been twisted over time by tectonic forces. But Nutman and colleagues discovered the mounds within a portion of the belt that appears relatively unaltered, and the team presented numerous lines of evidence suggesting the structures were actually stromatolites (SN: 10/1/16, p. 7). If true, the discovery would push back the date of the earliest fossilized evidence of life by some 250 million years.
But that study, too, has been met with skepticism, including from Allwood. “The evidence that was presented was robust,” she says. “But there were a couple of things about the structures that seemed odd.” For one thing, stromatolites would all grow upward from the seafloor, forming cones that point in one direction. But one of the cone-shaped structures in the 2016 study was, curiously, oriented downward.
So Allwood took a closer look, traveling to Greenland in September 2016 to study the outcrops featuring the structures. With Greenland’s permission, Allwood’s team cut a chunk of rock off the end of one of the two sites where the structures had been found. “We took a slightly bigger sample than Nutman had taken,” she says. “That was a good thing, because it provided…
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