An Indian rock nearly 3.5 billion years old has preserved a chemical signature that researchers attribute to very ancient microscopic life.
The rock comes from the Singhbhum craton in eastern India. It is a chert, a rock rich in silica. It contains thin, dark carbon-rich layers alternating with lighter layers. This arrangement resembles the mats formed by communities of microbes living on top of one another.
Layered rocks in India show potential signs of microbial life dating back 3.5 billion years.
To determine when this material formed, the team studied tiny zircon crystals found in the rock. These minerals contain uranium, which gradually transforms into lead. By measuring their proportions, researchers obtain a kind of geological clock. The zircons indicate an age of 3.497 billion years, with an uncertainty of 5 million years.
The scientists also analyzed the carbon it contains. Carbon exists in several forms, called isotopes. Organisms that capture carbon generally favor lighter carbon-12 over carbon-13. This preference leaves a measurable signature after they disappear.
In the Indian samples, this signature reaches approximately -30.9‰ for the isotope ratio studied. According to the authors, this value is consistent with a biological origin. Raman spectroscopy analyses also indicate that a significant portion of the carbon remains in the form of kerogen, a carbonaceous material derived from ancient organic compounds.
Caution is nevertheless necessary with rocks this old. Over billions of years, heat, pressure, and fluid circulation can transform their contents. Some mechanisms that do not involve living organisms can also produce clues resembling biological traces. That is why the combination of dating, layered organization, and carbon composition matters more than any single clue considered in isolation.
Potentially older clues exist, notably in rocks from Greenland or in Australian crystals. Their interpretation remains debated. Here, the main interest lies in the direct link established between a precisely dated rock and a carbon signature attributed to living organisms.
The researchers also estimate that the carbon composition is compatible with already sophisticated carbon-fixation mechanisms 3.5 billion years ago.
Independent analyses will now need to test the dating of the deposit and its biological interpretation. Nearby layers could also reveal other microscopic structures or chemical signatures that would strengthen or qualify this scenario.