Stromatolites are layered carbonate/silicate rocks. Their creation is most commonly attributed to cyanobacteria, which create stromatolites by trapping and binding of sediments. People do not know for sure, but they believe that one method cyanobacteria create stromatolites is that "each cyanobacteria cell produces and secretes a sticky film of mucus that traps the sediment. The sediment is then bound together with mucus, and the cyanobacteria grows over the grains, towards the sun."

Fossil stromatolites are arguably our earliest record of life on Earth. Some are more than 3.5 billion years old! (See Dating Rocks to find out how scientists know how old the fossils are.) Stromatolites are extremely significant for the theory of evolution because "if the stromatolites were formed by microbes, then life must have adapted to normal, non-extreme, environments even as early in the planet's history as 3.4 billion years ago. Earth is about 4.5 billion years old. Furthermore, life by that time would have already diversified enough to form complex ecosystems."

The heyday of stromatolites was during the Precambrian period, although they continued to survive afterwards, mostly only in barely habitable regions where other organisms couldn't reach them. The Earth mainly consisted of simple single-celled organisms during this time, before the Cambrian Explosion.
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Today, they can be still found in hypersaline lakes (lakes with high concentrations of salt) and marine lagoons, where the conditions are too extreme for animal grazing. One known place that still have "living" stromatolites is Western Australia.