Around 3.7 billion years ago, Earth as we know it was still in progress. Asteroids bombarded its surface. On land, mountains rose and small areas of shallow water formed. But was that long-gone water a proving ground for the first remnants of life on Earth? As The New York Times’ Nicholas Wade reports, a newly-discovered fossil that could be Earth’s oldest is shedding new light—and plenty of controversy—on the ancient origins of our planet.

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Australian and British scientists have discovered fossils in the Isua Greenstone Belt of Greenland that they claim to be the oldest ever found on Earth—a find so significant, they sat on the discovery for four years to allow enough time for verification. Now, they've finally published their research in the journal Nature.

The fossils are called stromatolites, which are layers of ancient microorganisms that grew in shallow water. The surface of the colony traps sand, which is eventually incorporated into their mat-like layers—the ancient remnants of which are recorded in the geologic record. Oddly enough, stromatolites are older than the world’s oldest rocks, since scientists think that the rocks they co-existed with (Earth’s oldest) have been crushed and destroyed by plate tectonic and erosion. The stromatolites in question were discovered in southwest Greenland, which is already home to some of Earth’s oldest rocks.

As Wade reports, it’s likely that scientists will debate many aspects of the find. Since the fossils are 220 million years older than any others yet found, they challenge scientific assumptions about how life formed on Earth.

The fossils' current estimated age means they formed toward the end of a period called the Late Heavy Bombardment, when the just-formed planet was continually pelted with asteroids and comets. But scientists are still debating how intense this bombardment was and whether it would even be possible for life to form, writes Wade. The other option is that the microbes crept in just after the bombardment ended. If that’s true, it means that life must have evolved much faster than previously thought—in just 100 million years.

If life sprung up on Earth this quickly, then perhaps another planetary neighbor could also have supported life at some point. Mars is thought to have been strikingly similar to Earth during the Late Heavy Bombardment, so it’s possible that the red planet generated life of its own during this time.

Since the discovery is so explosive, it will doubtless generate plenty of controversy. For one, natural abiotic processes could produce structures that appear to be stromatolites, reports Ed Yong at the Atlantic. Additionally, the rocks in the Isua Greenstone Belt are highly deformed and most have been twisted and smashed under high temperatures and heat.

To support their assertion that these wavy layers were once creatures, the researchers studied the chemistry of the rocks to tease out the signatures of life. “The chemical evidence could be interpreted as signs of life, but there’s always been some element of doubt,” lead author of the study Allen Nutman tells Yong. “But what we have now is something very different—something tangible and visible you can see, rather than a reading that’s come out of an instrument.”

Another concern is the difficulty in dating the most ancient objects on Earth. The scientists used radiometric dating to determine the stromatolites' age, Joel Achenbach reports for The Washington Post, a method that relies on measuring the proportion of radioactive elements in the rocks.

In an article on the find in Nature, University of Washington geobiologist Roger Buick tells Alexandra Witze that he has “about 14 queries and problems that need addressing before I believe it.” But if it is true, it might be time to update our vision of that roiling, immature Earth.