Stromatolite, a term derived from the ancient Greek words stroma (layer or stratum) and lithos (rock), describes a layered sedimentary structure created by microbial activity. These accretionary formations are considered the oldest visible ecosystems on Earth, offering a window into the planet’s earliest life forms. A stromatolite is fundamentally a lithified, or rock-hardened, structure, but its formation depends entirely on the activity of thriving communities of microorganisms.
The Mechanism of Formation and Structure
The formation of a stromatolite begins with a microbial mat, a cohesive, multi-layered sheet of microorganisms living in a shallow-water environment. These mats are primarily composed of photosynthetic organisms, such as cyanobacteria, which secrete a sticky, mucilaginous substance around their cells. This adhesive matrix traps and binds fine sedimentary grains, like sand and silt, that settle out of the water column. As the sediment accumulates, it threatens to bury the microbes and block access to sunlight.
In response to burial, the microbial community must actively grow upward through the newly trapped sediment layer to re-establish a surface exposed to light. This vertical migration forms a new microbial layer above the old sediment, effectively binding the loose particles into a stable, cohesive sheet. The continuous repetition of this process—trapping sediment, upward growth, and subsequent binding—results in the characteristic fine layering, or lamination, seen within the rock structure. The resulting shapes, which often include domal, conical, or columnar forms, reflect the patterns of microbial growth as the community tracks the sunlight.
The final structure is not the living mat itself but the resulting mineral deposit, which is cemented by calcium carbonate or other minerals precipitated by the microbes’ metabolic activity. Photosynthesis by cyanobacteria removes carbon dioxide from the surrounding water, which changes the water chemistry and promotes the precipitation of calcium carbonate around the microbial filaments. This biomineralization process fossilizes the layered structure, permanently recording the growth of the microbial mat.
Modern Living Colonies Versus the Fossil Record
Ancient stromatolites were once globally abundant, peaking in diversity and distribution during the Precambrian Eon, a time that spans from Earth’s formation up to about 541 million years ago. These microbial structures were a dominant feature in shallow marine environments worldwide. However, their prominence dramatically declined with the emergence and diversification of grazing and burrowing organisms, particularly during the Cambrian Period. The appearance of these complex life forms meant that the soft, nutrient-rich microbial mats were either consumed or mechanically disturbed, preventing the long-term, layered accretion necessary for stromatolite formation.
Today, actively growing stromatolites are rare and are relegated to niche, extreme environments where competing organisms are excluded. The most famous modern examples are found in hypersaline marine environments, such as Hamelin Pool in Shark Bay, Western Australia, where high salt concentrations deter most marine grazers. Living colonies also persist in other locations with unusual water chemistry, including certain alkaline lakes and coastal lagoons in the Bahamas.
The fossil record of stromatolites stretches back over 3.5 billion years, making them some of the oldest physical evidence of life on Earth. These fossilized remains demonstrate that microbial ecosystems were widespread across the ancient planet, unlike their sparse modern counterparts. The contrast between ancient abundance and the few, specialized modern locations highlights a significant shift in Earth’s biosphere, driven by the evolution of more complex, multicellular life.
Role in Earth’s Early Evolution
Stromatolites hold immense geological significance because they record the presence of life at a time when no other visible evidence of organisms existed. The oldest confirmed examples date back to the Archean Eon, providing tangible proof of microbial communities thriving in the planet’s primordial oceans.
The organisms responsible for building these structures, particularly the cyanobacteria, played a transformative role in shaping the planet’s atmosphere. Through oxygenic photosynthesis, these microbes used sunlight, water, and carbon dioxide to create energy, releasing free molecular oxygen as a waste product. For nearly two billion years, the cumulative effect of this microbial oxygen production slowly began to change the chemistry of the oceans and the atmosphere.
This gradual, massive accumulation of oxygen led to the Great Oxidation Event (GOE), a profound environmental change that occurred roughly 2.4 to 2.1 billion years ago. Before the GOE, Earth’s atmosphere contained almost no free oxygen, making it an anaerobic world. The oxygen released by the stromatolite builders initially reacted with iron dissolved in the oceans, leading to the deposition of banded iron formations, before finally saturating the water and escaping into the atmosphere. This transition from an anaerobic to an aerobic world paved the way for the evolution of all oxygen-breathing life.