biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary ...

, a kingdom is the second highest

taxonomic rank In biological classification, taxonomic rank is the relative level of a group of organisms (a taxon) in an ancestral or hereditary hierarchy. A common system consists of species, genus, family, order, class, phylum, kingdom, domain. While ...

, 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 * ...

. Kingdoms are divided into smaller groups called

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...

. Traditionally, some textbooks from the United States and Canada used a system of six kingdoms (

Animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

ia,

Plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...

ae,

Fungi A fungus ( : 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, separately fr ...

Protist 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 e ...

Archaea Archaea ( ; singular archaeon ) is 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 Archaeba ...

/Archaebacteria, and

Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

/Eubacteria) while textbooks in Great Britain, India, Greece, Brazil and other countries use five kingdoms only (Animalia, Plantae, Fungi, Protista and Monera). Some recent classifications based on modern

cladistics Cladistics (; ) 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 is typically shared derived cha ...

have explicitly abandoned the term ''kingdom'', noting that some traditional kingdoms are not

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 gr ...

, meaning that they do not consist of all the descendants of a common ancestor. The terms ''

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

'' (for plants), ''

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is '' flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. ...

'' (for animals), and, in the 21st century, ''

funga In life sciences ''funga'' is a recent term for the kingdom fungi similar to the longstanding fauna for animals and flora for plants. The term was considered to be urgently needed in order to simplify projects oriented toward implementation of edu ...

'' (for fungi) are also used for life present in a particular region or time.

Definition and associated terms

When

Carl Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, ...

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 ag ...

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 ...

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

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 appropriat ...

. Later two further main ranks were introduced, making the sequence kingdom, phylum or division,

family 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 the family is to maintain the well-being of its members and of society. Idea ...

and

. In 1990, the rank of

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 memb ...

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 exa ...

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 Peripatetic school of ...

(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 Aristotle's biology, texts on biolo ...

'', while his pupil

Theophrastus Theophrastus (; grc-gre, Θεόφραστος ; c. 371c. 287 BC), a Greek philosopher and the successor to Aristotle in the Peripatetic school. He was a native of Eresos in Lesbos.Gavin Hardy and Laurence Totelin, ''Ancient Botany'', Routle ...

(–) wrote a parallel work, the '' Historia Plantarum'', on plants.

(1707–1778) laid the foundations for modern

biological nomenclature Nomenclature codes or codes of nomenclature are the various rulebooks that govern biological taxonomic nomenclature, each in their own broad field of organisms. To an end-user who only deals with names of species, with some awareness that species ...

, now regulated by the

Nomenclature Codes Nomenclature codes or codes of nomenclature are the various rulebooks that govern biological taxonomic nomenclature, each in their own broad field of organisms. To an end-user who only deals with names of species, with some awareness that species ...

, in 1735. He distinguished two kingdoms of living things: ''Regnum Animale'' ('

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

kingdom') and ''Regnum Vegetabile'' ('vegetable kingdom', for

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...

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 ...

s in his

classification system Classification is a process related to categorization, the process in which ideas and objects are recognized, differentiated and understood. Classification is the grouping of related facts into classes. It may also refer to: Business, organizat ...

, 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 microbiologist and microscopist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the ...

, 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. The society fulfils a number of roles: promoting science and its benefits, re ...

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 John Joseph Hogg (born 19 March 1949) is a former Australian politician who served as a Senator for Queensland from 1996 to 2014, representing the Labor Party. He served as President of the Senate from 2008 to 2014. Early life Hogg was bor ...

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. He discovered, described and named thousands of new s ...

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 e ...

'', 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 micr ...

revealed important distinctions between those organisms whose cells do not have a distinct

nucleus Nucleus ( : 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 Nucl ...

(

prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Con ...

s) and organisms whose cells do have a distinct nucleus (

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

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 ' ...

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 of prokaryotic organisms; as a revised phylum Monera of the Protista, it included organisms now classified as

and

. 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'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

. 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 contribu ...

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. 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, Otto Kandler, and Mark Wheelis in 1990 that divides cellular life forms into three domains, namely Archaea, Bacteria, and Eukaryota or Eukarya. The key difference ...

of Archaea, Bacteria, and Eukaryota.

