In terms of biochemical and intracellular organization, living systems can be divided into two major “Superkingdoms,” eukaryotes and prokaryotes.

Eukaryotes, comprising the more advanced and later evolving Superkingdom, include unicellular or multicellular organisms (viz., members of the Kingdoms Protista, Fungi, Plantae and Animalia) characterized by nucleus-, mitochondrion-, and (in plants and some protists) chloroplast-containing cells that are capable typically of mitotic cell division. Paleontologic evidence indicates that the eukaryotic cell originated during the Middle Proterozoic, probably about 1.4 to 1.5 Ga ago (Schopf and Oehler, 1976).

Prokaryotes, comprising the more primitive and earlier evolving Superkingdom, include microbial microorganisms (viz., members of the Kingdom Monera: bacteria, cyanobacteria, archaebacteria, and prochlorophytes) characterized by cells that lack membrane-bound nuclei, mitochondria, chloroplasts, and similar organelles and that reproduce by non-mitotic and non-meiotic division. Some authors (e.g., Woese and Fox, 1977) subdivide prokaryotes (monerans) into two kingdoms, the Kingdom Archaebacteriae (including methanogenic, extremely halophilic and some thermoacidophilic bacteria) and the Kingdom Eubacteriae (including all non-archaebacterial prokaryotes), based on the chemistry of their cell walls, membranes, transfer RNA's and RNA polymerase subunits. Paleontologic evidence indicates that prokaryotes originated early in Earth history - the group was extant, morphologically varied and evidently physiologically advanced at least as early as 3.3 to 3.5 Ga ago (Schopf and Packer, 1987).