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Scientists often categorize both living and non-living things in order to better understand the world and its inhabitants. Living organisms may be grouped into kingdoms. Although no longer in use, the kingdom Moneran was long used as a classification for creatures with no nucleus. The single celled organisms in this kingdom were known as monerans, or prokaryotes.
The practice of grouping living organisms in biology is called taxonomy. In the 19th and most of the 20th centuries, scientists distinguished biological organisms as either plants, animals, or microscopic single-celled organisms that lay somewhere between the two. A scientist named Ernst Haeckel dubbed the latter group Protista, and included Monera as one of eight divisions in this grouping.
Monerans were differentiated from their counterparts by some significant differences. In most cells, a nucleus is the central life force. It guides the cell in food breakdown, growth, and reproduction. Monerans carry out these processes without the aid of a nucleus or other complex structures known as organelles. Rather, they rely on molecules found inside the cell.
These cells can survive on their own, although they may be found in clusters. The process of reproduction in monerans is also different, as they do not undergo cell mitosis like most other cells. Rather, they multiply via binary fission, or a simple splitting of the cell.
Two further divisions distinguish monerans from each other: bacteria and cyanobacteria. Bacteria can be found almost everywhere in the world, and they survive by adhering to surfaces with a sticky cell wall and thus gaining food and moisture. These creatures are also highly durable, as they can survive in temperatures and conditions nearly uninhabitable to other living organisms. For example, they can breathe without the use of oxygen. Cyanobacteria, on the other hand, are more plant-like as they undergo photosynthesis and provide an important food source in the world’s oceans.
By the mid-20th century, scientists had separated the Moneran as a completely separate entity from Protista. In this new system, any single-celled organism lacking a nucleus was considered a Moneran whereas any similar organism with complex enclosed nucleus structures — or organelles — retained the Protista name. Animalae, Plantae, and Fungi rounded out this five-kingdom system of classification.
In 1991, a new system of taxonomy gained general recognition among the scientific community. Under this new system, the kingdom Monera was discarded in favor of two separate groupings: archaea and bacteria. While most scientists accepted this new system, a few holdouts have retained the old system.