A team of researchers in South Africa put a bit of rock under a microscope and found the remains of 3.42-billion-year-old life. Those fossils—the squiggly, microscopic remains of organisms that subsisted on methane—broaden the scope of what habitats were suitable for life on Earth during the Archean Eon.
Life originated long before the Cambrian Explosion of 541 million years ago, which heralded a new era of more complex undersea life. But the life before it (“Precambrian” life) looked a lot less alive to the naked eye. The oldest signs of life on Earth are found in 3.5-billion-year-old rocks called stromatolites, piles of petrified accretions of biofilms. The newly discovered microfossils date to nearly the same time but represent a different form of life, a sort of microbe that thrived in a submarine hydrothermal system. The team’s results were published today in Science Advances.
“We found exceptionally well-preserved evidence of fossilised microbes that appear to have flourished along the walls of cavities created by warm water from hydrothermal systems a few meters below the seafloor,” said Barbara Cavalazzi, a geobiologist at the University of Bologna and lead author of the paper, in a university release. “Sub-surface habitats, heated by volcanic activity, are likely to have hosted some of Earth’s earliest microbial ecosystems and this is the oldest example that we have found to date.”
The microfossils Cavalazzi’s team found are ossified filaments; under a microscope, they look like cracks and fuzzy splotches. The fossils are composed of carbon sheathes that ensconce central matter that is distinctly separate from the external material, indicative of a cell wall surrounding intracellular stuff. It’s not terribly surprising to find life around a hydrothermal system; today, hydrothermal vents teem with otherworldly life. Such sites are not only interesting for understanding the origins of life on Earth but also for determining the conditions that could host life on other worlds, like Mars or the moons Europa and Enceladus.
The fossils’ age was based on the age of the rocks they were found in, which had been determined in previous studies using zircon chronology. Other work had identified microfossils from the Archean Eon in the same belt of stone, so the team had good reason to think they might find something exciting here. In the paper, the team notes that the microfossils are putative, meaning it’s possible they’re something else and not a record of living things. They could’ve been artifacts of the hydrothermal system, something that may simply look like life. With lifeforms so old and primitive, it’s not as simple as looking for skeletons or fossilized footprints.
In these hydrothermal systems, cold seawater mixes with heated subsurface fluids, creating a chemical soup. Today, this soup makes a happy home for tube worms and crabs. Back in the Archean, such complex life didn’t exist, but microbes living on methane would’ve had a field day. Concentrations of nickel in the organic compounds in the fossil deposit are consistent with the nickel content found in modern microbes, the researchers noted, which are anaerobic (they survive without oxygen) and metabolize methane.
“The authors have marshaled an impressive array of observations and present a persuasive case that the filaments were once ancient microbes that inhabited fractures prior to their entombment by silica cement,” said Birger Rasmussen, a geobiologist at the University of Western Australia who was not affiliated with the recent paper. He noted that some will have questions about the shape of the filaments, and other experts may want more evidence that the microfossils were hollow, which would indicate the filaments’ cell structure.
“These quibbles aside, this finding is certainly intriguing and points to the possibility of microbial life inhabiting the nooks and crannies beneath Earth’s surface more than 3.42 billion years ago,” Rasmussen said. “Time will tell whether they are embraced as probable or possible microfossils, or something else entirely.”