Sometime in Earth's distant past, nearly 4 billion years ago, our planet's earliest lifeforms somehow coalesced from primordial chemistry.
Not much remains of the earliest organisms that inhabited Earth. Time and geology have changed whatever remains almost beyond recognition.
But in a few rare, ancient stone formations around the world, layers can be found that scientists believe are the fossilized remnants of microbial mats – the slimy biofilms in which single-celled organisms house their colonies.
Some of these traces of life have been dated to as early as 3.7 or 3.8 billion years ago, based on the age of the surrounding rock layer.
Now, however, a team of scientists led by geologist Trisrota Chaudhuri of the Geological Survey of India has managed to directly date material embedded within what the evidence suggests are the remnants of an ancient microbial community that lived and died 3.5 billion years in the past.
"Finding both well-preserved biological material and datable zircons in the same rock is extremely rare in the early history of the Earth," Chaudhuri told ScienceAlert.
"When this is possible, the age of the rock and the evidence of life can be linked directly, providing much robust age and isotope-based evidence for ancient life."
Time, weather, and geology are not kind to once-living material – and the further back in time you look, the scantier the fossil record grows. It's rare to find fossilized bone from just a few million years ago; the older, invertebrate record is even sparser.
However, with the right set of conditions, traces of dead microbes can be rapidly entombed in silica, preserving them as thin layers of carbon that can survive even as the surrounding rock is transformed by heat and pressure.
So when Chaudhuri and her colleagues found a carbonaceous layer sandwiched in an ancient striped chert within the Singhbhum Craton, one of Earth's oldest surviving blocks of continental crust, their spidey senses started tingling.
"At first, I did not expect to find evidence of ancient life," she said. "I was interested to know the source and the geochemical pathway via which this carbon was preserved in these rocks."
Carbon this ancient could have several possible sources. It could have been produced by non-biological processes, such as volcanism, or it could be the remains of something that was once alive.
To show that the carbonaceous layer could be the remains of ancient microbes, the researchers had to establish three things: First, that the carbon was indigenous to the rock, rather than introduced during a later geological event; that it could have had a biological origin; and that its structure was consistent with a microbial mat.
First, they used a technique called Raman spectroscopy – basically, using a laser to light up the carbon to probe its structure, which in turn reveals its formation and alteration history.
This helped them determine that the maximum temperature the carbon had ever experienced in its 'lifespan' was between 324 and 369 degrees Celsius (615-696 degrees Fahrenheit).
It sounds hot, but it's within the range in which biological signatures can remain preserved. Meanwhile, carbon trapped in later veins of quartz had been heated to much higher temperatures. The relatively cooler temperatures of the finely layered carbon suggest it was part of the original rock rather than squirted in during a later hot-fluid event.
Then the researchers looked at the carbon isotopes.
"Microorganisms preferentially use the lighter 12C isotope during metabolism (a biological process to trap carbon and generate energy as a by-product), leaving their organic matter depleted in 13C," Chaudhuri explained.
Sure enough, the material the researchers sampled was strongly depleted in carbon-13, with an isotopic signature characteristic of ancient biological carbon, which indicates something was likely alive.
Finally, the carbon occurred in super-fine, repeated layers alternating with silica-rich material, forming a laminated structure consistent with the remains of a microbial mat.
Each one of these on its own would be insufficient to identify the material as biological in origin; but, taken together, they present a compelling argument.
"We had to use several independent lines of evidence to establish that the carbon was biological in origin," Chaudhuri told ScienceAlert.
That, finally, brings us to the dating, and this part is incredibly nifty.
Embedded and interspersed with the laminated carbon layers, the researchers found tiny crystals of zircon – a mineral that allows for incredibly precise dating.
That's because as zircon forms, it takes up uranium but strongly rejects lead. Uranium radioactively decays into lead at very precise, known rates. So most of the lead that accumulates inside the crystal comes from uranium decay.
Two uranium isotopes and their subsequent decay into lead are used for zircon dating. Scientists can look at the ratio of uranium to lead in a zircon sample and make a very precise estimate of when that zircon crystal formed.
The researchers analyzed eight zircon crystals in the carbon-bearing rock. Four had been disturbed over their immense geological history, but the remaining four gave matching ages clustered tightly around 3.5 billion years.
But simply finding ancient zircons in the carbon layer wasn't enough. The crystals themselves could have been older than the sediment, eroded from another rock before becoming embedded in it.
Instead, several features of the zircons indicated that they were fresh volcanic material deposited as the carbon-bearing chert was forming. That means their age could pin down the age of the deposit itself: 3.497 billion years, give or take about 5 million.
"Erosion of detrital zircons should produce multiple, pre-existing older ages," Chaudhuri explained. "Detrital zircons, due to erosional abrasion, are rounded in nature."
The Bhitardari zircons, by contrast, were tiny, elongated, needle-like crystals.
"We interpret them as zircon crystallized from the volcanism and trapped within the chert when the rock was being formed," she said.
This doesn't make the Bhitardari material the oldest evidence of life ever found. Scientists have reported possible traces hundreds of millions of years older.
What makes this discovery so exciting is the strength of the link between the biological evidence and its age. Rather than inferring the age from the surrounding geology, the researchers had a radiometric clock literally embedded in the same deposit.
"This is important because it helps us determine when life was already present on the early Earth, giving us a clearer picture of how soon life emerged and how Earth's earliest environments may have supported it," Chaudhuri said.
Related: Gobsmacking Study Finds Life on Earth Emerged 4.2 Billion Years Ago
Earth 3.5 billion years ago was a very different place from the lush, temperate world we live in today. Oxygen levels were extremely low, and hot, volcanic, and hydrothermal processes dominated.
"What we find most remarkable is that our study again confirms that primitive life thrived in a chemically extreme world 3.5 billion years ago. These microorganisms lived alongside active volcanism, in an ocean rich in iron and silica," Chaudhuri said.
"The Bhitardari carbonaceous chert again confirms that life was not simply surviving on a quiet early Earth; microbial ecosystems were already occupying and adapting to dynamic volcanic environments remarkably early in Earth's history."
The findings have been published in the Proceedings of the National Academy of Sciences.
This article was fact-checked by Rachel Garner and edited by Clare Watson. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.