Stromatolites or stromatoliths (are layered bio-chemical accretionarystructures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria. Fossilized stromatolites provide ancient records of life on Earth by these remains, some of which date from more than 3.5 billion years ago. Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that are formed above water, where rock meets air, by repeated colonization of the rock by endolithic lichens.
Stromatolites can now be found today as black and gooey mats on the bottom of lakes, and also as rock clusters in the shallow oceans.
Stromatolites were an important part of nature a long time ago. If they weren't around, we wouldn't exist! The microorganisms inside them produced oxygen as a waste product, and the oxygen created the atmosphere. So, the Stromatolites are responsible for our existence.