Stromatolites are biogenic sedimentary rocks. They arise from the binding of sediment particles or the precipitation of substances dissolved in water as a result of the growth and metabolism of microorganisms. They are mostly made of limestone.
The oldest known fossils are mostly stromatolites, and they could therefore provide clues to how life evolved from very simple to more complex forms. Today, stromatolites are only found in niches unsuitable for other life, such as continental salt and soda lakes.