Two kingdoms of life The classification of living things into animals and plants is an ancient one.
Aristotle (384–322 BC) classified animal species in his
History of Animals, while his pupil
Theophrastus ( – ) wrote a parallel work, the
Historia Plantarum, on plants.
Carl Linnaeus (1707–1778) laid the foundations for modern
biological nomenclature, now regulated by the
Nomenclature Codes, in 1735. He distinguished two kingdoms of living things:
Regnum Animale ('
animal kingdom') and
Regnum Vegetabile ('vegetable kingdom', for
plants). Linnaeus also included
minerals in his
classification system, placing them in a third kingdom,
Regnum Lapideum.
Three kingdoms of life '' with plants. In 1674,
Antonie van Leeuwenhoek, often called the "father of microscopy", sent the
Royal Society of London a copy of his first observations of microscopic single-celled organisms. Until then, the existence of such microscopic organisms was entirely unknown. Despite this, Linnaeus did not include any microscopic creatures in his original taxonomy. At first, microscopic organisms were classified within the animal and plant kingdoms. However, by the mid–19th century, it had become clear to many that "the existing dichotomy of the plant and animal kingdoms [had become] rapidly blurred at its boundaries and outmoded". In 1860
John Hogg proposed the
Protoctista, a third kingdom of life composed of "all the lower creatures, or the primary organic beings"; he retained Regnum Lapideum as a fourth kingdom of minerals. In the 1960s,
Roger Stanier and
C. B. van Niel promoted and popularized Édouard Chatton's earlier work, particularly in their paper of 1962, "The Concept of a Bacterium"; this created, for the first time, a rank above kingdom—a
superkingdom or
empire—with the
two-empire system of prokaryotes and eukaryotes.
Five kingdoms The differences between
fungi and other organisms regarded as plants had long been recognised by some; Haeckel had moved the fungi out of Plantae into Protista after his original classification, The resulting five-kingdom system, proposed in 1969 by Whittaker, has become a popular standard and with some refinement is still used in many works and forms the basis for new multi-kingdom systems. It is based mainly upon differences in
nutrition; his Plantae were mostly multicellular
autotrophs, his Animalia multicellular
heterotrophs, and his Fungi multicellular
saprotrophs. The remaining two kingdoms, Protista and Monera, included unicellular and simple cellular colonies. Following publication of Whittaker's system, the five-kingdom model began to be commonly used in high school biology textbooks.
Six kingdoms In 1977,
Carl Woese and colleagues proposed the fundamental subdivision of the prokaryotes into the Eubacteria (later called the Bacteria) and Archaebacteria (later called the Archaea), based on
ribosomal RNA structure; This six-kingdom model is commonly used in recent US high school biology textbooks, but has received criticism for compromising the current scientific consensus. But the division of prokaryotes into two kingdoms remains in use with the recent
seven kingdoms scheme of Thomas Cavalier-Smith, although it primarily differs in that Protista is replaced by
Protozoa and
Chromista. As mitochondria were known to be the result of the
endosymbiosis of a
proteobacterium, it was thought that these amitochondriate eukaryotes were primitively so, marking an important step in
eukaryogenesis. As a result, these amitochondriate protists were separated from the protist kingdom, giving rise to the, at the same time, superkingdom and kingdom
Archezoa. This superkingdom was opposed to the
Metakaryota superkingdom, grouping together the five other eukaryotic kingdoms (
Animalia,
Protozoa,
Fungi,
Plantae and
Chromista). This was known as the
Archezoa hypothesis, which has since been abandoned; later schemes did not include the Archezoa–Metakaryota divide. Cavalier-Smith no longer accepted the importance of the fundamental Eubacteria–Archaebacteria divide put forward by Woese and others and supported by recent research.
Summary The kingdom-level classification of life is still widely employed as a useful way of grouping organisms, notwithstanding some problems with this approach: • Kingdoms such as Protozoa represent
grades rather than
clades, and so are rejected by
phylogenetic classification systems. • The most recent research does not support the classification of the eukaryotes into any of the standard systems. In 2009, Andrew Roger and Alastair Simpson emphasized the need for diligence in analyzing new discoveries: "With the current pace of change in our understanding of the eukaryote tree of life, we should proceed with caution." Kingdoms are rarely used in academic phylogeny and are more common in introductory education, where 5–6 kingdom models are preferred. == Beyond traditional kingdoms ==