Monerans

Monera are bacteria and other mostly tiny, single-celled organisms whose genetic material is loose in the cell. The genetic material of plants, animals, and other eukaryotes (true nucleus), on the other hand, is held in the cell's nucleus. While the Monera were briefly understood to be one of five biological kingdoms, it is now understood to comprise two domains: the bacteria and the archaea. The Monera kingdom included most organisms with a prokaryotic cell organization (that is, no nucleus). For this reason, the kingdom was sometimes called Prokaryota or Prokaryotae.

Recent DNA and RNA sequence analyses has demonstrated that there are two major groups of prokaryotes, the Bacteria and Archaea, which do not appear to be closer in relationship to each other than they are to the Eukaryotes. Thus, Monera has since been divided into Archaea and Bacteria, forming the more recent six-kingdom system and three-domain system. All new schemes abandon the Monera and now treat the Bacteria, Archaea, and Eukarya as separate domains or kingdoms.

Prior to the five-kingdom model with its Monera kingdom, these organisms were classifies as two separate divisions of plants: the Schizomycetes (bacteria) were considered fungi, and the Cyanophyta were considered blue-green algae. The latter are now considered a group of bacteria, typically called the cyanobacteria and are now known not to be closely related to plants, fungi, or animals.

Further Classification
Based on molecular phylogeny studies, Carl Woese proposed that the prokaryotes (monerans) be divided into two separate groups: Bacteria and Archaea. In Carl Woese's 1990 proposed phylogeny, these three kingdoms are all rooted in a universal common ancestor and this is the most widely accepted categorical phylogeny accepted today. However, the most modern interpretation for these three kingdoms is the "Universal and Eukaryote Phylogenetic Tree" based on 16s rDNA, as presented in the Tree of Life Web Project.

Bacteria and Archaea
Eubacteria and Archaebacteria differ most noticeably in the environments they are able to inhabit. Eubacteria encompass the vast majority of bacteria that come into contact with humans. The bacteria that live within and around humans, such as Escherichia coli and those of the genus Salmonella, are Eubacteria. Archaebacteria live in much harsher conditions, such as in acidic hot springs and at depths of a mile below the arctic ice.

These groups were later renamed to Bacteria and Archaea, which might lead to some confusing situations, as the common use of the word "bacteria" in the English language (originally) simply refers to prokaryote microorganisms, or in other words monerans.

History
- Traditionally organisms were classified as animal, vegetable, or mineral as in Systema Naturae. After the discovery of microscopy, attempts were made to fit microscopic organisms into either the plant or animal kingdom. In 1866 Ernst Haeckel proposed a three kingdom system which added Protista as a new kingdom that contained most microscopic organisms. One of his eight major divisions of Protista was called Moneres. Haeckel's Moneres subcategory included known bacterial groups such as Vibrio. Haeckel's Protista kingdom also included eukaryotic organisms now classified as Protist. It was later decided that Haeckel's Protista kingdom had proven to be too diverse to be seriously considered one single kingdom. - In 1969, Robert Whittaker published a proposed five kingdom system for classification of living organisms. Whittaker's system placed most single celled organisms into either the prokaryotic Monera or the eukaryotic Protista. The other three kingdoms in his system were the eukaryotic Fungi, Animalia, and Plantae.