Stromatolites are ancient, layered rock formations that hold a unique place in Earth’s history as some of the planet’s earliest evidence of life. These structures, often appearing as mounds or columns, represent the collective activity of microscopic organisms that shaped the environment billions of years ago. Their study, particularly in locations like Australia, offers a remarkable glimpse into the conditions of the early Earth and the evolution of life.
What Are Stromatolites?
Stromatolites are sedimentary structures built by microbial mats, primarily composed of photosynthetic microorganisms known as cyanobacteria. These bacteria form sticky biofilms on submerged surfaces. As they grow, these microbial mats trap and bind sediment particles from the surrounding water, such as sand and other mineral grains.
The trapped sediment, along with minerals precipitated by the microbes, creates successive layers that gradually harden. This continuous process results in the distinct layered appearance characteristic of stromatolites. They can grow to a meter or more in height and exhibit various shapes including columns, cones, and domes. While once widespread, modern living stromatolites are now rare, making fossilized forms a significant record of ancient life.
Australia’s Living Fossils: Key Sites and Significance
Australia is home to some of the world’s most significant stromatolite sites, including both living and ancient fossilized structures. Hamelin Pool, within Shark Bay in Western Australia, hosts the most extensive living marine stromatolite system globally. These modern stromatolites thrive due to unique hypersaline conditions, where the seawater is approximately twice as salty as normal ocean water. This extreme salinity limits grazing animals and other organisms that would otherwise compete with or consume the cyanobacteria, allowing these ancient life forms to flourish undisturbed.
Shallow, warm waters and restricted circulation, caused by a bar at the bay’s entrance and high evaporation rates, contribute to this elevated salinity. These living stromatolites provide a direct example of processes that occurred billions of years ago, earning them the moniker “living fossils.” They grow very slowly, with a 1-meter structure potentially taking 2,000 to 3,000 years to form.
Beyond living examples, Western Australia’s Pilbara Craton contains some of Earth’s oldest fossilized stromatolites. The Strelley Pool Chert in the Pilbara region holds evidence of Early Archaean life in the form of an ancient stromatolitic carbonate platform. These structures, found in areas like the Dresser Formation, are approximately 3.4 to 3.5 billion years old. Their preservation offers detailed insights into the morphology and environmental conditions of early microbial life. Pilbara stromatolites show various forms, including flat, dome-shaped, and conical structures, indicating diverse microbial communities and ancient environments, possibly remnants of volcanic hot springs.
Unlocking Earth’s Ancient Past
Stromatolites offer insights into Earth’s ancient past, revealing how early life transformed the planet. The cyanobacteria within these formations were among the first organisms capable of photosynthesis, a process that consumes carbon dioxide and releases oxygen. This biological activity led to the gradual oxygenation of Earth’s atmosphere, an event known as the Great Oxygenation Event.
Over a billion years, this continuous oxygen production increased atmospheric oxygen levels from less than 1% to over 20%. This change was important for the evolution of more complex, oxygen-dependent life forms. Studying the layered structures and chemical composition of fossilized stromatolites allows scientists to reconstruct ancient ecosystems and climates, including water depth, sediment influx, and hydrothermal activity. These serve as geological indicators, helping researchers understand the conditions that allowed life to emerge, flourish, and shape the planet we inhabit today.