3.5-Billion-Year-Old Rock in India Yields the Oldest Directly Dated Biosignature

A team of researchers reports finding what may be some of the earliest evidence of microbial life on Earth, preserved in a 3.5-billion-year-old, carbon-rich layered rock called chert. The findings, led by Trisrota Chaudhuri, were published in the Proceedings of the National Academy of Sciences.

The researchers mapped and sampled a folded chert layer in the Singhbhum Craton in eastern India, containing thin, repeating layers of silica and carbonaceous material that resemble the remains of a microbial mat — a multilayered sheet of ancient microscopic organisms, mostly bacteria and archaea. Using uranium-lead dating on zircon crystals within the rock, which the team says formed from volcanism occurring alongside the sediment's deposition, the researchers determined the rock is about 3.497 billion years old.

To determine whether the carbon in the rock was ancient and biological, rather than the result of later alteration, the team measured carbon isotope ratios and used Raman spectroscopy. The ratio of carbon isotopes was consistent with biological materials, and Raman measurements showed the carbon was largely poorly ordered kerogen, an organic material found in sedimentary rocks. The researchers describe the find as, to their knowledge, the oldest directly dated rock with a confirmed biosignature.

Earlier geological studies from Greenland, South Africa and Western Australia have suggested life existed at least 3.4 to 3.8 billion years ago, though evidence from some of those sites is debated because heat, fluids and deformation can alter rocks in ways that mimic biological signatures. The researchers say further work, including independent testing of whether the zircons accurately date the chert's deposition, is needed to confirm the results as the oldest evidence of a biosignature.