Indian scientists searching for clues to the oldest signs of life on Earth have discovered evidence of organic matter in a 3.5-billion-year-old sample of Paleoarchean volcanic–hydrothermal chert.
If confirmed, the team said the ancient chert would represent one of the oldest signs of life emerging on Earth, less than a billion years after the planet’s formation.
Searching for the Oldest Signs of Life on Earth
According to a press release announcing the discovery, some of the earliest known traces of life on Earth have typically been preserved as carbonaceous matter. Although many have been discovered in marine and hydrothermal vents, others have been unearthed in black cherts and volcanic-sedimentary rock formations.
“Kerogen, insoluble organic matter preserved in sediments, represents some of the earliest known terrestrial biosignatures which have provided important insights into the origin of life and planetary habitability,” the researchers write.
However, they also note, directly dating these ancient materials is often complicated by a lack of “suitable geochronometers.” Instead, dating ancient cherts and other early traces of life on Earth has relied on indirect constraints from surrounding materials. These include dating the ages of cross-cutting lithologies.
Ancient Organic Material Dated to 3.497 Billion Years Ago
To date, the newly discovered 3.5-billion-year-old chert, T. Mark Harrison, Trisrota Chaudhuri, and colleagues examined samples of “banded” cherts collected from the Singhbhum Craton in eastern India. Under microscopic analysis, the team found that the cherts contained “submillimeter-scale layers” alternating between carbonaceous matter and quartz-rich sedimentary material.
“The chert exhibits micron-scale alternating layers of carbonaceous and siliceous materials,” the study authors explained.
Outcrop photo showing dark carbonaceous bands in chert. Image Credit: Trisrota Chaudhuri.
To date the material, the team analyzed a specific combination of zircon grain characteristics. This analysis indicated that the grains were “emplaced” as part of a ‘tuff.’ The team said a tuff is a volcanic ashfall “that settled on the sediments at the time of their deposition.”
Next, they tested the lead isotopes found within the zircon crystals. The team said this analysis of the material in the banded cherts suggested the organic cherts formed about 3.497 billion years ago. This date places them in the Paleoarchean Eon.
“Here, we report a 3.5-billion-year-old carbonaceous chert from eastern India containing what are interpreted to be contemporaneous tuffaceous zircons,” they write.
The team said the carbon isotope analysis yielded a value “consistent with a biological origin of relatively sophisticated metabolism.”
“Isotope data of carbonaceous matter (CM) together with stratigraphic relationships indicate a biogenic origin of the kerogen, marking this, to our knowledge, as the oldest directly dated rock with a confirmed biosignature,” they write.
Among the Earliest Known Occurrences of Biologically Generated Organic Matter
When discussing the significance of the findings, the team said the ancient Indian cherts “count among the earliest known occurrences of biologically generated organic matter.”
The research team also noted that the findings suggest Archaean-age cherts “may contain records of early life and further suggest that key biological processes were active on Earth by around 3.5 billion years ago.”
The study “Direct dating of 3.5 Ga biogenic carbon in a microbial mat remnant, Singhbhum Craton, India,” by Trisrota Chaudhuri et al. was published in the Proceedings of the National Academy of Sciences.
Christopher Plain is a Science Fiction and Fantasy novelist and has spent the last six years as Associate News Editor and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him at christopher@thedebrief.org