Stromatolites are rock-like buildups of microbial mats that form in limestone-forming environments. They typically form by the trapping and precipitation of mineral particles by communities of microorganisms such as cyanobacteria (pond scum).
Stromatolite-building communities include the oldest known fossils, dating back some 3.5 billion years when the environments of Earth were too hostile to support life as we know it today.
Stromatolites are the only fossils we can find for the first 7/8th of the history of life on earth. They show us the role that ancient cyanobacteria ("pond scum") played in the evolution of life on earth and in shaping earth's environments. The fossil record of stromatolites is astonishingly extensive, spanning 4 billion years of geological history with the forming cyanobacteria possibly having occupied every conceivable environment that ever existed.
Cyanobacteria are conjectured to have been the predominant form of life on early earth for more than 2 billion years, and were likely responsible for the creation of earth's atmospheric oxygen, consuming CO2 and releasing O2 through their photosynthetic metabolism.
Creation of the modern atmosphere by cyanobacteria is perhaps the most critical event in the history of earth. It powered the Cambrian explosion and subsequent evolution of the aerobic forms of life, including all animals.
