How long has Earth harbored life? Chemical signatures found in hardy microscopic crystals called zircons point to a beginning about 4.1 billion years ago. But finding fossilized remains of microbes—undoubtedly the creature of the day—is a far more difficult task. 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. The find may help guide scientists searching for life on other planets.
“It’s pretty impressive that anything remotely like a stromatolite is being found [in these rocks],” says Abigail Allwood, a geologist with NASA’s Jet Propulsion Laboratory in Pasadena, California, who was not involved in the study. “It’s another opportunity for us to sharpen our skills and develop rigorous techniques for the search for life.”
Earth was a tempestuous place during the Archean Eon, the name geologists give to the planet’s second act, which lasted from about 4 billion to 2.5 billion years ago. At its start, Earth was just half-a-billion years old and still fiery with remnant heat from its formation. Volcanoes were abundant, and the planet was likely too hot for plate tectonics, which may not have existed for another few hundred million years. Asteroids bombarded its surface. The atmosphere was a heady brew of methane and ammonia, with no free oxygen. The sun was fainter, shining at only about 75% of its current brightness.
If any hardy life did manage to succeed during that time, finding its traces in the Archean rock record seems unlikely. There are few outcrops of Archean rock on Earth, and they tend to be heavily metamorphosed—twisted and deeply altered by heat and pressure. Yet there have been some lucky finds: evidence for 3.4-billion-year-old fossil cells on an ancient Australian shoreline and, famously, 3.4-billion-year-old stromatolites in western Australia’s Strelley Pool Chert, which Allwood identified in 2006.
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.
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 Nature 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.
“This region just happened to avoid strong changes just by luck,” says Martin Van Kranendonk, a geologist at the University of New South Wales, Kensington, in Australia and a co-author on the paper.
The new outcrops included two sites with distinctive shapes—cones and peaks made up of thin, regular internal layers. The shapes are set against a distinctive background in the rock with a different texture and chemical makeup, the team reports. Further analyses of rare-earth elements suggest that the rocks were deposited in shallow ocean waters—just like modern microbial mats made by bacteria living in today’s oceans. Taken together, the team says, the evidence points strongly to a microbial origin for the structures—and that would make them the oldest fossilized life yet discovered on Earth.
Allwood agrees that the shapes are suggestive and interesting. 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. But those structures are at least widespread, spread out across 10 kilometers of outcrop. The new discoveries are tiny, with few structures to study, she notes. And although the overall shapes of the stromatolites have survived—perhaps against all odds—many of the textural and chemical details within them have degraded significantly.
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. Scientists had thought it unlikely that 3.7-billion-year-old rocks on Mars would have hosted life because the Red Planet—like Earth—was just coming out of a period of heavy asteroid bombardment. But if the Greenland structures are biological, Allwood says, they suggest life could have emerged in that time, and that standing bodies of shallow water would be the likeliest place to look. And because Mars lacks plate tectonics, its rocks haven’t been heavily metamorphosed by internal heat and pressure, as they were on Earth. Allwood says that means ancient signs of life might still linger.