What are the Main Categories of Organism?
Organisms are usually categorized according to the three-domain system, introduced by Carl Woese in 1990. It replaces the older five-kingdom system of organism classification. In the Woese classification, the three domains of life are: Bacteria, Archaea, and Eucaryota. Also under consideration is the addition of a fourth domain, Acytota, meaning “without cells”, to represent viruses and other cell-less pseudo-organisms such as prions. Even more speculative is a fifth domain organisms, Nanobiota, which may represent extremely small, life-like filamentary structures found in some minerals. More recently, in 2002, some scientists have regarded Archaea and Eucaryota as part of the same domain, Neomura. The most important division in life is generally regarded as that between prokaryotic organisms (cells with nuclei) and eukaryotic organisms (with nuclei). A slightly less significant, but still very important, distinction is that between unicellular and multicellular organisms, and between
