The planet's oldest bedrock
Near the edge of Greenland's vast ice sheet lies the Isua Greenstone Belt, a 35-kilometer-long stretch of some of the oldest rocks on Earth. These formations of metamorphosed volcanic and sedimentary rock date back between 3.7 and 3.8 billion years to the Eoarchean era. Geologists study this remote region because it shows the planet's early history. Most rocks this old have been so altered by heat and pressure over billions of years that their original features are gone. A few sections of the Isua belt, however, are unusually well-preserved, sparking a decades-long hunt for the world's most ancient signs of life.
The search began with chemistry. In the 1990s, researchers reported finding graphite particles within Isua rocks. This graphite contained a ratio of carbon isotopes—specifically, a low amount of carbon-13 compared to carbon-12—that suggests biological processes like photosynthesis. Living organisms preferentially use the lighter carbon-12 isotope, so graphite formed from ancient organic matter should have this "isotopically light" signature. Skeptics, however, argue that non-biological processes can also create this isotopic imbalance. They suggest that hot fluids reacting with minerals deep in the Earth's crust, a process called metasomatism, could have formed the graphite without any involvement from life.
Fossils or folded rocks?
The debate intensified in 2016. A team led by Allen Nutman of the University of Wollongong announced the discovery of what appeared to be fossilized stromatolites in 3.7-billion-year-old Isua rocks. Stromatolites are layered, mound-like structures built by communities of microbes in shallow water. If confirmed, these one-to-four-centimeter-high structures would predate the previous oldest stromatolite fossils from Western Australia by 220 million years. The discovery was made in an area recently exposed by melting snow.
Just two years later, a counterclaim was published by a team including Abigail Allwood, a NASA astrobiologist who had previously confirmed the ancient Australian stromatolites. After visiting the Isua site, Allwood’s team concluded that the conical shapes were not fossils. Their three-dimensional analysis suggested the structures were the product of geological deformation—the rocks had simply been folded and stretched by immense heat and pressure long after they first formed. They pointed to a lack of fine, internal layering characteristic of true stromatolites and chemical signatures inconsistent with a shallow marine environment. The original team maintains their interpretation, arguing that the intense geological history of the region could have distorted the fossils' finer features. The debate continues, with research teams making joint helicopter trips to the remote outcrop to examine the evidence together.