HOME

TheInfoList




In
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
, a kingdom (
Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in relation with") is "an appa ...

Latin
: ''regnum'', plural ''regna'') is the second highest
taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are gro ...
, 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 (
Animal Animals (also called Metazoa) are organisms that form the Animalia. With few exceptions, animals , , are , can , and grow from a hollow sphere of , the , during . Over 1.5 million animal have been —of which around 1 million are —b ...

Animal
ia,
Plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

Plant
ae,
Fungi A fungus (plural: fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

Fungi
,
Protista A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym f ...
,
Archaea Archaea ( ; singular archaeon ) constitute a 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 pa ...

Archaea
/Archaebacteria, and
Bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

Bacteria
/Eubacteria) while textbooks in Great Britain, India, Greece, Brazil and other countries use five kingdoms only (Animalia, Plantae, Fungi, Protista and
Monera Monera (/məˈnɪərə/) (Greek - μονήρης (monḗrēs), "single", "solitary") is a biological kingdom that is made up of prokaryote A prokaryote () is a Unicellular organism, single-celled organism that lacks a cell nucleus, nucleus, a ...
). Some recent classifications based on modern
cladistics Cladistics (; ) is an approach to biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying groups of biological organisms based on shared ch ...

cladistics
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. The informal terms ''
flora Flora is all the plant life present in a particular region or time, generally the naturally occurring (indigenous (ecology), indigenous) native plant, native plants. Sometimes bacteria and fungi are also referred to as flora, as in the terms ' ...

flora
'' (plants), ''
fauna Fauna is all of the animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular r ...

fauna
'' (animals), and, in the 21st century, '' funga'' are also used.


Definition and associated terms

When
Carl Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his Nobility#Ennoblement, ennoblement as Carl von Linné#Blunt, Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomi ...

Carl Linnaeus
introduced the rank-based system of
nomenclature Nomenclature (, ) is a system A system is a group of interacting Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, a ...

nomenclature
into biology in 1735, the highest rank was given the name "kingdom" and was followed by four other main or principal ranks:
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
,
order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the habit of achieving a ...
,
genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
and
species In biology, a species is the basic unit of biological classification, 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 individu ...

species
. Later two further main ranks were introduced, making the sequence kingdom, phylum or division, class, order,
family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society. Ideally, families would off ...
, 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 A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all its lineag ...

Protostomia
and
Deuterostomia Deuterostomia (; in Ancient Greek, Greek) are animals typically characterized by their anus forming before their mouth during embryogenesis, embryonic development. The group's sister clade is Protostomia, animals whose digestive tract developmen ...

Deuterostomia
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 A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental quest ...

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 by the , who had studied at in ...
'', while his pupil
Theophrastus Theophrastus (; grc-gre, Θεόφραστος ; c. 371c. 287 BC), a Greek native of Eresos Eresos (; el, Ερεσός; grc, Ἔρεσος) and its twin beach village Skala Eresou are located in the southwest part of the Greek island of Le ...

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 Nobility#Ennoblement, ennoblement as Carl von Linné#Blunt, Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomi ...

Carl Linnaeus
(1707–1778) laid the foundations for modern
biological nomenclature Biology is the natural science that studies life and living organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, prope ...
, now regulated by the
Nomenclature Codes Nomenclature codes or codes of nomenclature are the various rulebooks that govern biological taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classifica ...
, in 1735. He distinguished two kingdoms of living things: ''Regnum Animale'' ('
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
kingdom') and ''Regnum Vegetabile'' ('vegetable kingdom', for
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
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 Categorization is the human ability and activity of recognizing shared features or similarities between the elements of the experience of the world (such as Object (philosophy), objects, eve ...
, 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 Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () *Dutch language , spoken in Belgium ...
, 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 Charles II of ...
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 Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology that stu ...

