Rock in Eastern India Holds Oldest Directly Dated Trace of Life, at 3.5 Billion Years

Researchers say a single carbon-bearing rock from eastern India carries "compelling evidence" that microbial life existed there at least 3.5 billion years ago. The study, published in the peer-reviewed Proceedings of the National Academy of Sciences, describes it as — to the authors' knowledge — the oldest directly dated rock with a confirmed biosignature. The team includes Trisrota Chaudhuri of the Geological Survey of India, Kolkata, and Mark Harrison of the University of California, Los Angeles.

The rock is a black-and-white banded chert from Bhitardari in the Singhbhum Craton, an ancient, stable block of continental crust spanning parts of Jharkhand and Odisha. The region has long drawn geologists because it preserves some of India's oldest rocks, recording crust that formed more than four billion years ago. Chert — a hard, fine-grained rock made largely of silica — can preserve organic matter for billions of years, but cannot itself be dated directly.

To date it, the team extracted zircons: hardy crystals that act as natural clocks because they trap uranium, which decays to lead at a known rate. Four of the eight zircons analysed indicated an age of 3,497 million years, and because those crystals appear to be volcanic ash that settled as the chert formed, the researchers took that to be the rock's age as well. The carbon itself told the biological half of the story: its balance of carbon isotopes matches carbon captured by living cells, and the material is degraded organic matter rather than later inorganic contamination.

Earth formed about 4.54 billion years ago, so the trace comes from only about a billion years after the planet's birth — and if microbes were already established then, life itself must have begun earlier still. Older claims exist, including graphite in 3.7-to-3.8-billion-year-old Greenland rocks, but several have been disputed; the authors caution that their own case rests on chemistry and geological context.