Traces of life dating back 3.5 billion years have been found in India
In eastern India, scientists have studied rock dating back some 3.5 billion years and discovered features within it that may be linked to one of the earliest forms of life on Earth. The thin carbonaceous layers resemble the remains of microbial mats, and the isotopic composition of the carbon is consistent with biological processes. The authors consider the discovery to be one of the most reliably dated ancient biosignatures, although such claims require particularly thorough independent verification.
Researchers have studied a carbon-rich siliceous layer in the Singhbhum craton in eastern India.
The rock is a chert – a dense, siliceous sedimentary rock. Within it, the scientists observed repeating microscopic layers of silica and carbonaceous material.
According to the authors, this structure resembles the remains of an ancient microbial mat.
This is the term used to describe multilayered communities of microorganisms, primarily bacteria and archaea, which can form on the surface of sediment.
However, this does not refer to preserved cells of ancient bacteria. The scientists found specific geological and chemical features that are interpreted as possible evidence of microbial activity.
Details
One of the study’s main strengths is its dating.
Zircon crystals were found in the rock, the age of which was determined using the uranium-lead method.
The result was approximately 3.497 billion years.
The researchers believe that these zircons were formed as a result of volcanic activity at roughly the same time as the sediment was deposited. Therefore, their age can be used to estimate when the layer itself was formed.
If this interpretation is correct, the carbonaceous material was trapped within the rock almost 3.5 billion years ago.
A single age measurement was not sufficient.
The researchers also measured the carbon isotope ratio in the dark layers.
When assimilating carbon, living organisms usually prefer the lighter isotope — carbon-12. As a result, the organic matter they produce is relatively low in the heavier carbon-13.
It is precisely this isotopic signature that was detected in the Indian rock.
According to the authors of the study, this is consistent with the biological origin of the carbon.
Further data were obtained using Raman spectroscopy.
Analysis showed that a significant proportion of the ancient carbonaceous material has been preserved in the form of relatively poorly ordered kerogen.
Kerogen is a solid organic substance trapped within sedimentary rocks.
This is important because the Earth’s oldest rocks have been repeatedly subjected to heat, pressure and chemical processes over billions of years. Such processes can alter the original organic material so drastically that it becomes extremely difficult to distinguish a biological signal from a non-biological one.
The authors believe that, in this case, the combination of layer structure, isotopic data and the state of the carbon points to an ancient biosignature.
Despite their impressive age, the results require cautious interpretation.
The researchers did not detect individual microbial cells that could be directly identified as bacteria or archaea.
Nor does a light carbon isotope composition in itself constitute absolute proof of life: certain non-biological processes are capable of producing similar chemical signatures.
This is precisely why scientists rely on several indicators simultaneously — the rock’s structure, the geological context, the isotopic composition and the properties of the carbonaceous material.
Independent research teams will now need to verify both the biological interpretation of the discovery and the link between the dated zircons and the time of the layer’s formation.
The discovery should not be unquestionably labelled as the oldest evidence of life.
Previously, scientists had reported possible biosignatures dating back 3.7–3.8 billion years, as well as even older carbonaceous inclusions. However, some of these findings remain the subject of long-standing debate.
The distinctive feature of this new study lies elsewhere.
The authors claim to have obtained a directly dated rock containing preserved carbon with signs of biological origin. It is precisely this reliable temporal calibration of the biosignature that makes the study particularly interesting.
What was happening on Earth 3.5 billion years ago
The planet is approximately 4.5 billion years old.
Thus, if the researchers’ findings are confirmed, microbial communities existed as early as roughly one billion years after the Earth’s formation.
It was a completely different planet: there was almost no free oxygen in the atmosphere, the continents looked different, and complex animals and plants would not appear for billions of years yet.
Life was predominantly microscopic.
It was precisely these ancient microbial ecosystems that subsequently played a fundamental role in altering the chemical composition of the oceans and the atmosphere.
Why this is important
One of the key questions in modern science is exactly when life first emerged on Earth.
The further back researchers look into the Archaean era, the harder it becomes to distinguish genuine biological traces from structures and chemical signals that arose without the involvement of organisms.
The new discovery from India is significant precisely because it combines several independent pieces of evidence.
If the biological origin of the carbon is confirmed, the rock from the Singhbhum craton will become one of the most compelling pieces of evidence that complex microbial communities existed on Earth as far back as around 3.5 billion years ago.
Source
Study: Trisrota Chaudhuri et al. Direct dating of 3.5 Ga biogenic carbon in a microbial mat remnant, Singhbhum Craton, India.
Journal: Proceedings of the National Academy of Sciences (PNAS), 2026.