Five kingdoms

The differences between

fungi A fungus ( : 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, separately fr ...

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

. 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 provides organisms with nutrients, which can be metabolized to create energy and chemical structures. Failure to obtain sufficient ...

; 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 Wo ...

s, his Animalia multicellular

heterotroph A heterotroph (; ) 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 primary, secondary and tertiary consumers, but ...

s, and his Fungi multicellular

saprotroph Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...

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 biologist, and was the primary modern proponent for the significance of symbiosis in evolution. Historian Jan Sapp has said that "Lynn Ma ...

'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 comprise vegetation on Earth. Embryophytes () have a common ancestor with green algae, having emerged within the Phragmoplastophyta clade of green algae as siste ...

), 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). {, role="presentation" class="wikitable mw-collapsible" , + {{nowrap, Whittaker's five kingdom system (1969) , -style="font-size: 87%;vertical-align: top;background: white" , , Kingdom Monera *Branch Myxomonera **Phylum

Cyanophyta Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blue ...

**Phylum Myxobacteriae *Branch Mastigomonera **Phylum Eubacteriae **Phylum

Actinomycota The ''Actinomycetota'' (or ''Actinobacteria'') are a phylum of all gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to soi ...

**Phylum

Spirochaetae A spirochaete () or spirochete is a member of the phylum Spirochaetota (), (synonym Spirochaetes) which contains distinctive diderm (double-membrane) gram-negative bacteria, most of which have long, helically coiled (corkscrew-shaped or s ...

, , Kingdom

*Phylum

Euglenophyta Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typica ...

*Phylum

Chrysophyta Chrysophyta or golden algae is a term used to refer to certain heterokonts. It can be used to refer to: * Chrysophyceae (golden algae), Bacillariophyceae (diatoms), and Xanthophyceae (yellow-green algae) together. E.g., Pascher (1914). * Chrysoph ...

*Phylum

Pyrrophyta The dinoflagellates (Greek language, Greek δῖνος ''dinos'' "whirling" and Latin language, Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually consid ...

*Phylum

Hyphochytridiomycota Hyphochytrids are eukaryotic organisms in the group of Stramenopiles (Heterokonta). Characteristics They are distinguished by an anterior tinsel flagellum on their zoospores. Also they have a rhizoidal or hypha-like vegetative system (hence t ...

*Phylum

Plasmodiophoromycota The plasmodiophores (also known as plasmophorids or plasmodiophorids) are a group of obligate endoparasitic protists belonging to the subphylum Endomyxa in Cercozoa. Taxonomically, they are united under a single family Plasmodiophoridae, order P ...

*Phylum Sporozoa *Phylum

Cnidosporidia Cnidospora is a defunct subphylum of microscopic spore-forming parasites in the defunct phylum Protozoa. The subphylum was divided into two classes, the Myxosporidea (now classified as higher-animal Animals are multicellular, eukaryo ...

*Phylum Zoomastigina *Phylum Sarcodina *Phylum

Ciliophora The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...

, , Kingdom

Plantae Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...

*Subkingdom Rhodophycophyta **Phylum

Rhodophyta Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majorit ...

*Subkingdom Phaeophycophyta **Phylum

Phaeophyta Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and ...

*Subkingdom Euchlorophyta **Branch Chlorophycophyta ***Phylum

Chlorophyta Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to a ...

***Phylum

Charophyta Charophyta () is a group of freshwater green algae, called charophytes (), sometimes treated as a division, yet also as a superdivision or an unranked clade. The terrestrial plants, the Embryophyta emerged within Charophyta, possibly from terre ...

**Branch Metaphyta ***Phylum Bryophyta ***Phylum Tracheophyta , , Kingdom

*Subkingdom Gymnomycota **Phylum Myxomycota **Phylum Acrasiomycota **Phylum Labyrinthulomycota *Subkingdom Dimastigomycota **Phylum

Oomycota Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...

*Subkingdom Eumycota **Branch Opisthomastigomycota ***Phylum

Chytridiomycota Chytridiomycota are a division of zoosporic organisms in the kingdom Fungi, informally known as chytrids. The name is derived from the Ancient Greek ('), meaning "little pot", describing the structure containing unreleased zoöspores. Chytri ...

