In 2016, a series of unassuming stone shapes rocked the paleobiology world when they were declared the earliest fossilized life yet found. Standing up to 1.6 inches tall, the triangular forms line up like a string of inverted flags in an outcrop on the southwest coast of Greenland that dates back 3.7 billion years.
“If these are really the figurative tombstones of our earliest ancestors, the implications are staggering,” NASA astrobiologist Abigail Allwood wrote in a review article that accompanied the Nature study announcing the find. The microbes that made these fossils are over 200 million years older than the most widely accepted evidence of fossil life and would have lived a geologic blink of an eye after astroids had blasted Earth's early surface. Evidence of critters from this time would suggest that “life is not a fussy, reluctant, and unlikely thing,” Allwood wrote. “Give life half an opportunity, and it'll run with it.”
But even as Allwood penned these words, she had a nagging sense that something was amiss.
“I was initially quite enthusiastic,” she says. “But then I sort of was looking more closely and seeing some funny things.” So she collected her own set of rocks from the Greenland outcrop and analyzed their structure and chemistry. The results, published this week in Nature, paint an entirely different picture: The structures are not from microbial action, she reports, but from the more expected squishing and pulling of rocks as Earth's early surface took shape.
The University of Wollongong's Allen Nutman, who led the 2016 analysis of the structures, wholeheartedly disagrees with the new work. “This is a classic 'comparing apples to oranges' scenario, leading to the inevitable outcome that ours and their observations do not exactly match,” he says in an emailed statement.
This debate draws back the veil on the necessary analysis and reanalysis that happens in the scientific process, highlighting in this case just how difficult it can be to tease out details from ancient materials. Allwood sees this latest work as an important cautionary tale in our search for life on other planets, especially with the upcoming Mars 2020 rover set to seek traces of ancient life on the red planet.
“Give yourself the best chance possible while doing the work in the field,” she says. Returning to Mars—or even Greenland—is not easy, so careful analysis and review at every step of the process is vital to our collective understanding.
The section of rock that hosts the tiny structures is in what's known as the Isua supracrustal belt, which has long been covered with snow and ice. Thanks to Earth's warming climate, the snow cover there has thinned in recent years, and in 2012, a team of researchers led by Nutman spotted the unusual flame-like forms.
This series of rocks is an unlikely place for finding fossils—the crust there has been squished and folded under intense pressures and temperatures, and the team initially had their doubts. Over the next four years, they put samples of the rock through a battery of tests, analyzing both their chemical and physical structures. Rare-earth element analysis, carbon isotopic fingerprints, and remnants of internal laminations all led the team to conclude that the triangles were so-called stromatolites—fossilized traces of sun-loving cyanobacteria that still grow today in shallow seas.
These microbes, famous for pumping oxygen into early Earth's skies, often grow in mats. As sediments wash over their slimy surfaces, they must reach up to the sun to form new layers. The process results in finely laminated mounds and columns that have been found before in the fossil record. One particularly characteristic shape is a cone—precisely like the row of triangles preserved in the 3.7-billion-year-old rocks.
A paleo-Archean parade
It was these conical shapes, however, that first puzzled Allwood. The triangles seemed to be perfectly aligned, each sliced through at their peak by the weathered rock face.
“It was like their own paleo-Archean parade, and they had all their little toes lined up,” she says. But if the shapes had been randomly poking up from an ancient ocean floor, as you would expect for stromatolites, random parts of the cones should be cut through. So Allwood and her team visited the Greenland outcrop in 2016, and she was concerned immediately. On the front face of the rock, you can indeed see the tiny points. But things change when the rock is viewed from the side.
“It has a completely different fabric—dead straight, stretched out like chewing gum, elongate, tectonically deformed fabric,” she says. Not all the structures even point the same way; the occasional errant cone points downward, she notes, very unlike living microbial mats. The researchers then subjected their own samples of the rock to a battery of tests, and the results reveal a different image—one that requires no microbial activity.
Cutting to the point
The tiny triangles, she suggests, instead started as layers of rock. As they were compressed from the left and right, the layers buckled, forming pointed structures while elongating in the direction perpendicular to the pressure. This would mean the supposed microbial cones are actually ridges that stretch into the rock.
“It's a pretty classic indication of tectonic deformation,” she says. “It's not as visible in their sample because it's much smaller. It was cut very close to the edges of the stromatolite, and there's not much context above, below, or side to side.”
Ioan Lascu, a biominerologist at the Smithsonian's National Museum of Natural History who was not involved in either study, agrees with this assessment. “The way you section your rock, you can get a totally different picture depending on how your section cuts through the layers,” he says.
The chemistry of the rocks could also have non-microbial explanations, Allwood says. Detailed chemical maps reveal that the interiors of the forms hold silicon-rich minerals, while the outer layer Nutman and his team interpreted as stromatolite-like laminations is made of carbonate. Allwood suggests that this layer was formed not by life but by a carbonate-rich fluid that infiltrated the rocks after they were buried.
But for his part, Nutman takes issue with many points in the study. Allwood and her team collected their samples less than a foot and a half away from Nutman's original sample site. But Nutman says their collection site was simply too deformed to properly preserve delicate fossils. This alteration is also responsible for creating the downward-facing triangles, he says.
Plus, he's not convinced that traditional compression of rock layers could produce structures with pointed tops and flat bottoms. “With folding, both surfaces should be curvilinear,” he says via email. Also, all the rock layers surrounding the triangular forms should bow, “but they are flat,” he notes.
Allwood admits that such rock structures would be weird but not impossible. If the layers have different toughnesses squeezing from various sides won't necessarily produce even waves and bows. To create the surrounding flat layers, for instance, imagine smashing a stack of sandwich bread slices on top of a Toberlerone bar. The bread may evenly compress, producing flat bands, but the sturdier chocolate bar isn't going squash so easily.
“We shouldn't be surprised that there's debate,” says Yale biogeochemist Noah Planavsky, who wasn't involved in either study. The conclusions of the initial study represented a radical change in the timeline of life on Earth—and scientific debate is never solved by a single study.
Lascu agrees: “Who knows, maybe in two to three years another paper comes out and says this doesn't look at everything.”
For now, both Planavsky and Lascu agree strongly with the new work's conclusions. Planavsky particularly emphasizes the importance of the 3-D analysis, noting that the lack of such analysis in the 2016 study was “a real shortcoming.” And while Nutman continues to emphasize that the chemistry of the rocks points to carbonates forming in shallow seas, Planavsky stresses that chemistry alone is a tough sell to prove that these are stromatolites.
“The paleoenvironmental proxy they were using isn't really specific enough to try to resolve the key questions,” he says. In essence, it looks like something microbes made, but that doesn't mean they did it. Oftentimes, biological reactions simply speed up processes that could still happen without life.
“The way to settle this is not by me and him saying, he said, she said,” Allwood adds. Instead, the researchers are planning a workshop to host relevant experts next summer at the site in Greenland. This way, more people can stand on the outcrop and play geologic detective, reimagining the events that conspired billions of years ago to shape the rocks under their feet.
“I think it's the only way to solve it now,” Allwood says.