“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.”