**Branch

Amastigomycota Amastigomycota or Eufungi is a clade of fungi. It includes all fungi without flagella or centrioles, and with unstacked Golgi apparatus cisternae. Members of this clade are Dikarya and the traditional paraphyletic assemblage "Zygomycota", now ...

***Phylum

Zygomycota Zygomycota, or zygote fungi, is a former division or phylum of the kingdom Fungi. The members are now part of two phyla: the Mucoromycota and Zoopagomycota. Approximately 1060 species are known. They are mostly terrestrial in habitat, living ...

***Phylum

Ascomycota Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...

***Phylum

Basidiomycota Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Bas ...

, , Kingdom

Animalia Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

*Subkingdom Agnotozoa **Phylum Mesozoa *Subkingdom Parazoa **Phylum

Porifera Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

**Phylum

Archaeocyatha Archaeocyatha (or archaeocyathids 'ancient cups') is a taxon of extinct, sessile, reef-building marine sponges that lived in warm tropical and subtropical waters during the Cambrian Period. It is believed that the centre of the Archaeocyatha or ...

{{extinct *Subkingdom

Eumetazoa Eumetazoa (), also known as diploblasts, Epitheliozoa, or Histozoa, are a proposed basal animal clade as a sister group of the Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen ...

**Branch Radiata ***Phylum

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

***Phylum

Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and ...

**Branch

Bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...

***Grade Acoelomata ****Phylum

Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegm ...

****Phylum

Nemertea Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many h ...

or Rhynchocoela ***Grade

Pseudocoelomata The Aschelminthes (also known as Aeschelminthes, Nemathelminthes, Nematodes), closely associated with the Platyhelminthes, are an obsolete phylum of pseudocoelomate and other similar animals that are no longer considered closely related and have be ...

****Phylum

Acanthocephala Acanthocephala (Greek , ', thorn + , ', head) is a phylum of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses to p ...

****Phylum

Aschelminthes The Aschelminthes (also known as Aeschelminthes, Nemathelminthes, Nematodes), closely associated with the Platyhelminthes, are an obsolete phylum of pseudocoelomate and other similar animals that are no longer considered closely related and have b ...

****Phylum

Entoprocta Entoprocta (), or Kamptozoa , is a phylum of mostly sessile aquatic animals, ranging from long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that ...

or Kamptozoa ***Grade Coelomata ****Subgrade Schizocoela *****Phylum

Bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

Ectoprocta Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...

*****Phylum Brachiopoda *****Phylum Phoronida *****Phylum

Mollusca Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is es ...

*****Phylum Sipunculoidea *****Phylum

Echiuroidea Echiuroidea is an order of annelids in the class Polychaeta. Families *Suborder Bonelliida **Bonelliidae Lacaze-Duthiers, 1858Lacaze-Duthiers, H. (1858). Recherches sur la bonellie (''Bonellia viridis'') (1). ''Annales des Sciences Naturells, C ...

*****Phylum Annelida *****Phylum

Arthropoda Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin ...

****Subgrade Enterocoela *****Phylum Brachiata or Pogonophora *****Phylum

Chaetognatha The Chaetognatha or chaetognaths (meaning ''bristle-jaws'') are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and ca ...

*****Phylum

Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...

*****Phylum Hemichordata *****Phylum

Chordata A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...

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 through a pioneering phylogenetic taxonomy of 16S ribosomal RNA, ...

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 riboso ...

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.{{Cite journal , last=Case , first=Emily , date=2008-10-01 , title=Teaching Taxonomy: How Many Kingdoms? , url=https://eric.ed.gov/?id=EJ813862 , access-date=2020-07-28 , journal=

American Biology Teacher American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the "United States" or "America" ** Americans, citizens and nationals of the United States of America ** American ancestry, pe ...

, volume=70 , issue=8 , pages=472–477 , doi=10.2307/30163328 , jstor=30163328 , language=en 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 Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...

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'', ...

. {{clade , label1= Life , 1={{clade , 1={{clade , label1=Empire

Prokaryota A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...

