This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A team of researchers reports that they may have found rock containing some of the earliest forms of microbial life on Earth. Their study, published in the Proceedings of the National Academy of Sciences, describes the biosignatures found in this 3.5-billion-year-old, carbon-rich, layered rock, called chert.
Earth is about 4.5 billion years old. Unsurprisingly, scientists are interested in figuring out how and when life arose on the planet. While ancient rocks hold clues about early life, finding these clues is difficult because Earth's oldest rocks have been heated, squeezed and chemically altered.
Geological studies from Greenland, South Africa and Western Australia have suggested life existed at least 3.4–3.8 billion years ago. Still, evidence from some of these studies is debated, as scientists are uncertain whether heat, fluids and deformation may have changed the rocks in a way that altered the carbon thought to be biological in nature. Carbon consisting of a higher percentage of the carbon-12 isotope and a relative shortage of the heavier carbon isotope, carbon-13, is often linked to biological carbon-fixing processes.
Scientists have found early microbial traces in marine and hydrothermal environments, generally in the form of residual carbonaceous matter (CM), biominerals, microbial mats and layered rock structures built by tiny microbes. The new study reports evidence of microbial mats, which are multilayered sheets of ancient microscopic organisms, mostly bacteria and archaea. The study authors write, "Preservation of Paleoarchean microbial mats is rare due to subsequent deformation and metamorphism.
Thus, careful identification is required as abiotic processes may generate 'biomarker-like' signatures." The researchers mapped and sampled a folded, carbon-rich chert layer in the Singhbhum Craton in eastern India. The chert contained thin, repeating layers of silica and carbonaceous material, which the team says resemble the remains of a microbial mat. Zircon crystals within the rock could be dated using uranium-lead dating, which determined the rock to be around 3.497 billion years old.
The team interpreted this as the time of sediment deposition. They say sedimentation occurred alongside volcanism and the zircon crystals formed from this volcanic activity. To determine whether the carbon was ancient, rock-bound material or the result of later alteration, the team measured carbon isotope ratios and used Raman spectroscopy.
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