Ernst Haeckel
also proposed a third kingdom of life, the ''
Protista A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym f ...
'', 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 microscope A microscope (from grc, μικρός ''mikrós'' 'small' and ''skopeîn'' 'to look (at); examine, inspect') is a laboratory instrument used to examine objects that are too small to b ...

microscopy
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 single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contig ...
s) and organisms whose cells do have a distinct nucleus (
eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...

eukaryote
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 (/məˈnɪərə/) (Greek - μονήρης (monḗrēs), "single", "solitary") is a biological kingdom that is made up of prokaryote A prokaryote () is a Unicellular organism, single-celled organism that lacks a cell nucleus, nucleus, a ...
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 The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

Bacteria
and
Archaea Archaea ( ; singular archaeon ) constitute a 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 pa ...

Archaea
. 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 In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

Cyanobacteria
. 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 contrib ...
and
C. B. van Niel Cornelis Bernardus van Niel (November 4, 1897, Haarlem Haarlem (; predecessor of ''Harlem'' in English) is a city A city is a large human settlement.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. a ...
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 The three-domain system is a biological classification In biology Bi ...
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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, ...
of Archaea, Bacteria, and Eukaryota.


Five kingdoms

The differences between
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

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, 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 Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

Fungi
. 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 (biology), absorption, Assimilation (biology), assimilation, biosynthesis, catabolism and excretion. ...
; his Plantae were mostly multicellular
autotroph An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bo ...
s, his Animalia multicellular
heterotroph A heterotroph (; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...
s, and his Fungi multicellular
saprotroph Image:Hyphae.JPG, 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, whi ...
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 evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. ...

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 plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into ...

Embryophyta
), 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 A microbiologist (from Greek ) is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an B ...

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 A non-coding RNA (ncRNA) is an RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consis ...
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 precursor ...
scheme of Thomas Cavalier-Smith, although it primarily differs in that Protista is replaced by
Protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

Protozoa
and
Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
.


Eight kingdoms

Thomas Cavalier-Smith Thomas (Tom) Cavalier-Smith, Royal Society, FRS, Royal Society of Canada, FRSC, Natural Environment Research Council, NERC Professorial Fellow (21 October 1942 - 19 March 2021), was a Professor of Evolutionary Biology in the Department of Zoolog ...
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 microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an ...
and
Archaebacteria Archaea ( ; singular archaeon ) constitute a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified a ...
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 microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an ...
into two subkingdoms: Negibacteria (
Gram negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram stain, gram-staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan c ...
bacteria) and Posibacteria (
Gram positive 300px, Violet-stained gram-positive bacilli_.html" ;"title="bacillus_(shape).html" ;"title="gram-negative.html" ;"title="cocci and pink-stained gram-negative">cocci and pink-stained gram-negative bacillus (shape)">bacilli ">bacillus_(shape).html" ...
bacteria). Technological advances in electron microscopy allowed the separation of the
Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
from the
Plantae Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions ...
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 ( ...
instead of in the
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a s ...
. Moreover, only chromists contain . 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 mutualism (biology), mutualistic relationship. (The term endosymbiosis is from the Greek language, Greek: ...

endosymbiosis
of a
proteobacterium Proteobacteria is a major phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plu ...
, it was thought that these amitochondriate eukaryotes were primitively so, marking an important step in
eukaryogenesis Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
. 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 A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

Animalia
,
Protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

Protozoa
,
Fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

Fungi
,
Plantae Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions ...
and
Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
). 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
taxonomists In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...
.


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
alveolates The alveolates (meaning "with cavities") are a group of protist A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system tha ...
and the
rhizaria The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many foramini ...
ns 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 The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

Bacteria
(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 (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified a ...
and Posibacteria; 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 paraphyla (therefore "monophyletic" in the sense he uses this term) taxa, marking important innovations of biological significance (in regard of the concept of biological
niche Niche may refer to: Science *Developmental niche{{third-party, date=October 2020 The developmental niche is a theoretical framework for understanding and analyzing how culture Culture () is an umbrella term which encompasses the social behav ...

niche
). In the same way, his 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:
Hydrogenosome A hydrogenosome is a membrane-enclosed organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are par ...