, 1={{clade , 1=Kingdom

Eubacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were amon ...

(Bacteria) , 2=Kingdom Archaebacteria (Archaea) , 2={{clade , label1=Empire

Eukaryota Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

, 1={{clade , 1=Kingdom

or Protoctista , 2=Kingdom

ae , 3=Kingdom

, 4=Kingdom

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 discov ...

supported the consensus at that time, that the difference between

and Archaebacteria 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

into two subkingdoms:

Negibacteria 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 ...

(

Gram negative The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram. Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to th ...

bacteria) and

Posibacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...

(

Gram positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...

bacteria). Technological advances in electron microscopy allowed the separation of the

from the

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 ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

. 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 i ...

. 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 ...

of a

proteobacterium Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The ...

, it was thought that these amitochondriate eukaryotes were primitively so, marking an important step in

eukaryogenesis Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

. As a result, these amitochondriate protists were separated from the protist kingdom, giving rise to the, at the same time, superkingdom and kingdom

Archezoa Archezoa was a kingdom proposed in the 20th century by Thomas Cavalier-Smith (1942–2021), and was believed to encompass eukaryotes which did not have mitochondria (and are therefore ''amitochondriate'') or peroxisomes (e.g. '' Giardia''). The c ...

. This superkingdom was opposed to the

Metakaryota The superkingdom Metakaryota was defined by Thomas Cavalier-Smith as advanced eukaryotes resulting from the endosymbiosis of a proteobacterium, giving rise to the mitochondrion, by an archezoa Archezoa was a kingdom proposed in the 20th century ...

superkingdom, grouping together the five other eukaryotic kingdoms (

and

). This was known as the

Archezoa hypothesis Archezoa was a kingdom proposed in the 20th century by Thomas Cavalier-Smith (1942–2021), and was believed to encompass eukaryotes which did not have mitochondria (and are therefore ''amitochondriate'') or peroxisomes (e.g. '' Giardia''). The c ...

, which has since been abandoned; later schemes did not include the Archezoa–Metakaryota divide. {{clade , label1= Life , 1={{clade , 1={{clade , label1=Superkingdom

, 1={{clade , 1=Kingdom

, 2=Kingdom Archaebacteria , 2={{clade , label1=Superkingdom

‡ , 1={{clade , 1=Kingdom

‡ , 3={{clade , label1=Superkingdom

‡ , 1={{clade , 1=Kingdom

, 2=Kingdom

, 3=Kingdom

ae , 4=Kingdom

, 5=Kingdom

ia ‡ No longer recognized by

taxonomists In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...

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.{{cite journal , last=Cavalier-Smith , first=Thomas , author-link=Thomas Cavalier-Smith , year=2009 , title=Kingdoms Protozoa and Chromista and the eozoan root of the eukaryotic tree , journal=Biology Letters , volume=6 , issue=3 , pages=342–345 , doi=10.1098/rsbl.2009.0948 , pmc=2880060 , pmid=20031978Compared to the version Cavalier-Smith published in 2004, the alveolates 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 forami ...

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

(sole kingdom of empire

) was subdivided into two sub-kingdoms according to their membrane topologies: Unibacteria and

. Unibacteria was divided into phyla Archaebacteria and

; 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

("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, a concept for understanding the cultural context of child development * Ecological niche, a term describing the relational position of an organism's species *Niche differentiation, in ecology, the ...

). 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

archezoa Archezoa was a kingdom proposed in the 20th century by Thomas Cavalier-Smith (1942–2021), and was believed to encompass eukaryotes which did not have mitochondria (and are therefore ''amitochondriate'') or peroxisomes (e.g. '' Giardia''). The c ...

ns (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 found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecular hydrogen and ATP i ...

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

, like the phylum

Microsporidia Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered protozoans or pr ...

, were reclassified into kingdom

. Others were reclassified in kingdom

, like Metamonada which is now part of infrakingdom

Excavata Excavata is a major supergroup of unicellular organisms belonging to the 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 a variety of free- ...

