Differentiating among the Five Kingdoms

The biggest distinction used today in classifying life is the presence or absence of a nucleus and other membrane-bounded cell organelles such as mitochondria. Free-living organisms lacking such structures, and true chromosomes, are bacteria, or prokaryotes. They belong to the Kingdom Monera (Prokary-otae). Protoctists, the next kingdom of life to evolve, are composed of organisms that are either nucleated cells or colonial aggregations of such cells. As colonies of cells with nuclei, protoctists evolved on their own and into fungi, animals, and plants. The cells of these organisms have nuclei and mitochondria, and thus seem to come from a common microbial ancestor with nuclei and mitochondria in its cells. Perhaps the second biggest distinction to classify life forms, at least in the macroscopic kingdoms (fungi, plants, and animals), is nutrition. Fungi absorb their food, plants produce it, and animals consume it. (For a more detailed analysis of fungi, protoctists, and bacteria, see individual entries for those groups.) Additionally, plants and animals develop from embryos. Diverse embryo-forming animals, which range from beings smaller than pro-toctists to giant blue whales, show diverse forms of intelligence and behavior. Plants and animals (except for the sponges and Placozoa) possess tissues and organs. Algae and seaweeds are protoctists because, although green and plantlike, they do not form embryos.

Plants also all develop from multicellular structures enclosed in maternal tissue—that is, from embryos. Since embryos are sexually produced (or asexually produced in beings whose ancestors were sexual), all plants and animals may be regarded as being sexual or having sexual ancestors. The differentiation into specialized tissues, whether leaves and flowers of angiosperms, or the skin and lung tissues of primates, may be a legacy of sexual activities that evolved in protoctist ancestors. The prevailing plant-animal dichotomy mentioned above is so pervasive that neither plants nor animals have been deemed by the editors of this encyclopedia to merit their own separate entries. This in part reflects our bias as animals studying life zoologically from the inside: not only are we more apt to distinguish among those of our own largely mobile and animate kingdom, but we tend to confine the rest to our living antithesis, the slow-moving green world of plants. As our understanding of life and its diversity continues to grow, and life continues to evolve, our taxonomies are bound to change.

—Lynn Margulis and Dorion Sagan

See also: Archaebacteria; Bacteria; Botany; Coevo-lution; Coloniality; Ecology; Embryology; Evolution; Evolutionary Genetics; Fungi; Lichens; Museums and Biodiversity; Protoctists; Zoology

Bibliography

Barnes, R. S. K., ed. 1998. The Diversity of Living Organisms. Oxford, UK: Blackwell Science; Margulis, Lynn, Clifford Matthews, and Aaron Haselton. 2000. "Five-kingdom Classification Scheme: Superkingdom Prokaryota." In Environmental Evolution.

Appendix B. Cambridge, MA: MIT Press; Margulis, Lynn, and Karlene Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3d ed. New York: W. H. Freeman; Margulis, Lynn, Karlene V. Schwartz, and Michael Dolan. 1999. Diversity of Life: The Illustrated Guide to the Five Kingdoms. Sudbury, MA: Jones and Bartlett; Sagan, Dorion, and Lynn Margulis. 1993. Garden of Microbial Delights: A Practical Guide to the Subvisible World. Dubuque, IA: Kendall/Hunt.

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