Linnaeus sorted rocks into a kingdom of their own alongside plants and animals
How many kingdoms of life are there? Ask a textbook in the United States and it says six; one in Britain or India says five; one biologist's scheme says seven. When Carl Linnaeus started the system in 1735 he named two living kingdoms, plus a third one reserved for minerals.
A kingdom is the second rank in biological classification, just below domain and above phylum. Splitting life into animals and plants is ancient: Aristotle wrote about animals and his pupil Theophrastus produced a companion work on plants. Linnaeus formalized it in 1735 with an animal kingdom and a vegetable kingdom, and he put minerals in a third, Regnum Lapideum. He left out microscopic life entirely, even though Antonie van Leeuwenhoek had reported single-celled organisms to the Royal Society of London back in 1674.
Microbes broke the two-kingdom picture. John Hogg proposed a third living kingdom in 1860 while still keeping minerals as a fourth, and Ernst Haeckel suggested Protista in 1866 for primitive forms that were neither plant nor animal. In 1937 Édouard Chatton coined the words prokaryote and eukaryote for cells without and with a nucleus, and in 1938 Herbert Copeland gave the nucleus-free organisms their own kingdom, Monera. Robert Whittaker added fungi in 1969, sorting kingdoms mainly by how organisms feed, and his five-kingdom scheme became the high school standard, though some authors still used two kingdoms as late as 1975.
Genetics reshuffled things again. In 1977 Carl Woese and colleagues used ribosomal RNA to show that prokaryotes form two deeply separate groups, later named Bacteria and Archaea, which led to the three domains of life. The domain rank was placed above kingdom in 1990, and swapping Monera for Bacteria and Archaea produced the six kingdoms of American classrooms. Thomas Cavalier-Smith built a seven-kingdom scheme and later a revised six-kingdom one, and he rejected the idea that the bacteria–archaea split was fundamental.
Some modern classifications drop the word kingdom altogether, because several traditional kingdoms do not include all descendants of one common ancestor. The older language survives in other ways: flora and fauna describe the life of a region, and since the 21st century fungi have their own word, funga.
Source: Kingdom (taxonomy)