. Because Cavalier-Smith allows

paraphyly In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

, the diagram below is an 'organization chart', not an 'ancestor chart', and does not represent an evolutionary tree. {{clade , label1= Life , 1={{clade , 1={{clade , label1=Empire

, 1={{clade , 1=Kingdom

— includes Archaebacteria as part of a subkingdom , 2={{clade , label1=Empire

, 1={{clade , 1=Kingdom

— e.g.

Amoebozoa Amoebozoa is a major taxonomic group containing about 2,400 described species of amoeboid protists, often possessing blunt, fingerlike, lobose pseudopods and tubular mitochondrial cristae. In traditional and currently no longer supported c ...

Choanozoa Choanozoa is a clade of opisthokont eukaryotes consisting of the choanoflagellates (Choanoflagellatea) and the animals (Animalia, Metazoa). The sister-group relationship between the choanoflagellates and animals has important implications for th ...

, 2=Kingdom

— e.g. Alveolata, cryptophytes,

Heterokonta Heterokonts are a group of protists (formally referred to as Heterokonta, Heterokontae or Heterokontophyta). The group is a major line of eukaryotes. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which a ...

(

Brown Algae Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and p ...

Diatoms A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

etc.),

Haptophyta The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta (named for '' Prymnesium''), are a clade of algae. The names Haptophyceae or Prymnesiophyceae are sometimes used instead. This ending implies classification at ...

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 forami ...

, 3=Kingdom

ae — e.g.

glaucophyte The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments, less common today than they were during the Proterozoic. The stated number of speci ...

s, red and

green alga The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

land plants The Embryophyta (), or land plants, are the most familiar group of green plants that comprise vegetation on Earth. Embryophytes () have a common ancestor with green algae, having emerged within the Phragmoplastophyta clade of green algae as siste ...

, 4=Kingdom

, 5=Kingdom

ia {, role="presentation" class="wikitable mw-collapsible mw-collapsed" , + {{nowrap, Cavalier-Smith's six kingdom system (1998) , -style="font-size: 80%;vertical-align: top;background: white" , , {{nowrap, Kingdom

{{br Subkingdom

{{br Infrakingdom Lipobacteria{{br Superphylum Eobacteria{{br Phylum Heliobacteria{{br Phylum Hadobacteria{{br Subphylum

Chlorobacteria The Chloroflexota are a phylum of bacteria containing isolates with a diversity of phenotypes, including members that are aerobic thermophiles, which use oxygen and grow well in high temperatures; anoxygenic phototrophs, which use light for phot ...

{{br Subphylum Deinobacteria{{br Superphylum Endoflagellata{{br Phylum

{{br Subphylum Euspirochaetae{{br Subphylum Leptospirae{{br Infrakingdom Glycobacteria{{br Superphylum Pimelobacteria{{br Phylum

Sphingobacteria The FCB group is a superphylum of bacteria named after the main member phyla Fibrobacterota, Chlorobiota, and Bacteroidota. The members are considered to form a clade due to a number of conserved signature indels. Cavalier-Smith calls the e ...

{{br Subphylum Chlorobibacteria{{br Subphylum Flavobacteria{{br Phylum Eurybacteria{{br Subphylum Sclenobacteria{{br Subphylum

Fusobacteria Fusobacteriota are obligately anaerobic non-sporeforming Gram-negative bacilli. Since the first reports in the late nineteenth century, various names have been applied to these organisms, sometimes with the same name being applied to different s ...

{{br Subphylum Fibrobacteria{{br Phylum

{{br Subphylum

Gloeobacteria ''Gloeobacter'' is a genus of cyanobacteria. It is the sister group to all other cyanobacteria. ''Gloeobacter'' is unique among cyanobacteria in not having thylakoids, which are characteristic for all other cyanobacteria and chloroplasts. Inste ...

{{br Subphylum Phycobacteria{{br Phylum Proteobacteria{{br Subphylum Rhodobacteria{{br Infraphylum

Alphabacteria Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse an ...

{{br Infraphylum Chromatibacteria{{br Subphylum Thiobacteria{{br Superphylum

Planctobacteria The PVC superphylum is a superphylum of bacteria named after its three important members, Planctomycetota, Verrucomicrobiota, and Chlamydiota. Cavalier-Smith postulated that the PVC bacteria probably lost or reduced their peptidoglycan cell wal ...

