Domains of Life

Carl Linnaeus (1707–1778), Ernst Haeckel (1834–1919), C. B. van Niel (1897–1985), Roger Y. Stanier (1916–1982), Carl Woese (1928–2012), George E. Fox (b. 1945)


An impetus for classification came during the seventeenth century when new plants and animals were arriving in Europe. In 1735, Carl Linnaeus, a pioneer in the science of taxonomy (also called systematics), developed a hierarchical system of biological nomenclature in which the highest rank, inclusive of all lower levels, was the kingdom, and these were two: animal and vegetable (plant). With the growing realization that unicellular organisms were unaccounted for, in 1866 Ernst Haeckel proposed the addition of a third kingdom, Protista.

In the 1960s, Roger Y. Stanier and C. B. van Niel devised a four-kingdom classification system based on the distinction between prokaryotic and eukaryotic cells, with the latter having a cell membrane enclosing its nucleus. Furthermore, they proposed a higher and more inclusive rank termed superdomain or empire. The Empire Prokarya encompassed the Kingdom Monera (bacteria), and the Empire Eukarya included the Kingdoms Plantae, Animalia, and Protista.

Until the mid-1970s, all classifications were based on the outward appearance of cells, namely their anatomy, morphology, embryology, and cell structure. In 1977, Carl Woese and George E. Fox at the University of Illinois at Urbana-Champaign classified organisms based on a comparison of their genes at a molecular level. In particular, they compared the nucleotide sequences in a subunit of ribosomal rRNA, the molecules that undergo evolutionary changes. In 1990, they introduced the concept of three domains of cellular life: the Domain Archaea, a disparate collection of prokaryotic organisms, among the most ancient found on Earth and capable of adapting to extreme environments (extremophiles); the Domain Bacteria; and the Domain Eukarya, which was subdivided into Kingdoms Fungi (yeasts, molds), Plantae (flowering plants, ferns), and Animalia (vertebrates, invertebrates). More recently, their Protista Kingdom has been subdivided into more discrete kingdoms. The final chapter on classification has not been written, with systems proposed that contain two to eight kingdoms.

SEE ALSO: Prokaryotes (c. 3.9 Billion BCE), Eukaryotes (c. 2 Billion BCE), Fungi (c.1.4 Billion BCE), Linnaean Classification of Species (1735), Endosymbiont Theory (1967), Protist Taxonomy (2005).

The rainbow colors in the Grand Prismatic Spring in Yellowstone National Park, Wyoming—the world’s third largest hot spring—result from resident thermophilic microbes (extremophiles of the Kingdom Archaea) that favor temperatures ranging from 1,880°F (870°C) at the center to 1,470°F (640°C) at the rim.