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Stromatolites are the oldest known fossilized remains of life on Earth. They are layered mounds or columnar structures formed by the growth of microorganisms, mainly cyanobacteria, in shallow water environments.

Cyanobacteria are photosynthetic bacteria that can manufacture their food through the process of photosynthesis. They are the primary contributors to the formation of stromatolites. These microorganisms grow in thin layers on surfaces submerged in water. They produce a sticky layer referred to as a biofilm, which helps in trapping and binding the sediments from the surrounding environment.

As cyanobacteria perform photosynthesis, they release oxygen. This oxygen combines with dissolved minerals in the water, leading to the precipitation of minerals such as calcium carbonate. The precipitated minerals are deposited within the biofilm, along with the trapped sediments. This leads to the formation of a thin layer of mineralized sediment on the surface of the cyanobacteria. Once this layer is formed, the cyanobacteria migrate upward to re-establish a new colony on top of the newly-formed layer. This is because they need sunlight for photosynthesis. This process of layer formation and upward migration of the cyanobacteria continues over long periods, eventually leading to the formation of distinct layers visible in the structure of stromatolites.

The repeated process of layer formation and migration of cyanobacteria in shallow water environments eventually leads to the creation of the stromatolite structure. These structures can grow over time to be large mounds or columns, depending upon the specific environmental conditions. Stromatolites act as a record of ancient life on Earth and their presence also helps us understand the geological and environmental conditions during their formation.