By Ed Yong
Back then, the landscape would have been very different. The Earth was a hellish place that had only just acquired a firm crust. Its atmosphere was devoid of oxygen, and it was regularly pelted with asteroids. There were no reindeer, whales, polar bears, or lichen. But according to new research, there was life.
In a rock formation called the Saglek Block, Yuji Sano and Tsuyoshi Komiya from the University of Tokyo found crystals of the mineral graphite that contain a distinctive blend of carbon isotopes. That blend suggests that microbes were already around, living, surviving, and using carbon dioxide from the air to build their cells. If the two researchers are right—and claims about such ancient events are always controversial—then this Canadian graphite represents one of the earliest traces of life on Earth.
Those fossils, from the Isua Belt in southwest Greenland, are stromatolites—layered structures created by communities of bacteria. And as I reported last year, their presence suggests that life already existed in a sophisticated form at the 3.7-billion-year mark, and so must have arisen much earlier. And indeed, scientists have found traces of biologically produced graphite throughout the region, in other Isua Belt rocks that are 3.8 billion years old, and in hydrothermal vents off the coast of Quebec that are at least a similar age, and possibly even older.
“The emerging picture from the ancient-rock record is that life was everywhere,” says Vickie Bennett from Australian National University, who was not involved in the latest study. “As far back as the rock record extends—that is, as far back as we can look for direct evidence of early life, we are finding it. Earth has been a biotic, life-sustaining planet since close to its beginning.”
This evidence hinges on a quirk of chemistry. Carbon comes in two stable isotopes—carbon-12, which is extremely common, and carbon-13, which is rarer and slightly heavier. When it comes to making life, carbon-12 is the more pliable building block. It’s more reactive than its heavier cousin, and so easier to transform into molecules like carbohydrates and proteins.
But are those graphite grains the same age? The rocks around them are metamorphic—they’ve been warped and transformed at extreme temperatures and pressures. During that process, and all the subsequent geological tumult that this region has experienced, it’s possible that much younger graphite somehow infiltrated the older rock, creating a false signal of early life.
To rule out that possibility, the Tokyo team looked at the structure of the graphite grains. The more orderly and crystalline those structures, the hotter the grains were when they formed. Based on that relationship, the team calculated the graphite was created at temperatures between 536 and 622 Celsius—a range that’s consistent with the temperatures at which the surrounding metamorphic rocks were transformed. This suggests that the graphite was already there when the rocks were heated and warped, and didn’t sneak in later. It was truly OG—original graphite.
There’s still room for doubt, though. Given how ancient these rocks are, and how much geological tumult they have experienced, it’s hard to fully exclude the possibility that the graphite got there later. Also, other processes that have nothing to do with living things could potentially change the ratio of carbon-12 and carbon-13. It’s concerning that the ratio varies a lot in the samples that the Tokyo team analyzed, says Andrew Knoll from Harvard University. But he also says that the team has been careful, and their combined evidence “makes a strong case that life existed on earth nearly 4 billion years ago.”
“The authors have done as many checks as they could for whether they are indeed analyzing 3.95-billion-year-old graphite rather than later contamination,” adds Elizabeth Bell, a geochemist from the University of California, Los Angeles. “They make a plausible case that the graphite is original.”
Bell herself found the oldest graphite that’s been measured to date. It lurked within a 4.1-billion-year-old zircon gemstone from Western Australia, and also contained a blend of isotopes that hinted at a biological origin. That discovery is also controversial, especially since the graphite was completely cut off from its source environment, making it hard to know the conditions in which it was formed.
Still, all of this evidence suggests Earth was home to life during its hellish infancy, and that such life abounded in a variety of habitats. Those pioneering organisms—bacteria, probably—haven’t left any fossils behind. But Sano and Komiya hope to find some clues about them by analyzing the Saglek Block rocks. The levels of nitrogen, iron, and sulfur in the rocks could reveal which energy sources those organisms exploited, and which environments they inhabited. They could tell us how life first lived.