Stromatolites are among the most ancient records of life preserved in Earth’s geological history. These layered, rock-like structures represent the fossilized remains of microbial communities that flourished in the early oceans. They provide physical evidence of a time when the only life forms were microscopic, offering a direct window into the antiquity of biological processes.
What Stromatolites Are
Stromatolites are accretionary structures formed by the growth of successive layers of microbial mats, primarily composed of photosynthetic cyanobacteria. These dome, column, or sheet-like structures are the result of biological activity influencing physical sedimentation in shallow water environments. The sticky, extracellular polymeric substances secreted by the microorganisms create a cohesive film that traps fine sediment grains carried by water currents.
As sediment accumulates, the microbes must migrate upward toward the light to continue photosynthesis. This upward growth and subsequent trapping of new sediment create the characteristic fine, laminated layers, known as laminae, visible in a cross-section. The continuous trapping and binding of sediment, combined with the precipitation of minerals like calcium carbonate through metabolic processes, result in the lithified, rock-hard structure recognized as a stromatolite.
Dating the Oldest Fossil Evidence
Determining the precise age of the oldest stromatolite fossils is a complex challenge due to the intense heat and pressure that ancient rocks have endured. The most widely accepted evidence comes from the Dresser Formation in the Pilbara Craton of Western Australia, where layered structures are preserved in rocks dated to about 3.48 billion years ago. These structures exhibit the conical and domal shapes typical of biologically formed stromatolites, providing strong, though debated, evidence for life in the Archean Eon.
A potentially older, but more controversial, claim exists for fossils found in metasedimentary rocks in the Isua Supracrustal Belt of Western Greenland. Researchers argue that these microstructures represent stromatolites dating back approximately 3.7 billion years. Dating such ancient formations requires scientists to rely on radiometric dating of surrounding volcanic and igneous rock layers to constrain the maximum age of the fossils. While the 3.48 billion-year-old Australian fossils are accepted as the earliest definitive structures, the 3.7 billion-year-old Greenlandic findings push the timeline for microbial life further back.
Their Role in Earth’s Oxygenation
The cyanobacteria that built these ancient stromatolites were responsible for Earth’s first global-scale environmental transformation through the process of oxygenic photosynthesis. For nearly two billion years, these microbes continuously converted sunlight and carbon dioxide into energy, releasing free oxygen as a waste product. Initially, this oxygen reacted with dissolved iron in the oceans, forming massive deposits of Banded Iron Formations, effectively scrubbing the gas from the water.
Once the oceans became saturated with oxygen, the gas began to build up in the atmosphere, leading to the Great Oxidation Event (GOE) around 2.4 billion years ago. This rise in atmospheric oxygen changed the planet from an anoxic world to an oxygen-rich one. The influx of oxygen was catastrophic for the anaerobic life forms that dominated, but it paved the way for the evolution of oxygen-breathing organisms and complex life.
Where Stromatolites Live Today
Despite their former global dominance, living stromatolites are now extremely rare, surviving only in a handful of geographically isolated environments. Their decline began with the evolution of grazing organisms, such as snails and other invertebrates, which feed on the delicate microbial mats. Today, stromatolites are generally confined to habitats where the extreme conditions naturally exclude these grazers.
The most famous modern example is found in Hamelin Pool at Shark Bay, Western Australia, where the water salinity is nearly twice that of normal seawater. This hypersaline environment is too harsh for most marine animals, allowing the slow-growing cyanobacteria to thrive and build their characteristic structures without predation. Other living examples exist in unique settings, such as high-salinity lagoons in the Bahamas and cold, freshwater lakes in Canada. These modern colonies serve as living laboratories, enabling researchers to study the formation processes that shaped Earth’s earliest ecosystems.