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In
biology Biology is the natural science that studies life and living organisms, including their physical structure, chemical processes, molecular interactions, physiological mechanisms, development and evolution.Based on definition from: Despite the c ...
, kingdom (
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the dominant language ...
: ''regnum'', plural ''regna'') is the second highest
taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only about biological ...
, just below
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function *Doma ...
. Kingdoms are divided into smaller groups called phyla. Traditionally, some textbooks from the United States and Canada used a system of
six kingdoms#REDIRECT Six Kingdoms ...
(
Animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a ...
ia,
Plantae Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi wer ...
,
Fungi A fungus (plural: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, which is separa ...
,
Protista A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exclus ...
,
Archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
/Archaebacteria, and
Bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres ...
/Eubacteria) while textbooks in Great Britain, India, Greece, Brazil and other countries use five kingdoms only (
Animalia Animals (also called Metazoa) are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a ...
,
Plantae Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi wer ...
,
Fungi A fungus (plural: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, which is separa ...
,
Protista A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exclus ...
and
Monera Monera () (Greek - μονήρης (monḗrēs), "single", "solitary") is a kingdom that contains unicellular organisms with a prokaryotic cell organization (having no nuclear membrane), such as bacteria. They are single-celled organisms with no t ...
). Some recent classifications based on modern
cladistics Cladistics (, from Greek , ''kládos'', "branch") is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships ...
have explicitly abandoned the term ''kingdom'', noting that the traditional kingdoms are not
monophyletic 300px, A cladogram of the primates, showing a ''monophyletic'' taxon: ''the simians'' (in yellow); a ''paraphyletic'' taxon: ''the prosimians'' (in cyan, including the red patch); and a ''polyphyletic'' group: ''the night-active primates, i.e., ...
, meaning that they do not consist of all the descendants of a common ancestor.


Definition and associated terms

When
Carl Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement as Carl von LinnéBlunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the modern sys ...
introduced the rank-based system of
nomenclature Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal conventions of everyday speech to the internationally agreed ...
into biology in 1735, the highest rank was given the name "kingdom" and was followed by four other main or principal ranks: class, order,
genus Genus (plural genera) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses, in biology. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...
and
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate sexe ...
. Later two further main ranks were introduced, making the sequence kingdom, phylum or division, class, order,
family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of ...
, genus and species. In 1990, the rank of
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function *Doma ...
was introduced above kingdom. Prefixes can be added so ''subkingdom'' (''subregnum'') and ''infrakingdom'' (also known as ''infraregnum'') are the two ranks immediately below kingdom. Superkingdom may be considered as an equivalent of domain or empire or as an independent rank between kingdom and domain or subdomain. In some classification systems the additional rank ''branch'' (Latin: ''ramus'') can be inserted between subkingdom and infrakingdom, e.g.,
Protostomia Protostomia is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's members ...
and
Deuterostomia Deuterostomia ( in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some examples ...
in the classification of Cavalier-Smith.


History


Two kingdoms of life

The classification of living things into animals and plants is an ancient one.
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Lyceum, the Peripatetic schoo ...

Aristotle
(384–322 BC) classified animal species in his ''
History of Animals ''History of Animals'' ( grc-gre, Τῶν περὶ τὰ ζῷα ἱστοριῶν, ''Ton peri ta zoia historion'', "Inquiries on Animals"; la, Historia Animalium, "History of Animals") is one of the major texts on biology by the ancient Greek p ...
'', while his pupil
Theophrastus Theophrastus (; grc-gre, Θεόφραστος ''Theόphrastos''; c. 371c. 287 BC), a Greek native of Eresos in Lesbos,Gavin Hardy and Laurence Totelin, ''Ancient Botany'', Routledge, 2015, p. 8. was the successor to Aristotle in the Peripatetic ...

