The Rock Bottom of Origins

AS A YOUNG geology student in Durgapur, West Bengal, in the 1980s, Rajat Mazumdar now recalls with affection how taken up he was with the debate around an ancient rock sample that had been discovered in Odisha. “The scientists who had made the discovery [from Presidency College in Kolkata] believed that they had found a sample that was part of an ancient crust and at least around 3.8 billion years old. Everyone back then was talking about it,” he says. But after the findings were published, several scientists were sceptical of the claim. And subsequent investigations revealed that the sample was old, but not that old. “There had been some sort of a miscalculation and it was found that it was actually about 3.3 billion years old. But it became a sentimental thing for me,” he says. “I told myself I must find the oldest rock in India.”

Much later, in 2010, Mazumdar, a professor now, was in the very area, Champua, where the rock sample had been discovered in the 1980s. He was accompanied by a student, Trisrota Chaudhuri. Using clues gleaned from journals and research papers, Mazumdar and Chaudhuri were trying to find rocks as old as the ones found in the 80s. “We had descriptions and some landmarks, but this was all from the pre-GPS age. So it was in a language like ‘from this PWD bungalow, walk such and such kilometres south or north from it’,” Mazumdar recalls.

Mazumdar was then part of an international UNESCO project tasked with finding rocks belonging to the Paleoproterozoic era. This is the period spanning 2.5 to 1.6 billion years ago, when continents first began to stabilise and, according to some, the planet had 20- hour days and 450 days a year, thanks to its rotational rate. Knowing this could be his best bet, he had picked the Champua area.

Champua is a remote and rural location in Odisha, bordering Jharkhand, about four-five hours by road from Jamshedpur. It is replete with deep forests, hills, valleys, and large and small rocks that appear everywhere. According to Mazumdar, over the years, several of these rocks taken back to laboratories have been throwing up very early dates, indicating that they could’ve been part of a very ancient crust.

“It’s really a nice little place for geologists,” he says. “It’s not been the point of focus [for geologists globally] like Jack Hills [in western Australia, an area which has resulted in several important geological studies] or places in Canada. But the clues have all been there [in Champua], and we geologists from India have been finding some interesting things here and there.”

For the next year, over several trips, each lasting a few days, Mazumdar and Chaudhuri covered the area on foot, scanning it for any rock that might look old. To an untrained eye, these rocks might appear unimpressive, Chaudhuri says. “There’s nothing special about them at all. But us geologists, we can tell,” she says. “So we just walked and browsed.”

Every trip resulted in bags of rock pieces, 10-20 kg each, which they either sent through buses or carried with them in a cab, back to their labs in Kolkata. Once in their labs, Mazumdar and Chaudhuri managed to retrieve zircon crystals from these samples. Zircons are tiny crystalline minerals, barely spanning the width of a human hair but near indestructible. They form in magmas and incorporate other minerals within their crystal structures. And even though rocks perish over time, zircon crystals can survive for billions of years, offering us a rare glimpse into the earliest pages of Earth’s history.

Mazumdar and Chaudhuri were quite certain that they had found some interesting samples. But the technology and machinery to deal with zircon minerals, to date them or analyse their isotopic and elemental nature, is expensive and unavailable in India. For the next eight years, Mazumdar says, he approached foreign labs in Australia, Japan, Canada, the US and Germany to have a look at their samples, but failed to interest them. “The whole process is quite expensive. Most of these labs are already committed to their projects. And they are usually not too interested to check samples from areas that haven’t previously produced anything interesting,” he says.

Mazumdar eventually managed to convince a lab in China—the Beijing SHRIMP (Sensitive High Resolution Ion Microprobe) Centre under Yusheng Wan at the Chinese Academy of Geological Sciences— that was willing to analyse the samples on a collaborative basis. But Yusheng had a condition. “He was very clear. He was willing to do it, but said he would examine only four samples,” Mazumdar says. He jumped at the opportunity.

Close to seven years since he first discovered the samples, last year Mazumdar got a call from Beijing. It was Yusheng Wan. “He said, ‘You were right, they are very, very old,’” Mazumdar says.

Their findings were recently published in Scientific Reports, a journal from the publishers of Nature. Three of the samples turned out to be 3.4 billion, 3.7 billion and 4.03 billion years old. And the fourth sample, found to be 4.24 billion years old, was the second oldest material ever found on this planet.

Earth came into existence around 4.5 billion years ago, along with the rest of the solar system, from the wreckage of an exploded star. But this early Earth was no Eden. For the next half-billion years, as the story goes, Earth was covered with gurgling oceans of magma and continuously pummelled by meteorites and cosmic detritus. One such strike, possibly with another planet, led to a chunk of our planet splintering off to become the moon. For those years, Earth was hot and inhospitable, with no liquid water or any possibility of life. This period is considered to be such a hellish one that geologically the era is named after the Greek word for the underworld—Hadean.

Life could emerge—as simple single- celled prokaryotic cells, as the theory goes—no earlier than 3.8 billion years ago, once the violent bombardment of meteorites had ceased, the oceans of magma had receded, and water finally appeared.

