Stromatolites are identified partly by what they look like — convex-upward structures and wavy laminations that reflect microbial communities growing toward sunlight — and partly by their geochemistry. The sulfur isotopes in the barite veins within the Dresser Formation stromatolites support a biological origin. Most scientists accept their biogenicity. The structures are found in the Pilbara Craton, in a geological unit that has not been substantially metamorphosed or deformed, which is why the evidence survives. Other ancient rock formations that might hold earlier traces — such as the Isua Supracrustal Belt in Greenland at 3.7 billion years, or the Nuvvuagittuq Belt in Quebec at up to 4.28 billion years — exist, but their rocks have been altered enough that the biological interpretation remains contested. The Dresser Formation fossils are the oldest that are broadly accepted.

The Jack Hills, also in Western Australia, have yielded something even older: biologically fractionated graphite inside a single zircon grain, suggesting life may have existed 4.1 billion years ago. A zircon grain is not a stratigraphic unit — it is a single mineral crystal — so the evidence, while intriguing, cannot yet be placed in a geological context the way the Dresser Formation fossils can. The Apex chert rocks from the Pilbara Craton, dated to 3.465 billion years, also contain microfossils of prokaryotic filaments. Together, the Pilbara rocks contain a layered record of early life that spans hundreds of millions of years of the Archean eon. The craton's survival — its resistance to the plate-tectonic recycling that consumed most of Earth's early crust — is why this record exists at all.

Photosynthetic bacteria — cyanobacteria — built the early stromatolites and released oxygen as a metabolic byproduct. For hundreds of millions of years, most of that oxygen was immediately consumed by dissolving iron in the oceans, oxidising it into the iron oxide deposits now mined as ore across the Pilbara. Around 2.3 billion years ago, when the oceanic iron was effectively exhausted, oxygen began accumulating in the atmosphere. The Great Oxidation Event, as it is called, made aerobic life possible. Every animal that has ever existed owes its oxygen-breathing metabolism to the work of bacteria that looked nothing like animals. The stromatolites in the Dresser Formation are not merely old fossils. They are, in a meaningful sense, the reason the rest of life happened.

Stromatolites are not only fossils. Living stromatolites grow today in Shark Bay, approximately 800 kilometres south of the Pilbara on the Western Australian coast — a UNESCO World Heritage site where hypersaline conditions exclude the grazing organisms that would otherwise consume the microbial mats. The living structures build at rates of a fraction of a millimetre per year. They look like stubby grey columns in shallow water, unremarkable until you know what you are seeing: the same biological process that first built Earth's oxygen, continuing at a pace so slow it barely registers against a human lifetime. The contrast between Shark Bay's living mats and the Dresser Formation's ancient stone record — 3.48 billion years apart — is not a contrast between life and death. It is a continuity.

The origin of life on Earth is not fully explained by the fossil record. Stromatolites prove that photosynthetic bacteria existed 3.48 billion years ago, but photosynthesis is a sophisticated metabolic process that almost certainly evolved from simpler precursors. Molecular clock models — using the genomes of modern organisms to extrapolate backward — suggest the last universal common ancestor, the LUCA, may have lived 4.477 to 4.519 billion years ago. That is very close to the formation of the oceans themselves, around 4.4 billion years ago, suggesting that life may have arisen almost as soon as liquid water was stable. The Pilbara's rock record does not reach back that far. But it holds the oldest material that does.