Pre-Cambrian eraall geological time prior to the beginning of the CAMBRIAN PERIOD 590 million years ago. Very few fossils are known from this time and the fossil record effectively begins with the trilobites and brachiopods of the Cambrian. However, impressions of soft-bodied animals such as medusae, sea pens, annelids and some arthropods have been found in Pre-Cambrian rocks dated between 640 and 580 million years old. Bacteria may have existed for up to 3,800 million years and blue-green algae (CYANOBACTERIA) for 3,500 million years.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005