In the soft reduced sediments of the continental shelf, below the oxygen minimum zone (OMZ) of the eastern South Pacific (ESP), peculiar microbial communities have been disclosed which include a variety of large prokaryotes, protists and small metazoans. Dominant among the prokaryotes are large multi-cellular filamentous bacteria which, according to their size range, are roughly divided into megabacteria and macrobacteria . The former group is made up of a few species of Gamma Proteobacteria of the genera Thioploca and Beggiatoa and the second group includes a diversity of phenotypes. Protists include ciliates, flagellates, and foraminifers and the metazoans are mostly nematodes and small polychaetes. A significant similarity has been found in the exploitation of the area/volume relationship among these large bacteria and their fossil analog forms as described from pre-Cambrian rocks. For the same reason, the latter have mostly been referred to as algae or cyanobacteria in the literature. The presence of these seemingly ancient bacteria in the sediments of the oxygen minimum zones of the ESP, one of the most productive but also ecologically most inefficient marine ecosystems of the world, suggests that such setting must have prevailed throughout the geological history of the planet allowing for their survival and further that it might be considered an analog of Proterozoic ocean conditions. These non-cyanobacterial communities offer an alternative hypothesis to students of the evolution of life on Earth and may be of special interest to astrobiologists looking for life or traces of life in terrestrial or extraterrestrial environments since these do not necessarily imply a photosynthesis-based metabolism.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.