Theophrastus
(c. 371–c. 287 BC) wrote a parallel work, the '' Historia Plantarum'', on plants.
Carl Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement as Carl von LinnéBlunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the modern sys ...
(1707–1778) laid the foundations for modern
biological nomenclature Biology is the natural science that studies life and living organisms, including their physical structure, chemical processes, molecular interactions, physiological mechanisms, development and evolution.Based on definition from: Despite the c ...
, now regulated by the
Nomenclature Codes#REDIRECT Nomenclature codes#REDIRECT Nomenclature codes {{R from other capitalisation ...
{{R from other capitalisation ...
, in 1735. He distinguished two kingdoms of living things: ''Regnum Animale'' ('
animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a ...
kingdom') and ''Regnum Vegetabile'' ('vegetable kingdom', for
plant Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi wer ...
s). Linnaeus also included
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure, that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
s in his
classification system Classification is a process related to categorization, the process in which ideas and objects are recognized, differentiated and understood. See Classification (general theory) It may also refer to: Business, organizations, and economics * Clas ...
, placing them in a third kingdom, '' Regnum Lapideum''.


Three kingdoms of life

In 1674,
Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch businessman and scientist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as "the Father of ...
, often called the "father of microscopy", sent the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. Founded on 28 November 1660, it was granted a royal charter by King Charles I ...
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 ad becomerapidly 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 1866,
Ernst Haeckel Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist, naturalist, eugenicist, philosopher, physician, professor, marine biologist, and artist who discovered, described and named thousands of new spe ...
also proposed a third kingdom of life, the ''
Protista A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exclus ...
'', for "neutral organisms" or "the kingdom of primitive forms", which were neither animal nor plant; he did not include the Regnum Lapideum in his scheme. Haeckel revised the content of this kingdom a number of times before settling on a division based on whether organisms were unicellular (Protista) or multicellular (animals and plants).


Four kingdoms

The development of
microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micro ...
revealed important distinctions between those organisms whose cells do not have a distinct
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
(
prokaryote A prokaryote is a typically unicellular organism that lacks a nuclear membrane-enclosed nucleus. The word ''prokaryote'' comes from the Greek (, 'before') and (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connections". Pearson Education ...
s) and organisms whose cells do have a distinct nucleus (
eukaryote Eukaryotes () are organisms whose cells have a nucleus enclosed within a nuclear envelope. Eukaryotes belong to the domain Eukaryota or Eukarya; their name comes from the Greek εὖ (''eu'', "well" or "good") and κάρυον (''karyon'', "nu ...
s). In 1937
Édouard Chatton Édouard Chatton (; 11 October 1883 – 23 April 1947) was a French biologist who first characterized the distinction between the prokaryotic and eukaryotic cellular types. Chatton coined the terms and published them first in his 1937 paper "Pa ...
introduced the terms "prokaryote" and "eukaryote" to differentiate these organisms. In 1938, Herbert F. Copeland proposed a four-kingdom classification by creating the novel Kingdom
Monera Monera () (Greek - μονήρης (monḗrēs), "single", "solitary") is a kingdom that contains unicellular organisms with a prokaryotic cell organization (having no nuclear membrane), such as bacteria. They are single-celled organisms with no t ...
of prokaryotic organisms; as a revised phylum Monera of the Protista, it included organisms now classified as
Bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres ...
and
Archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
. Ernst Haeckel, in his 1904 book ''The Wonders of Life'', had placed the blue-green algae (or Phycochromacea) in Monera; this would gradually gain acceptance, and the blue-green algae would become classified as bacteria in the phylum
Cyanobacteria Cyanobacteria , also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' comes from their color ( el, κυανός, translit=kyanós, translation=blue), giving them their ot ...
. In the 1960s,
Roger Stanier Roger Yate Stanier (22 October 1916 – 29 January 1982) was a Canadian microbiologist who was influential in the development of modern microbiology. As a member of the Delft School and former student of C. B. van Niel, he made important contributi ...
and
C. B. van Niel Cornelis Bernardus van Niel (November 4, 1897, Haarlem – March 10, 1985, Carmel, California) was a Dutch-American microbiologist. He introduced the study of general microbiology to the United States and made key discoveries explaining the che ...
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 The two-empire system (two-superkingdom system) was the top-level biological classification system in general use before the establishment of the three-domain system. It classified cellular life into Prokaryota and Eukaryota as either "empires" or " ...
of prokaryotes and eukaryotes. The two-empire system would later be expanded to the
three-domain system The three-domain system is a biological classification introduced by Carl Woese ''et al.'' in 1990 that divides cellular life forms into ''archaea'', ''bacteria'', and ''eukaryote'' domains. The key difference from earlier classifications is the sp ...
of Archaea, Bacteria, and Eukaryota.


