Stromatolites are laminated sedimentary deposits created by benthic (bottom-dwelling) microbial organisms, primarily bacteria and cyanobacteria but also eukaryotic algae, capable of trapping, binding, and cementing sediment grains in thin layers. These kinds of organisms remain an important constituent in many marine communities, including reefs, but are easily overlooked because of their tiny size, unfamiliar morphology, uncertain taxonomic affinities, and obscurity in recrystallized or metazoan-dominated samples. Stromatolites first appeared on Earth 3.5 billion years ago and were the earliest ecosystems to evolve on the planet. Their abundance in the Precambrian geologic record indicates microorganisms were the only common forms of life for more than 3 billion years.