ABSTRACT
The cyanobacteria are exceedingly ancient organisms, identifiable in rocks dating from the first thousand million years of the earth’s history. As cyanobacterial colonies occur in shallow water, they appear in the fossil record in sedimentary rocks deposited in shallow seas and lakes. The older rocks containing cyanobacteria are the cherts, generated from silt, sand, and mud by heat and pressure over the large extent of geological time. The cyanobacterial colonies called stromatolites appear in rocks as fossilized mushroom shapes and sheets in widely distributed locations around the world. One of the bestknown stromatolite formations is the Gunflint chert of the Lake Erie region of North America, which dates from 2.09 billion years before the present. The oldest described in detail are the Apex cherts of Western Australia, dated to approximately 3.5 billion years before the present. As the earth’s crust dates to approximately 4.5 billion years before the present, cyanobacteria are among the very earliest life forms (Thorpe, Hickman et al. 1992; Schopf 2000). These rocks have been shown to contain fossil evidence of a wide range of both filamentous and spherical organisms, many identical in size and shape to current cyanobacteria (Schopf 2000). Isotopic ratio data from carbon within these and other cherts show evidence of photosynthetic activity, as living organisms incorporate carbon 12 preferentially to carbon 13 and residues of the organic carbon from the organisms remain in the rocks, providing a ratio of the isotopes characteristic of photosynthetic life (Strauss, Des Marais et al. 1992).
