The taxon Monera was first proposed as a phylum by Ernst Haeckel in 1866. Subsequently, the phylum was elevated to the rank of kingdom in 1925 by Édouard Chatton. The last commonly accepted mega-classification with the taxon Monera was the five-kingdom classification system established by Robert Whittaker in 1969.
Under the three-domain system of taxonomy, introduced by Carl Woese in 1977, which reflects the evolutionary history of life, the organisms found in kingdom Monera have been divided into two domains, Archaea and Bacteria (with Eukarya as the third domain). Furthermore, the taxon Monera is paraphyletic (does not include all descendants of their most-recent common ancestor), as Archaea and Eukarya are currently believed to be more closely related than either is to Bacteria. The term "moneran" is the informal name of members of this group and is still sometimes used (as is the term "prokaryote") to denote a member of either domain.[1]
Most bacteria were classified under Monera; however, Cyanobacteria (often called the blue-green algae) were initially classified under Plantae due to their ability to photosynthesize
Haeckel's classification
Traditionally the natural world was classified as animal, vegetable, or mineral as in Systema Naturae. After the development of the microscope, attempts were made to fit microscopic organisms into either the plant or animal kingdoms. In 1675, Antonie van Leeuwenhoek discovered bacteria and called them "animalcules", assigning them to the class Vermes of the Animalia.[3][4][5] Due to the limited tools — the sole references for this group were shape, behaviour, and habitat — the description of genera and their classification was extremely limited, which was accentuated by the perceived lack of importance of the group.[6][7][8]
Ten years after The Origin of Species by Charles Darwin, in 1866 Ernst Haeckel, a supporter of evolution, proposed a three-kingdom system that added the Protista as a new kingdom that contained most microscopic organisms.[2] One of his eight major divisions of Protista was composed of the monerans (called Moneres by Haeckel), which he defined as completely structure-less and homogeneous organisms, consisting only of a piece of plasma. Haeckel's Monera included not only bacterial groups of early discovery but also several small eukaryotic organisms; in fact the genus Vibrio is the only bacterial genus explicitly assigned to the phylum, while others are mentioned indirectly, which led Copeland to speculate that Haeckel considered all bacteria to belong to the genus Vibrio, ignoring other bacterial genera.[7] One notable exception were the members of the modern phylum Cyanobacteria, such as Nostoc, which were placed in the phylum Archephyta of Algae (vide infra: Blue-green algae).
The Neolatin noun Monera and the German noun Moneren/Moneres are derived from the ancient Greek noun moneres, which Haeckel stated meant "simple";[2] however, it actually means "single, solitary".[9] Haeckel also describes the protist genus Monas in the two pages about Monera in his 1866 book.[2] The informal name of a member of the Monera was initially moneron,[10] but later moneran was used.[1]
Due to its lack of features, the phylum was not fully subdivided, but the genera therein were divided into two groups: