At first, Abigail Allwood saw nothing wrong with Allen Nutman’s claims.
In August 2016, Nutman, a geologist from the University of Wollongong, announced that he and his colleagues had found the world’s oldest fossils in an outcrop in Greenland. The team discovered rows of inch-high conical humps embedded in 3.7-billion-year-old rocks, and interpreted them as stromatolites—layered mounds created by colonies of ancient marine bacteria. The oldest accepted stromatolites, from the Pilbara region of Australia, are 3.5 billion years old. Nutman’s finds were 200 million years older.
His team published the results in the journal Nature, and Allwood, a NASA geologist, wrote a commentary to accompany them. She was largely positive and explained that the finds pushed the origin of life on Earth nearer to the very origin of the planet itself. If the interpretation was accurate, it suggested that life is likely not a fussy phenomenon but one that will take the opportunity to emerge if given half a chance. “It was a pretty exciting discovery, and the impact would have been huge,” she tells me. “The paper seemed fine. The interpretation seemed fine. Nothing seemed amiss.”
She found it odd that the stromatolites were all neatly lined up in the rock face, like a Toblerone bar. “Every one of these conical structures was bisected through its apex,” she says. “It seemed miraculous, as if they had all been on parade back then.”
Intrigued, she decided to check out the structures for herself. Nutman turned down her invitation to accompany her because of Greenland’s famously unpredictable weather. But Allwood was lucky and arrived to find days of bright, dreamy conditions. She and her colleague Minik Rosing chartered a helicopter to the same site, cut a sample of the stromatolites from the rock, and flew back the next day. It was a short trip, but enough to convince Allwood that the so-called stromatolites are nothing of the kind.
Instead, she argues that they’re the product of billions of years of geological upheaval, which warped, crushed, and stretched the ancient rocks to create structures that look like the work of living things—but are not. She has published her findings in a new paper in the same journal.
Stromatolites form because microbe colonies ensconce themselves in sticky mucus that traps minerals from the surrounding seawater. These minerals accumulate in layers that, over time, harden and fossilize, creating a wide range of columns, cones, and domes. Regardless of the final shape, stromatolites should always grow upward from the seafloor. “And the first thing we noticed in the field was that there were one or two that point downward,” Allwood says.
When she cut one of the so-called stromatolites out of the outcrop, she realized that it wasn’t a cone at all. It’s actually a ridge, one that extends several inches into the rock. Forget the Toblerones; think instead of a row of speed bumps. It’s much easier to produce such structures through geological processes than through biological ones. And the rocks have clearly gone through a lot. They have been scrunched up in one direction to produce the ridges, and pulled in another to produce features that Allwood compares to “stretched-out chewing gum.”
As you might imagine, Nutman and his colleagues aren’t happy with this interpretation. “We expected that our publication would generate new investigations and looked forward to the results,” says Vickie Bennett of Australian National University, who worked with Nutman in Greenland. “But their cursory investigation, based on a less-than-one-day field trip to the outcrops, which were partially covered in snow at the time, only serves to confuse our earlier work.” She and Nutman also contend that Allwood focused on a peripheral part of the outcrop that had been more heavily deformed over geological time rather than the better-preserved central regions. “They basically did not look at the same rocks,” Bennett says.
Allwood counters that the region she studied was less than a meter away from one of the sites in the original paper. And as to the brevity of her trip, “One day was enough to disprove them,” she says. “Frankly, more shame on them that they didn’t figure it out in the many years they spent there.
“In their defense,” she adds, “Greenland fieldwork is very hard to do. But in our defense, that’s why having good weather and good exposure allowed us to see everything quite clearly.”
Back in their lab, she and her colleagues analyzed the chemical composition of the Greenland rock and found more evidence to support their interpretation. True stromatolites, like those in Australia, should have internal layers, but the Greenland ones don’t. Instead, they are almost pure silicon on the inside, with rims of dolomite minerals separating them from the overlying rock. These structures weren’t the work of microbes, Allwood says. Instead, they were created when fluids containing dolomite minerals seeped into lumps of silicon and crystallized, “like chocolate soaking into a vanilla sponge.”
Nutman contends that internal layers, though not as well preserved as those in younger Australian stromatolites, are there in other Greenland samples. And he points to the distinct chemistry of the cones. They have lower concentrations of titanium and potassium than the surrounding rocks, and also unusual levels of yttrium and other rare elements that are indicative of seawater. These signatures suggest that the cones weren’t just random bits of rock that were folded into stromatolite-esque shapes, but the work of marine microbes yanking bits of minerals out of the ocean.
Not so, Allwood says. The titanium and potassium don’t mean anything: They are also depleted in other parts of the outcrop that lie outside the cones. And as for the yttrium and other rare elements, a more detailed analysis shows that they’re concentrated in microscopic specks of mica and quartz—minerals that likely formed in the rocks at later stages of their existence. “It’s got nothing to do with biology,” Allwood says. (Nutman doesn’t find this plausible, and notes that other rocks from the same area don’t have the same signature. “We stand by our interpretation,” he says.)
Phoebe Cohen, a paleontologist from Williams College who wasn’t involved with either study, thinks Allwood’s interpretation is more likely. “Exceptional claims require exceptional data to back them up, and while the original [team] did a good job garnering evidence for their claim, it wasn’t entirely convincing,” she says. “This follow-up study is exactly what I would have hoped for. I’m sure this will not be the last time that an ‘oldest evidence of life’ paper is refuted by further research, but I look forward to someday being convinced!”
Bennett argues that the controversy changes little, since there are other lines of evidence pointing to the existence of life more than 3.6 billion years ago. “It is becoming difficult to dispute the presence of ancient life as far back as the beginning of the rock record on Earth,” she says. And again, Allwood disagrees. Beyond the Australian stromatolites, which are less than 3.5 billion years old, “we don’t have any other unequivocal evidence,” she says. “It doesn’t mean that there weren’t microbes around, but we can’t hang our hat on that fact.”
Allwood is a lead investigator on NASA’s Mars 2020 mission, and one of the instruments she used to study the Greenland rocks will be strapped to a rover, to look for signatures of ancient life. But if it’s this hard for two sets of respected and well-trained people to agree about rocks on Earth, how much deeper will the disagreements be about evidence on other planets?
“It’s not impossible,” Allwood says. The problem isn’t necessarily one of distance, but one of collaboration. Nutman’s paper and her rebuttal have five co-authors apiece, and only two members from each team actually went to Greenland. Everyone else worked with samples or from photos. “Until other people can get a team out there, it’s my word against Allen’s,” Allwood says. “But when exploring Mars, we’ll have hundreds of the world’s best scientists looking over the rover’s shoulder.”
To her, the big lesson is to prioritize fieldwork. The Mars 2020 rover is meant to collect samples that later missions will bring back to Earth to be analyzed. But to interpret those samples properly, the team will need to make the most of the rover’s eyes and instruments to carefully study the Martian landscape. “We can’t kick the can down the road,” Allwood says. “Any question you want to address with samples when you get back, you need to have a good handle on it when you’re there.”
To aid those efforts, she wants to build a virtual-reality setup that will allow scientists to examine Martian landscapes in a more immersive way, rather than straining over photos. “When Mars teams get together, it’s science by Webex and PowerPoint,” she says. But if her plans work, “at the end of each week, people will put on their goggles and be able to stand on-site at the outcrops.”
I ask if she’s talking about Mars or Greenland.
“I was talking about Mars,” she says. “But you just gave me an idea.”
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