Stromatolites, often called “living fossils,” are some of the most ancient life forms on our planet, providing a direct link to what life was like billions of years ago. While once widespread, stromatolites are now exceptionally rare. One of the few places on Earth where these ancient life forms still thrive in a marine environment is Shark Bay in Western Australia.
What Are Stromatolites?
Stromatolites are not individual organisms but are layered, rock-like structures built by communities of microscopic life. They are formed by cyanobacteria, a type of photosynthesizing microbe. These microbes form vast, sticky mats in shallow water, which trap and bind particles of sediment in a slow cycle of growth and burial.
As one layer of sediment covers the microbial mat, the cyanobacteria must grow upwards through it to reach sunlight for photosynthesis. This upward migration creates a new living layer on top of the old. Over time, this repeated layering builds the distinctive dome-shaped or cauliflower-like structures. A structure just a meter high could be several thousand years old, with only the thin top layer being actively alive, while the underlying mass is the lithified remnant of past microbial communities.
The Significance of Ancient Stromatolites
Fossilized stromatolites are one of the earliest and most widespread records of life on Earth, with some examples dating back 3.5 billion years. Their importance extends beyond their age, as they fundamentally changed the planet’s environment. The cyanobacteria that build these structures were among the first organisms to perform oxygenic photosynthesis, releasing oxygen as a byproduct.
For billions of years, this oxygen gradually accumulated in the oceans and then the atmosphere. This transformation is known as the Great Oxidation Event, which occurred roughly 2.3 to 2.5 billion years ago. Before this, the atmosphere contained very little free oxygen, and these microbes created the conditions that allowed for the evolution of more complex, air-breathing life.
Why Shark Bay is a Unique Habitat
Stromatolites flourish in Shark Bay’s Hamelin Pool due to unusual environmental conditions. The water here is hypersaline, meaning it is about twice as salty as normal seawater. This is caused by a seagrass bank that restricts water flow and a high rate of evaporation.
This extremely salty environment is inhospitable to most marine creatures that would graze on the microbial mats. In most of the world’s oceans, the evolution of such grazing animals led to the widespread decline of stromatolites. The harsh conditions of Hamelin Pool create a refuge, protecting the cyanobacteria from predation and allowing the structures to grow undisturbed.
Visiting the Hamelin Pool Stromatolites
Hamelin Pool is a protected Marine Nature Reserve within the Shark Bay World Heritage Area. A 200-meter boardwalk leads visitors out over the shallow water, allowing observation of the dark, rounded stromatolites on the seafloor. Interpretive signs along the way help explain their formation and history.
Visitors must stay on the designated paths to protect the fragile structures. The boardwalk was damaged by Cyclone Seroja in 2021. While it is closed for repairs, the stromatolites can still be viewed from the shore, especially at low tide.