Fossil and modern stromatolites: evidence of autotrophic cyanobacteria.

Each stromatolite is a rocklike structure, typically 1 metre in diameter. (a) Section of a fossilized stromatolite. These layers are mats of mineralized cyanobacteria, one layer on top of the other. The existence of fossil stromatolites provides evidence of early autotrophic organisms, which produced organic molecules and oxygen near the beginning of the history of life on Earth. (b) Modern stromatolites that have formed in western Australia. (c) Ancient stromatolites on Lake Superior's north shore, near Schreiber, Ontario.