Brief History of the Five Kingdom Scheme

"Some people dismiss taxonomies and their revisions as mere exercises in abstract ordering," writes Stephen Jay Gould, "a kind of glorified stamp collecting of no scientific merit and fit only for small minds who need to categorize their results. No view could be more false and more inappropriately arrogant. Taxonomies are reflections of human thought; they express our most fundamental concepts about the objects of our universe. Each taxonomy is a theory about the creatures it classifies." Early humans did not distinguish well between alive and not alive, considering moving astral bodies alive, and wind and other aspects of nature to be inhabited by unseen spirits (here we can see the origins of religion). A primordial tripartite division, into plants, animals, and minerals, is still a norm among educated people. (That is despite the fact that plants and animals have mineral parts, such as the calcium phosphate of bones, and that many minerals, such as limestone, were once produced in or by the living tissues of organisms really neither plant nor animal.) With increasing study, living things have become divided into more subtle categories. French zoologist Georges Cuvier classified all animals, including microbes, into four phyla. The Swiss taxonomist Linnaeus, who invented the Latin binomial nomenclature still used and familiar to us as species names (for example, Homo sapiens [humans], Canis familiaris [dogs]), put all simple animals together into Vermes—that is, worms. In the nineteenth century photosynthetic protists were still considered one-celled animals by zoologists, while botanists claimed them for their own as tiny plants.

The imaginative German champion of Darwin, Ernst Haeckel (1834-1919), proposed a new kingdom to make room for the microbes, which he correctly saw were more general than plants and animals rather than specialized versions of them. First he lumped them together into the Kingdom Monera; as his ideas developed, he alternately included, then excluded, cells with nuclei (eukaryotes) within the Monera. Later, in 1956, H. F. Copeland, a biologist at Sacramento City College in California, clearly separated bacteria and cells with nuclei in a four kingdom system. In Copeland's taxonomy, initially overlooked, the first organisms to evolve, bacterial cells, were split apart from their symbiotic evolutionary products, the amoebas and other cells with nuclei collectively known as protoctists (familiar today in forms such as algae, Para-mecia, and slime molds). Not until the 1960s and 1970s, with the arrival of molecular biology and more powerful forms of microscopy, did it become clear that microbes are distinct not only from plants and animals but also from the smaller nucleiless bacteria. The first essentially modern taxonomy to be accepted long after he suggested it in 1956 was put forth by Cornell University biologist R. H. Whittaker.

Whittaker, a student of desert and forest ecosystems, found bacteria and fungi to be so distinct from plants that treating them as such was intellectually unacceptable. He argued for the five kingdom system with Protista (single cells) rather than Protoctista as his kingdom of miscellaneous microbial eukaryotes. The modern five kingdom system is essentially identical, except that multicelled eukaryotes that do not develop from embryos and are not fungi are now called protoctists, with the informal term "protist" being reserved for single-celled and other microscopic organisms in the kingdom. (The term protoctista, for "organisms that are clearly neither animals nor plants," was first coined by British biologist John Hogg.)

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