An international collaboration found the fossils of methane-cycling microbes from 3.42 billion years ago, providing new insights on the potential habitability, or ability to sustain life, of early Earth.
Led by the researchers from the University of Bologna, the international team discovered the fossilized remains in a hydrothermal system deep under the seafloor. These methane-cycling microbes are now the oldest evidence for this lifeform, expanding mankind's understanding of potentially habitable environments in early Earth and could set precedents for assessing the possibility of similar conditions in other planets like Mars.
Details of the discovery appear in the latest Science Advances, July 14, in an article titled "Cellular remains in a ~3.42-billion-year-old subseafloor hydrothermal environment."
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Basic Life Forms Push Frontiers of Habitability
The researchers found the ancient methane-cycling microbes embedded within two thin layers of a rock sample extracted from the Barberton Greenstone Belt in South Africa. This particular excavation site, seated near the border of Eswatini and Mozambique, is home to some of the oldest and best-preserved samples of sedimentary rocks found on Earth.
In the microfossils, researchers found carbon-rich outer layers and a core of unique chemical and structural properties consistent with the structure of cell walls or the membrane that surrounds cytoplasmic or intracellular material.
"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," explains Prof. Barbara Cavalazzi, lead author of the study from the Department of Biological, Geological, and Environmental Sciences at the University of Bologna, in a university press 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."
Researchers also explained that cool seawater interacting with warmer hydrothermal fluids under the surface would have encouraged chemical reactions, with slight variations creating habitats such as those that allowed the methane-cycling microbes from 3.42 billion years ago to thrive.
Methane as an Indicator of Habitability, in Earth and Elsewhere
Additional chemical analysis revealed that the filaments also contained the elements believed to be the building blocks of life. For example, the amount of nickel present in organic compounds also support the notions of these primordial metabolism processes being consistent with the nickel content found in extant microbes known as archaea that can live without oxygen, using methane instead. With the discovery of the methane-cycling microbes, it could further our understanding of the environments that host these lifeforms.
In another study, the discovery of methane in Enceladus - the sixth-largest moon of Saturn - raised the debate on whether the satellite could also host life. A recently published study in the Nature Astronomy journal proposed a mathematical model to quantify the odds of lifeforms similar to methanogenic archaea on Earth thriving in Enceladus. While it does not conclusively show whether life is present, its assessment of the conditions necessary shows that, if met, similar life forms are possible on the Saturn moon.
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