Indian rock yields oldest directly dated traces of microbial life: study

Researchers say they have found “compelling evidence” from multiple sources that microbial life existed in what is now eastern India at least 3.5 billion years ago. Typically these involve finding fossils or traces of organic chemicals but in this case they rely on converging chemical clues, according to a paper that appeared in the peer-reviewed Proceedings of the National Academy of Sciences.

The source of their discovery is a single carbon-bearing rock in the Singhbhum Craton, which is an ancient, stable block of continental crust in eastern India, spanning parts of Jharkhand and Odisha. It has long interested geologists because it holds some of the oldest rocks in India, recording crust that formed more than 4 billion years ago.

“To our knowledge, (this) is the oldest directly dated rock with a confirmed biosignature,” say the authors, who include Trisrota Chaudhuri, of the Geological Survey of India, Kolkata, and Mark Harrison, of the Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles.

The rock — a “black-and-white” banded chert from Bhitardari — pairs a firm age with several independent chemical and structural signs that its carbon was made by living organisms. Chert is a hard, fine-grained rock made largely of silica that can preserve organic matter for billions of years, but cannot be dated directly. So the researchers extracted zircons — hardy crystals that act like natural clocks as they trap uranium that decays to lead at a known rate. Of eight zircons analysed, four indicated an age of 3,497 million years. Because those zircons appear to be volcanic ash that settled as the chert formed, the team took that to be the rock’s age, too.

They then argued the carbon was biological. Its balance of carbon isotopes matches carbon captured by living cells. They also found the material was degraded organic matter rather than later inorganic contamination.

Scientists bother with such questions because the earth formed about 4.54 billion years ago, so a 3.5-billion-year-old trace of life comes from only about a billion years after the planet’s birth and may open new hypotheses on evolution. If microbes were already established, life itself must have begun earlier still. That bears on whether life is a freak accident or something that emerges readily on habitable worlds.

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.