A team of international researchers has discovered the fossilized remains of methane-feeding microbes that lived in a hydrothermal system beneath the seafloor 3.42 billion years ago.
The study, published in the journal Science Advances, analyzed microfossil specimens in two thin layers within a rock collected from the Barberton Greenstone Belt in South Africa. This region, near the border with Eswatini and Mozambique, contains some of the oldest and best-preserved sedimentary rocks found on our planet, dating back 3.2 to 3.5 billion years. The researchers studied veins of light-colored chert deposited by geothermal fluids in former fractures of the seabed.
Dr. Barbara Cavalazzi, the lead author of the study, said: "We found exceptionally well-preserved evidence of fossilized microbes that appear to have flourished along the walls of cavities created by warm water from hydrothermal systems a few meters below the seafloor. 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 interaction of cooler sea-water with warmer subsurface hydrothermal fluids would have created a rich chemical soup, with variations in conditions leading to multiple potential micro-habitats. The clusters of filaments were found at the tips of pointed hollows in the walls of the cavity, whereas the individual filaments were spread across the cavity floor.
Chemical analysis shows that the filaments include most of the major elements needed for life. The concentrations of nickel in organic compounds provide further evidence of primordial metabolisms and are consistent with nickel-content found in modern microbes, known as Archaea prokaryotes, that live in the absence of oxygen and use methane for their metabolism.
"Although we know that Archaea prokaryotes can be fossilized, we have extremely limited direct examples. Our findings could extend the record of Archaea fossils for the first time into the era when life first emerged on Earth," said Cavalazzi.
The microfossils are the oldest evidence for this type of life and expand the frontiers of potentially habitable environments on the early Earth. Stromatolites - algal mats living in shallow water - from Australia are among the oldest direct evidence of life found on Earth, dating back at least 3.5 billion years. Certain carbon isotopes signatures found in rocks from Greenland, dating back 3.4 to 3.6 billion years, might result from biological activity.
She added: "As we also find similar environments on Mars, the study also has implications for astrobiology and the chances of finding life beyond Earth."