Five kingdoms

The differences between
fungi A fungus (plural: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, which is separa ...
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, but was largely ignored in this separation by scientists of his time. Robert Whittaker recognized an additional kingdom for the
Fungi A fungus (plural: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, which is separa ...
. 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 Nutrition is the biochemical and physiological process by which an organism uses food to support its life. It includes ingestion, absorption, assimilation, biosynthesis, catabolism and excretion. The science that studies the physiological proces ...
; his Plantae were mostly multicellular
autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", 3r ...
s, his Animalia multicellular
heterotroph A heterotroph (; from Ancient Greek "other" and "nutrition") is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are prim ...
s, and his Fungi multicellular
saprotroph Mycelial cord made up of a collection of hyphae; an essential part in the process of saprotrophic nutrition, it is used for the intake of organic matter through its cell wall. The network of hyphae is referred to as a mycelium, which is fundamen ...
s. The remaining two kingdoms, Protista and Monera, included unicellular and simple cellular colonies. The five kingdom system may be combined with the two empire system. In the Whittaker system, Plantae included some algae. In other systems, such as
Lynn Margulis Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolutionary theorist, biologist, science author, educator, and science popularizer, and was the primary modern proponent for the significance of symbio ...

Lynn Margulis
's system of five kingdoms, the plants included just the land plants (
Embryophyta The Embryophyta () or land plants are the most familiar group of green plants that form vegetation on earth. Embryophyta is a clade within the Phragmoplastophyta, a larger clade that also includes several green algae groups (including the Charophy ...
), and Protoctista has a broader definition. Following publication of Whittaker's system, the five-kingdom model began to be commonly used in high school biology textbooks. But despite the development from two kingdoms to five among most scientists, some authors as late as 1975 continued to employ a traditional two-kingdom system of animals and plants, dividing the plant kingdom into subkingdoms Prokaryota (bacteria and cyanobacteria), Mycota (fungi and supposed relatives), and Chlorota (algae and land plants).


Six kingdoms

In 1977,
Carl Woese Carl Richard Woese (; July 15, 1928 – December 30, 2012) was an American microbiologist and biophysicist. Woese is famous for defining the Archaea (a new domain of life) in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique he pionee ...

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 Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal D ...
structure; this would later lead to the proposal of three "domains" of life, of Bacteria, Archaea, and Eukaryota. Combined with the five-kingdom model, this created a six-kingdom model, where the kingdom Monera is replaced by the kingdoms Bacteria and Archaea. 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 kingdomsThe phrase Seven Kingdoms can refer to: History *Seven Kingdoms of Kongo dia Nlaza, the precursors to the Kingdom of Kongo in Central Africa *Seven Warring States, the combatants from a turbulent period of Chinese history *Heptarchy, the precursors ...
scheme of Thomas Cavalier-Smith, although it primarily differs in that Protista is replaced by
Protozoa Protozoa (also protozoan, plural protozoans) is an informal term for a group of single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris. Historically, prot ...
and
Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids). It includes all protists whose plastids contain chlorophyll ''c'', such a ...
.