Hydrogenosome
s. 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 Microsporidia are a group of spore )'', growing on a thinning, thinned hybrid black poplar ''(populus, Populus x canadensis)''. The last stage of the moss#Life cycle, moss lifecycle is shown, where the sporophytes are visible before dispersi ...
, were reclassified into kingdom
Fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

Fungi
. Others were reclassified in kingdom
Protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

Protozoa
, like Metamonada which is now part of infrakingdom
Excavata Excavata is a major supergroup of unicellular organisms belonging to the Domain (biology), domain Eukaryota. It was first suggested by Simpson and Patterson in 1999 and introduced by Thomas Cavalier-Smith in 2002 as a formal taxon. It contains ...
. Because Cavalier-Smith allows
paraphyly In taxonomy 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 ...
, 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 The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

Bacteria
(=
Eubacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an ...
) and
Archaea Archaea ( ; singular archaeon ) constitute a 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 pa ...

Archaea
(=
Archaebacteria Archaea ( ; singular archaeon ) constitute a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified a ...
). This is based on the consensus in the Taxonomic Outline of Bacteria and Archaea (TOBA) and the
Catalogue of Life The Catalogue of Life is an online databaseAn online database is a database A database is an organized collection of data Data are units of information Information can be thought of as the resolution of uncertainty; it answers the q ...
.


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
grade Grade or grading may refer to: Arts and entertainment * Grade (band) Grade is a melodic hardcore band from Canada, often credited as pioneers in blending metallic hardcore with the hon and melody of emo, and - most notably - the alternating scr ...
s rather than
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
s, and so are rejected by
phylogenetic classification Phylogenetic nomenclature is a method 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 naming ...

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
cladistic Cladistics (; ) is an approach to Taxonomy (biology), biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically ...

cladistic
classification, where there is emphasis in arranging organisms into .


Three domains of life

From around the mid-1970s onwards, there was an increasing emphasis on comparisons of genes at the molecular level (initially
ribosomal Ribosomes () are macromolecular machines, found within all living cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids Amino acids are organic compounds that contain amino (–NH2) and Carboxylic ...

ribosomal
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...
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 In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

monophyletic
(''all'' descendants of a common ancestor) or
paraphyletic In taxonomy 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 ...

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 A microbiologist (from Greek ) is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an B ...

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
Eubacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an ...

Eubacteria
and
Archaebacteria Archaea ( ; singular archaeon ) constitute a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified a ...

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

In 2004, a review article by Simpson and Roger noted that the Protista were "a grab-bag for all
eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...

eukaryote
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 Animals (also called Metazoa) are multicellular Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the L ...
) are descended from the same ancestor as both the unicellular
choanoflagellate The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates having a funnel shaped collar of interconnect ...
s 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 A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
dubbed the
SAR supergroup SAR or Harosa (informally the SAR supergroup) is a clade A clade (), also known as a monophyletic group or natural group, is a group of organism In biology, an organism () is any organic, life, living system that functions as an ind ...
, 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 The International Committee on Taxonomy of Viruses (ICTV) authorizes and organizes the Taxonomy (biology), taxonomic classification of and the nomenclatures for viruses. The ICTV has developed a universal taxonomic scheme for viruses, and thus h ...
uses the taxonomic rank "kingdom" for the classification of viruses (with the suffix ''-virae''); but this is beneath the top level classifications of
realm A realm is a community or territory over which a sovereign Sovereign is a title which can be applied to the highest leader in various categories. The word is borrowed from Old French Old French (, , ; Modern French French ( or ...
and subrealm. There is ongoing debate as to whether
viruses A virus is a wikt:submicroscopic, submicroscopic infectious agent that Viral replication, replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, incl ...
can be included in the tree of life. The ten arguments against include the fact that they are obligate intracellular
parasites Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), adapted structurally to this w ...
that lack
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a phys ...

metabolism
and are not capable of 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 A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. By convention, sequences are usuall ...
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 ''Mimivirus'' is a genus of giant viruses, in the family ''Mimiviridae''. Amoeba serve as their natural hosts. This genus contains a single identified species named ''Acanthamoeba polyphaga mimivirus'' (APMV), which serves as its type species. I ...

Mimivirus
, that possess typical cellular genes.


See also

*
Cladistics Cladistics (; ) is an approach to biological classification In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...

Cladistics
*
Phylogenetics In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

Phylogenetics
*
Systematics Biological Biology is the natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), o ...
*
Taxonomy 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 ...


Notes


References


Further reading

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

istorical 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