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.

So living organisms concentrate carbon-12 in their cells—and when they die, that signature persists. When scientists find graphite that’s especially enriched in carbon-12, relative to carbon-13, they can deduce that living things were around when that graphite was first formed. And that’s exactly what the Tokyo team found in the Saglek Block—grains of graphite, enriched in carbon-12, encased within 3.95-billion-year-old rock.

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.”