Researchers have discovered fossilized cell remnants in rock that roughly 3.4 billion years ago was a hydrothermal vein—a crack in bedrock containing superheated water. The microfossils, described today (July 14) in Science Advances, support the theory that such veins were breeding grounds for Earth’s earliest lifeforms, as well as the idea that primitive microbes were methane producers.
“On the basis of very detailed chemical analyses [the] filamentous . . . structures are interpreted as methane-cycling microbes,” Malcolm Walter, an astrobiologist at the Australian Centre for Astrobiology who was not involved in the study, writes in an email to The Scientist. “This is a significant addition to the very rare early Archean microfossil record.”
Hydrothermal veins in rock contain magma-heated ground water that rises to the surface as hot springs or geysers on land or vents in the seabed, and are believed to be among the first places on Earth that life began. That’s because they are enriched with the types of chemical elements thought to “create an environment in which microbes could potentially originate,” says Barbara Cavalazzi, a geobiologist and astrobiologist at the University of Bologna. Specifically, she says, they provide the sort of chemical environment suitable for methanogens—microbes that generate methane—which are believed to be among the earliest forms of life.
The intervening billions of years since life’s infancy mean ancient hydrothermal veins are long gone, geological processes having crushed them, moved them, filled them with chert—a type of sedimentary rock—or all three. There are few places on Earth where it is possible to find well-preserved, fossil-filled chert, even fewer where the chert represent ancient hydrothermal veins, and fewer still where those veins date back to the Archean Eon, 3–4 billion years ago. One such place is the Barberton Greenstone Belt in the Makhonjwa Mountains of South Africa, where Cavalazzi and her team collected their 3.42-billion-year-old samples.
Other microfossils have been found in similarly aged or even older chert samples, but the appeal of the particular rocks Cavalazzi chose is that they originated from the subseafloor—the rock deep below the seabed. This would mean that the only type of microorganisms that could be present were ones that obtain energy though chemical processes (chemotrophs), such as methanogens. It ruled out the possibility of finding phototrophs, which convert light into energy and which are thought to have evolved more recently, Cavalazzi explains.
Cavalazzi’s team sliced the rock into sections 30–50 µm thick for viewing under the microscope. At the interface between the chert (which would once have been aqueous) and the host rock, they found tiny filamentous structures the size and shape of microbes, on average, 42 µm in length and 0.77 µm in diameter. Some were grouped together in formations resembling biofilms—carpets of microbes growing together on a surface.
The team also used mass spectrometry and a specialized type of imaging called Raman microspectroscopy to examine the chemical composition of the filaments and surrounding chert.
“I’m pretty well convinced,” says environmental chemist Eli Moore of Rowan University who was not involved in the research. “The [fossils’] morphology resembles cellular colonies, and then within the fossils they have high concentrations of carbon, nitrogen, and hydrogen, so it really looks like organic matter . . . most likely representing ancient cells.”
The analysis also showed the presence of nickel in the chert, which is “particularly cool,” says Moore, because nickel is an important metal cofactor in the biological process of microbial methanogenesis.
“The evidence is definitely strong” that these filaments are indeed fossilized Archean methanogens, and is more definitive than that gleaned from previously discovered microfossils, he says.
Earlier reports of filamentous microfossils had been debated as potential abiogenic biomorphs—that is, organic structures that look like cells but are produced as a result of geochemical, not biological, processes. “We were able to exclude any possibility that our structures were related to any abiotic process,” Cavalazzi says, because they have a different composition from abiogenic biomorphs and formed typically microbial-looking biofilms.
“We can’t be completely sure, one hundred percent” that these were once cells, “because we were not there when this stuff was happening,” says Cavalazzi. But taking all the data together, she continues, they “strongly support the biological origin of these structures.”