These are at best educated guesses. The early years of Earth’s history are still a mystery. The everyday churn of our planet for 4.5 billion years—the meteorite impacts and volcanic eruptions of our ancient past, the movement of our tectonic plates and the erosion of our rocks—has meant that nothing from this early period has survived. There are no primeval rocks or relics that could reveal how things exactly were. Except for zircons, that is.

“Zircons are like these tiny windows— the only window, really—to what was going on on our planet 4 billion years ago,” Chaudhuri says.

Zircon crystals often incorporate other minerals like silicon, oxygen and zirconium into their structures. They are used in imitation diamonds and jewellery. But some of them—like the ones discovered by Mazumdar and Chaudhuri—can be incredibly old and they contain within their tiny structures the stories of our planet’s earliest time.

While their original host rocks have long disintegrated, these tiny indestructible structures have remained, moved by rivers and streams and deposited into other locations or simply gotten embedded into other rocks, where they have remained for billions of years until now.

These tiny zircon crystals have now begun to tell us a very different story from the one we have heard for so many years. “We have always believed Earth was this hot and hellish place 4 billion years back. But some of these zircons, although it is still debated, are telling us it may have become cooler and wetter, even liveable long before,” Chaudhuri says.

Studies on ancient zircon crystals have revealed several interesting things. They have pointed to the presence of a stable primordial crust on the earth’s surface by 4.4 billion years ago and a distinctive oxygen isotopic signature that indicates the presence of water on Earth’s surface by 4.3 billion years ago. This is far from the picture painted of earth as a hot and molten world, which, let alone life, could not even support a crust of solid rock.

All of this now leads to a larger question. If Earth’s atmosphere had significantly cooled for the presence of a stable crust and for water, could life be far behind? The most explosive study in this field came in 2015, with the finding of a 4.1 billion-year-old biological signature within a zircon’s chemical composition. The journal Proceedings of the National Academy of Sciences published a study where researchers had found a 4.1 billion year old zircon mineral with a chemical composition that suggested organic life. If true, it meant that our earliest ancestors— in their single-celled microbial form—had emerged a long time before anyone had thought possible. And this emergence pre-dated the earliest undisputed fossil evidence for life—a fossilised mat of single cell microbes called stromatolites that grew in shallow seas and was discovered in a sandstone rock in Australia—by 600 million years.

In this study, the researchers had found tiny bits of potentially undisturbed graphite, a form of carbon, in a single 4.1 billion- year-old zircon crystal from Jack Hills in Western Australia. The graphite had a higher ratio of carbon-12 (light atoms) to carbon-13 (heavier ones). This suggests that the carbon had been processed by living organisms, because, according to geochemists, some of these life forms tend to incorporate more of the light carbon and less of the heavy one.

The researchers have themselves claimed that it is possible for non-biological processes to lead to such an isotopic ratio, but it is highly unlikely. “It’s not a smoking gun for there being life at 4.1 billion years,” the lead researcher Elizabeth Bell from the University of California then told Los Angeles Times. “But if you saw that same isotopic signature on the Earth today, you would say, ‘That is from a biogenic source’.”

SINCE THEN, THERE have been few more studies that have hinted at a very early emergence of life. Some Australian researchers claimed to have found 3.7-billion-year-old stromatolites, or fossil remains of microbes, in 2016. Last year, another group of researchers reported finding 4.28-billion-year-old fossilised microorganisms that once thrived underwater around hydrothermal vents in the Nuvvuagittuq Belt, which consists of very ancient rocks close to Quebec, Canada. Earlier this year, another study, published in Science Advances, found that two 4.5 billion-year-old meteorites which had separately crashed to Earth back in 1998, contained liquid water along with complex organic substances (hydrocarbons and amino acids) that may have been the ingredients for life.

All of this is still hotly contested. This early emergence of life—however tantalising— is still at best a claim. It may hint to an origin of life not as simple or recent as we have believed. But it is still a ‘might have been’ than a definite ‘life was here’.

Our best bet to get to the root of all this, Mazumdar says, lies in finding more of the oldest materials on earth. Mazumdar and Chaudhuri say this will be the next step in their research. They will be looking for more samples of ancient zircon crystals in India and examining their oxygen and carbon compositions for early signs of water and perhaps even life. “Even if we find water, it will be very significant. Because wherever there is water, there is usually life,” Chaudhuri says.

According to Mazumdar, while there are several capable geologists working in India, very few international agencies and researchers have attempted to look at rocks in India. “If you search, you will definitely get more [zircons here],” Mazumdar says. “But I think with this discovery [of the second-oldest zircon in Odisha], it will intensify work in this field in India. I’m sure geologists and the Geological Society of India are already looking more intently at Champua.” There is even talk of an institute, either an IIT or the GSI, he says, acquiring a machine for isotopic analyses of zircon minerals in India soon.

So what are the chances of laying one’s hands on a zircon mineral that contains evidence of early life? “Look at the Australian study [the 2015 study of a 4.1 billion-year-old zircon carrying carbon composition suggesting life]. It was a freak finding. To find such a zircon with such a carbon composition from among thousands of samples was really lucky,” Chaudhuri says. “And that’s one thing I have learnt with our current discovery. You need to be a sharp geologist, yes, but also very lucky.”

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