They could be the earliest fossils ever found. Structures discovered in 3.7-billion-year-old rocks in Greenland appear to be evidence of microbes living in a shallow sea on early Earth.
The structures, all no more than a few centimetres tall, look like stromatolites. These are layered mounds that were – and still are – formed by photosynthetic microbes living in water.
The fossils’ age makes them about 220 million years older than any previous fossil found. They come from a region of south-west Greenland called the Isua supracrustal belt.
“This is one of the extremely few places where this kind of feature could still be preserved in the rock record,” says Allen Nutman at the University of Wollongong, who led the team.
That’s because most rocks this old have been heavily altered by heat and plate tectonics in the billions of years since they formed. Those geological processes easily obliterate fossils, or leave them warped beyond recognition.
But a small section of the Isua region has escaped complete transformation. Its rocks have been only partially altered, allowing that area to retain some of its original characteristics – potentially including fossils, Nutman says.
Nutman and his colleagues found their possible stromatolites at two locations where the bedrock is now exposed, although in past years it was covered by snow all year round. Analysing samples in the lab showed that the structures have the same chemical signature as sea water, pointing to a marine origin. And their shape – bulging up from the sediment layer below, with hints of layers visible in the rock – mirrors that of modern stromatolites.
However, some doubts remain.
“These folks have done a very nice piece of work describing them, showing that they are very much like younger stromatolites,” says William Schopf at University of California Los Angeles.
The main problem, he says, is that the Isua rocks contain no fossil remains of microbes. “To seal the case, you’d really like to have the microorganisms preserved, but that’s not possible in this type of rock in that sort of setting.”
It’s plausible to conclude they are stromatolites, says David Wacey at the University of Western Australia in Perth, but more detailed testing is needed to completely rule out formation through physical or chemical processes.
“While we now know that the Isua material contains exciting sedimentary structures, we are still a little way from this being 100 per cent proof of life at 3700 million years ago,” Wacey says.
“It’s an extremely interesting find,” says Abigail Allwood at the Jet Propulsion Laboratory in Pasadena, California. But the altered nature of the structures, and their limited number, complicate attempts to figure out their origin, she says.
If the rocky formations are stromatolites, the microbes that created them were likely part of a larger community that had already evolved on Earth, Nutman says.
“What we have in Isua is just a tiny sample of any life that may have been around at that time,” he says. “It would be like going to somewhere on Earth now and picking up some shells from a beach and getting the impression, ‘Oh, that’s the full diversity of life that is on the planet’.”
Journal reference: Nature, DOI: 10.1038/nature19355