In his classification scheme, Linnaeus recognized only two kingdoms of living things: Animalia and Plantae. At the time, microscopic organisms had not been studied in detail. Either they were placed in a separate category called Chaos or, in some cases, they were classified with plants or animals. Then in the 1860s, the German investigator Ernst Haeckel proposed a three-kingdom system of classification. Haeckel’s three kingdoms were Animalia, Plantae, and Protista. Members of the kingdom Protista included the protozoa, fungi, bacteria, and other microorganisms. Haeckel’s system was not widely accepted, however, and microorganisms continued to be classified as plants (for example, bacteria and fungi) or animals (for example, protozoa).
In 1968, Robert Whittaker devised a system that was widely accepted by biologists for many years. Whittaker’s classification scheme recognized five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. Even though kingdoms Fungi, Plantae, Animalia, and, to a lesser extent, Protista remain intact today, kingdom Monera is no longer considered a valid category after genetic work shed new light on its phylogenic relatedness.
In the 1970s, genetic comparisons using small-subunit ribosomal RNA highlighted a major problem with grouping all prokaryotic cells together under the single kingdom of Monera. Some prokaryotic cells once considered bacteria were more closely related to eukaryotes. This required a larger phylogenic category to be created: the domain. Two of the three domains contain prokaryotic cells (domains Bacteria and Archaea), and all eukaryotic organisms reside under domain Eukarya.
Domain Bacteria includes the vast majority of “everyday prokaryotes,” the ones we interact with on a daily basis. This includes pathogenic strains that cause diseases and beneficial species that populate our foods and inhabit our bodies. Domain Archaea, on the other hand, includes those prokaryotes that prefer to live in very extreme environments, reminiscent of early Earth conditions (thus their name, the “archaic” prokaryotes). The third domain includes all life composed of eukaryotic cells. This vast array of life includes kingdoms Protista, Fungi, Plantae, and Animalia.
Kingdom Protista has itself been under intense revision, though it is still referred to as its own kingdom. The protists include protozoa, algae, and slime molds. The cells of these organisms are eukaryotic. They can be either unicellular or multicellular, and they may be autotrophic or heterotrophic. Eukaryotic organisms have a nucleus and membrane-bound organelles in their cytoplasm, possess multiple chromosomes, have large ribosomes, and reproduce by mitosis.
The kingdom Fungi includes the yeasts, molds, mildews, mushrooms, and other similar organisms. The cells of this kingdom are eukaryotic and heterotrophic. Yeasts are unicellular, whereas other species form long chains of cells and are called filamentous fungi. A cell wall strengthened by chitin is found in most members. Food is taken in by the absorption of small molecules from the external environment; thus, fungi are deemed absorptive heterotrophs.
The next kingdom is Plantae. Classified here are the mosses, ferns, and seed-producing and flowering plants. All plant cells are eukaryotic and autotrophic. The organisms synthesize their own foods by photosynthesis, and their cell walls contain cellulose. All the organisms are multicellular.
The final kingdom, Animalia, includes animals. Animals without backbones (invertebrates) and with backbones (vertebrates) are included here. The cells are eukaryotic; the organisms are heterotrophic. All animals are multicellular, and none has cell walls. In the kingdom Animalia, biologists classify such organisms as sponges, hydras, worms, insects, starfish, reptiles, amphibians, birds, and mammals. The feeding form is one in which large molecules from the external environment are consumed and then broken down to usable parts in the animal body.