Eight kingdoms

Thomas Cavalier-Smith Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 - 19 March 2021), was a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford. His research has led to discovery of a number o ...
supported the consensus at that time, that the difference between
Eubacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres ...
and
Archaebacteria Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
was so great (particularly considering the genetic distance of ribosomal genes) that the prokaryotes needed to be separated into two different kingdoms. He then divided
Eubacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres ...
into two subkingdoms: Negibacteria (
Gram negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the gram-staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall san ...
bacteria) and
Posibacteria 300px, Violet-stained gram-positive bacilli_.html" style="text-decoration: none;"class="mw-redirect" title="gram-negative">cocci and pink-stained bacilli_">gram-negative">cocci_and_pink-stained_gram-negative_bacillus_(shape)">bacilli_ In_bacte ...
(
Gram positive 300px, Violet-stained gram-positive bacilli_.html" style="text-decoration: none;"class="mw-redirect" title="gram-negative">cocci and pink-stained bacilli_">gram-negative">cocci_and_pink-stained_gram-negative_bacillus_(shape)">bacilli_ In_bacte ...
bacteria). Technological advances in electron microscopy allowed the separation of the
Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids). It includes all protists whose plastids contain chlorophyll ''c'', such a ...
from the
Plantae Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi wer ...
kingdom. Indeed, the chloroplast of the chromists is located in the lumen of the
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum (R ...
instead of in the
cytosol The cytosol, also known as intracellular fluid (ICF) or cytoplasmic matrix, or groundplasm, is the liquid found inside cells. It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into m ...
. Moreover, only chromists contain
chlorophyll c Chlorophyll ''c'' is a form of chlorophyll found in certain marine algae, including the photosynthetic Chromista (e.g. diatoms and brown algae) and dinoflagellates. It has a blue-green color and is an accessory pigment, particularly significant in ...
. Since then, many non-photosynthetic phyla of protists, thought to have secondarily lost their chloroplasts, were integrated into the kingdom Chromista. Finally, some protists lacking mitochondria were discovered. As mitochondria were known to be the result of the
endosymbiosis An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within", σύν ' ...

endosymbiosis
of a
proteobacterium Proteobacteria is a major phylum of Gram-negative bacteria. They include a wide variety of pathogenic genera, such as ''Escherichia'', ''Salmonella'', ''Vibrio'', ''Helicobacter'', ''Yersinia'', ''Legionellales'', and many others. Others are free- ...
, 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 Animals (also called Metazoa) are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a ...
,
Protozoa Protozoa (also protozoan, plural protozoans) is an informal term for a group of single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris. Historically, prot ...
,
Fungi A fungus (plural: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, which is separa ...
,
Plantae Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi wer ...
and
Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids). It includes all protists whose plastids contain chlorophyll ''c'', such a ...
). This was known as the Archezoa hypothesis, which has since been abandoned; later schemes did not include the Archezoa–Metakaryota divide. † No longer recognized by Taxonomy (biology), taxonomists.


Six kingdoms (1998)

In 1998, Cavalier-Smith published a six-kingdom model, which has been revised in subsequent papers. The version published in 2009 is shown below.Compared to the version Cavalier-Smith published in 2004, the Alveolata, alveolates and the rhizarians have been moved from Kingdom Protozoa to Kingdom Chromista. Cavalier-Smith no longer accepted the importance of the fundamental Eubacteria–Archaebacteria divide put forward by Woese and others and supported by recent research. The kingdom
Bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres ...
(sole kingdom of empire Prokaryota) was subdivided into two sub-kingdoms according to their membrane topologies: Unibacteria and Negibacteria. Unibacteria was divided into phyla
Archaebacteria Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
and
Posibacteria 300px, Violet-stained gram-positive bacilli_.html" style="text-decoration: none;"class="mw-redirect" title="gram-negative">cocci and pink-stained bacilli_">gram-negative">cocci_and_pink-stained_gram-negative_bacillus_(shape)">bacilli_ In_bacte ...
; the bimembranous-unimembranous transition was thought to be far more fundamental than the long branch of genetic distance of Archaebacteria, viewed as having no particular biological significance. Cavalier-Smith does not accept the requirement for taxa to be
monophyletic 300px, A cladogram of the primates, showing a ''monophyletic'' taxon: ''the simians'' (in yellow); a ''paraphyletic'' taxon: ''the prosimians'' (in cyan, including the red patch); and a ''polyphyletic'' group: ''the night-active primates, i.e., ...
("holophyletic" in his terminology) to be valid. He defines Prokaryota, Bacteria, Negibacteria, Unibacteria, and Posibacteria as valid Paraphyly, paraphyla (therefore "monophyletic" in the sense he uses this term) taxa, marking important innovations of biological significance (in regard of the concept of biological Ecological niche, niche). In the same way, his Paraphyly, paraphyletic kingdom Protozoa includes the ancestors of Animalia, Fungi, Plantae, and Chromista. The advances of phylogenetic studies allowed Cavalier-Smith to realize that all the phyla thought to be archezoans (i.e. primitively amitochondriate eukaryotes) had in fact secondarily lost their mitochondria, typically by transforming them into new organelles: Hydrogenosomes. This means that all living eukaryotes are in fact metakaryotes, according to the significance of the term given by Cavalier-Smith. Some of the members of the defunct kingdom Archezoa, like the phylum Microsporidia, were reclassified into kingdom
Fungi A fungus (plural: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, which is separa ...
. Others were reclassified in kingdom
Protozoa Protozoa (also protozoan, plural protozoans) is an informal term for a group of single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris. Historically, prot ...
, like Metamonada which is now part of infrakingdom Excavata. Because Cavalier-Smith allows paraphyly, the diagram below is an ‘organization chart’, not an ‘ancestor chart’, and does not represent an evolutionary tree.


Seven kingdoms

Cavalier-Smith and his collaborators revised their classification in 2015. In this scheme they reintroduced the division of prokaryotes into two kingdoms,
Bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres ...
(=
Eubacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres ...
) and
Archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
(=
Archaebacteria Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
). This is based on the consensus in the Bacterial taxonomy, Taxonomic Outline of Bacteria and Archaea (TOBA) and the Catalogue of Life.


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 evolutionary grade, grades rather than clades, and so are rejected by cladistics, phylogenetic classification systems. * The most recent research does not support the classification of the eukaryotes into any of the standard systems. , no set of kingdoms is sufficiently supported by research to attain widespread acceptance. 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."


Beyond traditional kingdoms

While the concept of kingdoms continues to be used by some taxonomists, there has been a movement away from traditional kingdoms, as they are no longer seen as providing a Cladistics, cladistic classification, where there is emphasis in arranging organisms into Clade, natural groups.


Three domains of life

From around the mid-1970s onwards, there was an increasing emphasis on comparisons of genes at the molecular level (initially ribosome, ribosomal ribonucleic acid, RNA genes) as the primary factor in classification; genetic similarity was stressed over outward appearances and behavior. Taxonomic ranks, including kingdoms, were to be groups of organisms with a common ancestor, whether monophyletic (''all'' descendants of a common ancestor) or paraphyletic (''only some'' descendants of a common ancestor). Based on such RNA studies,
Carl Woese Carl Richard Woese (; July 15, 1928 – December 30, 2012) was an American microbiologist and biophysicist. Woese is famous for defining the Archaea (a new domain of life) in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique he pionee ...

Carl Woese
thought life could be divided into three large divisions and referred to them as the "three primary kingdom" model or "urkingdom" model. In 1990, the name "domain" was proposed for the highest rank. This term represents a synonym for the category of dominion (lat. dominium), introduced by Moore in 1974. Unlike Moore, Woese et al. (1990) did not suggest a Latin term for this category, which represents a further argument supporting the accurately introduced term dominion. Woese divided the prokaryotes (previously classified as the Kingdom Monera) into two groups, called bacteria, Eubacteria and Archaea, Archaebacteria, stressing that there was as much genetic difference between these two groups as between either of them and all eukaryotes. According to genetic data, although eukaryote groups such as plants, fungi, and animals may look different, they are more closely related to each other than they are to either the Eubacteria or Archaea. It was also found that the eukaryotes are more closely related to the Archaea than they are to the Eubacteria. Although the primacy of the Eubacteria-Archaea divide has been questioned, it has been upheld by subsequent research. There is no consensus on how many kingdoms exist in the classification scheme proposed by Woese.


