The Bacteria That Built a 3.5-Billion-Year-Old Reef: Western Australia's Strelley Pool Stromatolites

The Bacteria That Built a 3.5-Billion-Year-Old Reef: Western Australia's Strelley Pool Stromatolites

On a rocky ridge in Western Australia's Pilbara region, a reef rises from the ancient landscape—not a coral reef, but a structure built by microbes. The Strelley Pool Chert preserves some of the oldest convincing evidence of life on Earth: layered mounds of sediment that microbes built 3.43 billion years ago.

A Microbial Reef in a Shallow Sea

The Strelley Pool formation is a band of sedimentary rock that outcrops across the East Pilbara Terrane, one of the oldest surviving fragments of Earth's crust. Around 3.43 billion years ago, this area lay beneath a shallow, warm sea. In those waters, communities of photosynthetic bacteria—cyanobacteria, or their ancient ancestors—grew in thin mats across the seafloor.

These microbes didn't build skeletons. Instead, they trapped and bound sediment grains from the water, creating sticky films that accumulated layer by layer. Over time, these microbial mats grew into domes and columns, some reaching a metre across. The result was a reef of living slime, a structure that looked nothing like the coral reefs that would appear billions of years later.

The Evidence in the Rock

What makes Strelley Pool special is not just the age of its stromatolites, but the quality of their preservation. The chert—a hard, fine-grained silica rock—has preserved microscopic details that other ancient stromatolite localities lack.

Geologists have found wavy lamination patterns that match modern microbial mats, as well as tiny filaments and spheroids that resemble bacteria. The structures show evidence of phototaxis—growth toward light—suggesting these microbes used sunlight for energy. Carbon isotope ratios in the rock point to biological activity, with a preference for the lighter carbon-12 isotope that living things favour.

These are not just shapes that look like fossils. They carry chemical and textural signatures that are hard to explain by any non-biological process.

A Window into Early Earth

The Strelley Pool stromatolites lived during the Archean Eon, a time when Earth's atmosphere contained almost no oxygen. The sun was dimmer, the moon closer, and the continents were small and unstable. Yet here, in that harsh world, life had already established itself and was building structures visible from space.

These microbial reefs were the dominant ecosystem on Earth for over two billion years. Their modern descendants still exist in places like Shark Bay, Western Australia, where living stromatolites grow in hypersaline water. The comparison is striking: the same shapes, the same layering, the same process of trapping and binding sediment, separated by 3.4 billion years.

What the Stromatolites Tell Us

The Strelley Pool formation challenges the idea that early life was limited to simple, single cells drifting in the ocean. These microbes organised themselves into complex communities, responded to their environment, and built structures that altered the landscape. They were not just surviving; they were engineering.

Some researchers argue that stromatolites like those at Strelley Pool may have been the first ecosystems to significantly affect global geochemistry. By fixing carbon and releasing oxygen as a byproduct of photosynthesis, these microbial cities began the slow process of transforming Earth's atmosphere—a transformation that would eventually make animal life possible.

Today, the Strelley Pool Chert lies exposed on a dry Australian ridge, far from the sea that once covered it. The reef is dead, turned to stone. But the shapes remain, as clear as the day they formed, waiting for anyone who knows how to read them.