ABSTRACT
Abstract Anaerobic prokaryotes have dominated life on the planet Earth for possibly more than a billion years before the increasing oxygen content forced them to retreat into niches, hostile for oxygenic organisms. These environments were provided by geological conditions and by oxygenic microorganisms. Eukaryotic species in concert with aerobic bacteria provided additional environmental facilities in which anaerobic forms could evolve. The diversity of morphologies, chemical and physiological traits represented among the recent anaerobic organisms are found in phylogenetic lineages that branched off early in the tree of ribosomal RNA gene sequences, which indicates that genetic diversity was already high 3.8 billion years ago. The majority of archaeal and bacterial lineages embrace anaerobic prokaryotes, though only few of them are defined solely by obligate and strictly anaerobic forms. This chapter will provide a summary on the diversity of anaerobes of the domain Bacteria, emphasising the classification at levels above the genus rank.