Eukaryotic supergroups

Image:Eukaryota tree.svg, upright=1.5, One hypothesis of eukaryotic relationships, modified from Simpson and Roger (2004). In 2004, a review article by Simpson and Roger noted that the Protista were "a Wastebasket taxon, grab-bag for all
eukaryote Eukaryotes () are organisms whose cells have a nucleus enclosed within a nuclear envelope. Eukaryotes belong to the domain Eukaryota or Eukarya; their name comes from the Greek εὖ (''eu'', "well" or "good") and κάρυον (''karyon'', "nu ...
s that are not animals, plants or fungi". They held that only monophyletic groups should be accepted as formal ranks in a classification and that – while this approach had been impractical previously (necessitating "literally dozens of eukaryotic 'kingdoms) – it had now become possible to divide the eukaryotes into "just a few major groups that are probably all monophyletic". On this basis, the diagram opposite (redrawn from their article) showed the real "kingdoms" (their quotation marks) of the eukaryotes. A classification which followed this approach was produced in 2005 for the International Society of Protistologists, by a committee which "worked in collaboration with specialists from many societies". It divided the eukaryotes into the same six "supergroups". The published classification deliberately did not use formal taxonomic ranks, including that of "kingdom". In this system the multicellular animals (Metazoa) are descended from the same ancestor as both the unicellular choanoflagellates and the fungi which form the Opisthokonta. Plants are thought to be more distantly related to animals and fungi. However, in the same year as the International Society of Protistologists' classification was published (2005), doubts were being expressed as to whether some of these supergroups were monophyletic, particularly the Chromalveolata, and a review in 2006 noted the lack of evidence for several of the six proposed supergroups. , there is widespread agreement that the Rhizaria belong with the Stramenopiles and the Alveolata, in a clade dubbed the SAR supergroup, so that Rhizaria is not one of the main eukaryote groups. Beyond this, there does not appear to be a consensus. Rogozin ''et al.'' in 2009 noted that "The deep phylogeny of eukaryotes is an extremely difficult and controversial problem." , there appears to be a consensus that the six supergroup model proposed in 2005 does not reflect the true phylogeny of the eukaryotes and hence how they should be classified, although there is no agreement as to the model which should replace it.


Viruses

The International Committee on Taxonomy of Viruses uses the taxonomic rank "kingdom" for the classification of viruses (with the suffix ''-virae''); but this is beneath the top level classifications of Realm (virology), realm and subrealm. There is ongoing debate as to whether viruses can be included in the tree of life. The ten arguments against include the fact that they are obligate intracellular parasites that lack metabolism and are not capable of Self-replication, replication outside of a host cell. Another argument is that their placement in the tree would be problematic, since it is suspected that viruses have arisen multiple times, and they have a penchant for harvesting nucleotide sequences from their hosts. On the other hand, arguments favor their inclusion. One comes from the discovery of unusually large and complex viruses, such as Mimivirus, that possess typical cellular genes.


See also

* Cladistics * Phylogenetics * Systematics * Taxonomy (biology), Taxonomy


Notes


References


Further reading

* Pelentier, B. (2007-2015). ''Empire Biota: a comprehensive taxonomy''

[Historical overview.] * Peter H. Raven and Helena Curtis (1970), ''Biology of Plants'', New York: Worth Publishers. [Early presentation of five-kingdom system.]


External links


A Brief History of the Kingdoms of Life
at Earthling Nature



{{DEFAULTSORT:Rank01 Kingdoms (biology), Biology terminology, Kingdom