To find more direct evidence, scientists searched for rocks untouched—or at least less altered by—metamorphism.That’s a needle-in-a-haystack sort of hunt, but a team led by geologist Allen Nutman of the University of Wollongong in Australia reports online this week in that is has finally struck pay dirt, thanks to an unseasonably early spring rainfall that washed away the snow from a promising, largely unmetamorphosed ISB outcrop in Greenland.But, she cautions, “it’s a chalk and cheese comparison with the stromatolites of western Australia—which are controversial in themselves.” Convincing scientists that the 3.4-billion-year-old western Australia stromatolites were made by microbes was a “hard sell,” she says.
Still, Allwood says, the discovery offers a tantalizing new “biosignature” for scientists hunting for signs of past life—whether on Earth or elsewhere.
On Mars, for example, scientists are unsure which sites have the best possible chance of finding life, a key target of space exploration vehicles like the Mars rover.
But the oldest known rocks on Earth are in Greenland, where a formation known as the Isua supracrustal belt (ISB) dates to between 3.7 billion and 3.8 billion years old.
Scientists have pored over the belt for signs of life, but until now found only indirect evidence—chemical signatures of carbon and sulfur isotopes that might have been the handiwork of microbes.
The reddish peaks in this 3.7-billion-year-old rock may be structures made by microbes in a shallow ocean—if so, they would be the earliest known evidence of life on Earth. Now, scientists say they have identified fossilized microbial mats, called stromatolites, in Greenland that date to about 3.7 billion years ago—nearly 300 million years older than the previous fossil record holder.