Scientists Discover Fossil Threads Carrying the Oldest Known Archaea Microbes

Threadlike filaments pressed in rock may be the remnants of archaea that burped methane near hydrothermal vents 3.42 billion years ago. If so, these strands in rock excavated in South Africa around a decade ago would provide the earliest direct evidence of a methane-based metabolism. Such ancient fossil filaments may contain clues about Earth’s early inhabitants and hint at where to look for extraterrestrial life. Scientists suspect that life on our planet could have arisen in such an environment.

Biologists have deduced that metabolisms based on munching or belching methane evolved early on, but don’t know exactly when. Previous research has found indirect evidence for methane-cycling microbes in the chemistry of fluid-filled pockets of ancient rocks from around 3.5 billion years ago. But that work didn’t find the actual microbes. With this fossil analysis, what we find, basically, is evidence of about the same age. But this is a cellular remain — it’s the organism.

The newly identified fossil threads have a carbon-based shell. That shell is different structurally from the preserved interior, suggesting a cell envelope enclosing the cells’ insides, the authors write. And the team found relatively high nickel concentrations in the filaments. The concentrations were similar to levels found in modern methane-makers, suggesting the fossils’ metal may come from nickel-containing enzymes in the microbes.

Yet the search for early life-forms has had its share of false signs, and some researchers aren’t convinced these fossils are the real deal. In silica-rich hydrothermal environments, the ingredients for structures that mimic cells mingle and can form life look-alikes through chemistry.

If the strands are ancient archaea, they’d become the earliest fossil evidence for this domain of life, predating specimens from less than 500 million years ago. And if such microbes evolved so quickly on Earth, within around 1 billion years of the planet’s origin, methane-cyclers may be more common than realized on other planets where liquid water has been around for a while.

CITATIONS

B. Cavalazzi et al. Cellular remains in a ~3.42-billion-year-old subseafloor hydrothermal environment. Science Advances. Published online July 14, 2021. doi: 10.1126/sciadv.abf3963.