In the history of life on Earth, the first chapter is still the most incomplete—and any good epic needs its origin story. The problem with finding that story is preservation. The earliest lifeforms were microscopic sacks of organic chemistry, so finding evidence for them, as far as needles in haystacks go, is not exactly equivalent to spotting a six-foot Apatosaurus bone. To make matters worse, most of the haystack has been burned to a crisp by geology since then.

Fossil evidence goes back about 3.5 billion years, with controversial isotopic signs that might signify life about 3.8 billion years ago (or perhaps even earlier). At this age, you run out of rocks. Although the planet is about 4.5 billion years old, very few rocks have survived for more than 3.5 billion years. The ones that have look their age, metamorphosed so much over the eons that signs of life might have been erased.

Still, the quest to push back the earliest evidence for life goes on. New finds are subject to rigorous debate, and researchers have to work hard to figure out whether a physical process could be responsible for a feature that has the appearance of a fossil.

In a newly published paper, a team led by University of Wollongong researcher Allen Nutman describes a find from Greenland it thinks represents a 3.7 billion-year-old microbial community. That would push back the earliest fossil evidence by more than 200 million years.

The find comes from a belt of ancient, metamorphosed sedimentary rocks in southwest Greenland that has long been targeted for this kind of thing. These same rocks are the source of the isotopic signal some have interpreted as indicative of nearly 3.8 billion-year-old life. More of the study area is now accessible, though, because the snow that perpetually blanketed it now melts off in the summer.

The researchers haven’t found microscopic fossils there, but they have found macroscopic structures they believe are stromatolites. Stromatolites are small, layered mounds of sediment built by mats of microbes living in shallow water. They’re still found in a few places today and can commonly be found throughout the geologic record due to their distinctive structure of fine, concentric layers. Stromatolites can range in size from a few centimeters across to more than a meter—all of which is significantly easier to spot than a single, fossilized microbe.

At their Greenland outcrop, the researchers found metamorphosed sedimentary rocks that would have been deposited in shallow water near shore. In two spots, there are several layers that each sport a few stromatolite-shaped bumps of sediment several centimeters across. Because of all the metamorphism these rocks have experienced, the original grains of sediment have congealed together into larger crystals of altered mineral, largely obscuring any possibility of visible stromatolite layering.

Nevertheless, by some chance, several of them somehow escaped with visible layering, and that layering is stromatolite-like. On the strength of that evidence, the researchers argue that these aren’t just sedimentary look-alikes. They also suggest that chemical analyses showing differences in trace element concentrations inside and outside the bumps match patterns seen in stromatolites elsewhere.

If they’re right, communities of microbes were building stromatolites fully 3.7 billion years ago. What’s more, the researchers argue that this would be consistent with the hypothesis that life actually arose more than 4 billion years ago—meaning life got going pretty quickly despite our planet's continuing bombardment by leftover debris from the formation of the Solar System.

But as with every purported discovery like this, many scientists will be skeptical. It took a long time for 3.4 and 3.5 billion-year-old stromatolites in Australia to be confirmed, for example. Paris Institute of Earth Physics researcher Pascal Philippot, who has worked on those oldest Australian stromatolites, told Ars that having only a few specimens to look at makes this tricky. “If you went to Australia or South Africa, you need to follow those stromatolite layers over tens, hundreds of meters—eventually kilometers—to be sure that they are real stromatolites,” Philippot said.

Saturated sediments can sometimes be squeezed around during earthquakes, for example, forming lumps that would have to be ruled out. And these Greenland specimens are found within a fold—an arch of rock layers deformed at high pressure. Within larger folds, you can sometimes find smaller ones, like the crinkled edges of lasagna noodles. That, too, is a potential source of confusion.

“I don’t want to be too skeptical, I like the idea of having some stromatolites preserved, but OK, maybe there is some new stuff that may come out in a few months or years arguing that maybe it’s not the case,” Philippot said. “It’s a natural object; it’s not physics, right? You don’t have only one variable; you have plenty, so you need to have a network of evidence in favor of the biogenic interpretation.”

Philippot worries that the team's chemical evidence could be misleading because of all the metamorphism that rock has seen. But if some organic carbon could be found, unlikely as that may seem, the case could become much stronger.

In the meantime, we at least have a serious candidate for the earliest page in the history of life. In an article accompanying the paper in Nature, Jet Propulsion Laboratory researcher Abigail Allwood writes, “If life could find a foothold here, and leave such an imprint that vestiges exist even though only a minuscule sliver of metamorphic rock is all that remains from that time, then life is not a fussy, reluctant and unlikely thing. Give life half an opportunity and it’ll run with it.”

Earth isn’t the only place that lesson might apply to, Allwood reminds us. “A plethora of Mars missions has shown that around the time that the Isua rocks [in Greenland] were forming, Mars did not look too different from Earth from a habitability perspective, with standing bodies of water at the surface.”

Nature, 2016. DOI: 10.1038/nature19355, 10.1038/nature19429 (About DOIs).