Stromatolites are among the oldest known physical evidence of life on Earth, representing ancient microbial structures that first appeared approximately 3.5 billion years ago. These layered, rock-like formations were built by communities of microorganisms in shallow, sunlit waters. During the Archean Eon, Earth’s atmosphere contained virtually no free oxygen, dominated instead by gases like methane and carbon dioxide. The proliferation of stromatolites signaled a profound transformation that would fundamentally alter the planet’s chemistry and atmosphere.
Oxygen The Key Contribution
The gas contributed by stromatolites that proved most important was molecular Oxygen (\(O_2\)). This gas was a waste product of the microorganisms’ metabolism, which employed oxygenic photosynthesis. This mechanism utilizes sunlight to convert carbon dioxide (\(CO_2\)) and water (\(H_2O\)) into energy-rich sugars.
The reaction releases molecular oxygen as a byproduct, a substance toxic to most existing life forms at the time. Earlier life forms relied on anaerobic processes, often using compounds like hydrogen sulfide or iron for energy generation in an oxygen-free environment. This biological activity systematically injected large quantities of a highly reactive gas into the oceans and atmosphere, eventually overwhelming the planet’s ability to absorb it.
Stromatolites and Cyanobacterial Producers
Stromatolites are laminated, sedimentary mounds built by sheets of microorganisms called microbial mats. These structures are created by the trapping, binding, and cementation of fine sediment particles by microbial secretions. This accretionary process results in the characteristic layered, dome-like or columnar shape seen in ancient fossils and rare modern examples, such as those in Shark Bay, Australia.
The primary organisms responsible for oxygen production within these mats are cyanobacteria, a type of photosynthetic bacteria. Sometimes referred to as blue-green algae, cyanobacteria are the biological engine of the stromatolite structure. Their ability to perform oxygenic photosynthesis changed the planet’s fate.
The physical structure provided a stable, vertically growing platform for the cyanobacteria to thrive in shallow water. As microbes grew toward the sunlight, they trapped sediment, forming new layers on top of the old. This continuous cycle allowed cyanobacteria to maintain access to sunlight while protecting the community beneath from harsh ultraviolet radiation. The sheer number of these structures, growing over hundreds of millions of years, allowed oxygen to gradually accumulate.
The Great Oxidation Event
The accumulation of biologically produced oxygen triggered the Great Oxidation Event (GOE), beginning around 2.4 to 2.5 billion years ago. This marks the point when free oxygen first built up significantly in the atmosphere. Before the GOE, oxygen released by cyanobacteria was consumed by chemical “sinks,” primarily reacting with dissolved minerals in the oceans and crust.
Once these sinks were saturated, excess oxygen escaped into the atmosphere. The consequences were catastrophic for the dominant anaerobic life forms, which had evolved in an oxygen-free world. Oxygen acted as a poison, leading to a widespread extinction event, often termed the “Oxygen Catastrophe.”
The rise in atmospheric oxygen set the stage for two major developments enabling the evolution of complex life. The first was the creation of the ozone layer in the upper atmosphere. Oxygen molecules were split by solar radiation and reformed into ozone (\(O_3\)), shielding the planet’s surface from damaging ultraviolet radiation. This protection eventually allowed life to colonize land.
The second development was aerobic respiration, a highly efficient form of energy generation. Organisms that evolved to use oxygen in their metabolism could generate significantly more energy. This energy surplus was a prerequisite for the evolution of larger, multicellular, and more complex life forms.
Geological Evidence of Atmospheric Change
Scientists have been able to reconstruct the timing of the GOE and the subsequent atmospheric changes through distinct physical signatures left in the rock record. One of the most compelling pieces of evidence is the existence of Banded Iron Formations (BIFs). These sedimentary rock layers date from about 3.7 to 1.85 billion years ago, characterized by alternating bands of iron-rich minerals, such as hematite, and iron-poor chert.
BIFs formed because the newly produced oxygen reacted with vast amounts of dissolved ferrous iron (\(Fe^{2+}\)) present in the anoxic early oceans. This reaction formed insoluble ferric iron oxide (\(Fe^{3+}\)), or rust, which precipitated out of the seawater and settled on the ocean floor. The alternating layers suggest a cyclical process of oxygen release and consumption.
The near-disappearance of BIFs from the geological record around 1.85 billion years ago signals that the oxygen sinks in the oceans had finally been depleted. Following this, the oxygen saturation front began to move onto the continents. The later appearance of “Red Beds”—terrestrial sedimentary rocks stained red by iron oxides—indicates that oxygen was present in the atmosphere at high enough concentrations to react with iron exposed on land surfaces. This transition from oceanic evidence (BIFs) to terrestrial evidence (Red Beds) provides a clear timeline for the planet’s transition from an anoxic to an oxygenated world.