THE BIOLOGY BOOK
Ernst Haeckel (1834–1919), Robert H. Whittaker (1920–1980)
Biologists are classifiers, but as experience has shown over almost two centuries, protists defy a simple classification that has withstood the test of time. To the ancient classification that all living organisms were either plants or animals, unicellular organisms were added as a third kingdom, which in 1866, Ernst Haeckel called protists, referring to their primitive forms. In 1959, the American plant ecologist Robert Whitaker proposed a five-kingdom and later a four-kingdom classification, one of which was Protista.
Until recently, protists were classified as one of the four kingdoms within the Domain Eukaryota, which consist of organisms that have membranes enclosing a true nucleus and intracellular organelles. Based on ultrastructure (organelles), biochemistry, and genetics, members of the plant, animal, and fungal kingdoms are considered monophyletic—that is, each group is derived from a single ancestor and all its descendants.
There are more than 200,000 species of protists inhabiting an environment in which water is present some or all the time. They are usually unicellular and differ in size and shape, method of reproduction, motility, and how they obtain their nutrition. But, based on DNA and ultrastructural studies, the protists are even more diverse than previously believed, with some more closely related to other kingdom members than to other protists. Protists do not share a common lineage (they are polyphyletic), and Protista is not really a kingdom but rather a catchall category of eukaryotic organisms that are not plants, animals, or fungi. Nevertheless, the name protist continues to be used as a shorthand designation for such organisms.
In 2005, the ecologist Sina M. Adl at Dalhousie University in Canada, proposed a classification that forgoes seeking hereditary relatedness and has informally placed all protists into five supergroups, with each subdivided into groups based on how they move and how they obtain their nutrition. In addition, an even simpler and more readily understood classification for protists is based on the following: Protozoa or animal-like, where members ingest food and are motile; Algae or plantlike, including organisms that manufacture their own food by photosynthesis; and Fungilike, which digest foods from their environment.
SEE ALSO: Eukaryotes (c. 2 Billion BCE), Leeuwenhoek’s Microscopic World (1674), Linnean Classification of Species (1735), Photosynthesis (1845), Endosymbiont Theory (1967), Domains of Life (1990).
A stromatolite reef in Cuatro Ciénegas, Mexico. Stromatolites, among the world’s oldest fossils, are formed by the accumulation of multiple layers of prokaryotic cyanobacteria (formerly called blue-green algae) and protists. Before their source was understood, they were referred to as “living rocks.”