{{br Phylum

{{br Subkingdom Unibacteria{{br Infrakingdom

{{br Phylum

{{br Subphylum Teichobacteria{{br Infraphylum

Endobacteria The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the ear ...

{{br Infraphylum Actinobacteria{{br Subphylum Togobacteria{{br Infrakingdom Archaebacteria{{br Phylum

Mendosicutes Archaea ( ; singular archaeon ) is 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 Archaeba ...

{{br Subphylum Euryarcheota{{br Infraphylum Halomebacteria{{br Infraphylum Eurytherma{{br Subphylum Sulfobacteria{{br , , {{nowrap, Kingdom

{{br Subkingdom

{{br Phylum Metamonada{{br Subphylum Eopharyngia{{br Subphylum Axostylaria{{br Phylum

Trichozoa Trichozoa is a group of excavates. "Fornicata" is a similar grouping, but it excludes Parabasalia.Tree
a ...

{{br Subphylum Anaeromonada{{br Subphylum Parabasala{{br Subkingdom

Neozoa The Scotokaryotes (Cavalier-Smith) is a proposed basal Neokaryote clade as sister of the Diaphoretickes. Basal Scotokaryote groupings are the Metamonads, the Malawimonas and the Podiata. In this phylogeny the Discoba are sometimes seen as paraph ...

{{br Infrakingdom Sarcomastigota{{br Phylum Neomonada{{br Subphylum

Apusozoa The Apusozoa are an Obazoa phylum comprising several genera of flagellate eukaryotes. They are usually around 5–20 μm in size, and occur in soils and aquatic habitats, where they feed on bacteria. They are grouped together based on the prese ...

{{br Subphylum Isomita{{br Subphylum

{{br Phylum

Cercozoa Cercozoa is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead defined by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major euk ...

{{br Subphylum

Phytomyxa ''Bradyrhizobium'' is a genus of Gram-negative soil bacteria, many of which fix nitrogen. Nitrogen fixation is an important part of the nitrogen cycle. Plants cannot use atmospheric nitrogen (N2); they must use nitrogen compounds such as nitrat ...

{{br Subphylum

Reticulofilosa Reticulofilosa is a grouping of Rhizaria. It includes Chlorarachnea ('' Chlorarachnion, Bigelowiella, Lotharella, Cryptochlora, Gymnochlora'') and Proteomyxidea Proteomyxidea is a class of Cercozoa. Although it is known to be paraphyleti ...

{{br Subphylum Monadofilosa{{br Phylum

Foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

{{br Phylum

{{br Subphylum Lobosa{{br Subphylum Conosa{{br Infraphylum

Archamoebae The Archamoebae are a group of protists originally thought to have evolved before the acquisition of mitochondria by eukaryotes. They include genera that are internal parasites or commensals of animals (''Entamoeba'' and ''Endolimax''). A few s ...

{{br Infraphylum

Mycetozoa Mycetozoa is a polyphyletic grouping of slime molds. It was originally thought to be a monophyletic clade, but recently it was discovered that protostelia are a polyphyletic group within Conosa. Classification It can be divided into dictyoste ...

{{br Superclass Eumyxa{{br Superclass Dictyostelia{{br Infrakingdom

Discicristata Discicristata is a proposed eukaryotic clade. It consists of Euglenozoa plus Percolozoa. It was proposed that Discicristata plus Cercozoa yielded Cabozoa. Another proposal is to group Discicristata with Jakobida into Discoba superphylum. See ...

{{br Phylum

Percolozoa The Percolozoa are a group of colourless, non-photosynthetic Excavata, including many that can transform between amoeboid, flagellate, and cyst stages. Characteristics Most Percolozoa are found as bacterivores in soil, fresh water and occasio ...

{{br Subphylum Tetramitia{{br Subphylum Pseudociliata{{br Phylum

Euglenozoa Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by three major clades, i.e., Kinetoplastea, D ...

{{br Subphylum Plicostoma{{br Subphylum Saccostoma{{br Infrakingdom Alveolata{{br Superphylum

Miozoa Myzozoa is a grouping of specific phyla within Alveolata, that either feed through myzocytosis, or were ancestrally capable of feeding through myzocytosis. Many protozoan orders are included within Myzozoa. It is sometimes described as a ph ...

{{br Phylum Dinozoa{{br Subphylum Protalveolata{{br Subphylum Dinoflagellata{{br Phylum Sporozoa{{br Subphylum Gregarinae{{br Subphylum

Coccidiomorpha Coccidia (Coccidiasina) are a subclass of microscopic, spore-forming, single-celled obligate intracellular parasites belonging to the apicomplexan class Conoidasida. As obligate intracellular parasites, they must live and reproduce within an a ...

{{br Subphylum Manubrispora{{br Superphylum Heterokaryota{{br Phylum

{{br Subphylum Tubulicorticata{{br Subphylum Epiplasmata{{br Subphylum Filocorticata{{br Infrakingdom Actinopoda{{br Phylum

Heliozoa Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes ( protists) with stiff arms (axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule-supported projections fr ...

{{br Phylum

Radiozoa The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The ela ...

{{br Subphylum Spasmaria{{br Subphylum

Radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The el ...

{{br , , {{nowrap, Kingdom

{{br Subkingdom Eomycota{{br Phylum Archemycota{{br Subphylum Dictyomycotina{{br Class Chytridiomycetes{{br Subclass Rumpomycetidae{{br Subclass Spizomycetidae{{br Class Enteromycetes{{br Subphylum Melanomycotina{{br Infraphylum Allomycotina{{br Class Allomycetes{{br Infraphylum Zygomycotina{{br Superclass Eozygomycetia{{br Class Bolomycetes{{br Class Glomomycetes{{br Superclass Neozygomycetia{{br Class Zygomycetes{{br Subclass Mucoromycetidae{{br Subclass Meromycetidae{{br Class Zoomycetes{{br Subclass Entomycetidae{{br Subclass Pedomycetidae{{br Superorder Trichomycetalia{{br Superorder Pyxomycetalia{{br Phylum

{{br Class Minisporea{{br Class

Microsporea Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered protozoans or pro ...

{{br Subclass Pleistophorea{{br Subclas Disporea{{br Subkingdom Neomycota{{br Phylum

{{br Subphylum Hemiascomycotina{{br Class

Taphrinomycetes The Taphrinomycetes are a class of ascomycete fungi belonging to the subdivision Taphrinomycotina. It includes the single order Taphrinales, which includes 2 families, 8 genera and 140 species In biology, a species is the basic unit of Ta ...

{{br Class Geomycetes{{br Class Endomycetes{{br Subclass Dipomycetidae{{br Subclass Saccharomycetidae{{br Subphylum Euascomycotina{{br Class

Discomycetes Discomycetes is a former taxonomic class of Ascomycete fungi which contains all of the cup, sponge and brain fungi and some club-like fungi. It includes typical cup fungi like the scarlet elf cup and the orange peel fungus, and fungi with fruit ...

{{br Subclass Calycomycetidae{{br Subclass Lecomycetidae{{br Subclass Pezomycetidae{{br Class Pyrenomycetes{{br Subclass Verrucomycetidae{{br Subclass Ostiomycetidae{{br Class Loculomycetes{{br Subclass Dendromycetidae{{br Subclass Loculoascomycetidae{{br Class Plectomycetes{{br Phylum

{{br Subphylum Septomycotina{{br Class Septomycetes{{br Subclass Sporidiomycetidae{{br Subclass Uredomycetidae{{br Subphylum

Orthomycotina Orthomycotina is a clade of fungi containing Agaricomycotina and Ustilaginomycotina, or all Basidiomycete fungi except Pucciniomycotina according to the 2007 fungal phylogeny "The Mycota: A Comprehensive Treatise on Fungi as Experimental System ...

{{br Superclass Hemibasidiomycetia{{br Class Ustomycetes{{br Superclass Hymenomycetia{{br Class Gelimycetes{{br Subclass

Tremellomycetidae The Tremellomycetes are a class of dimorphic fungi in the Agaricomycotina. Some species have gelatinous basidiocarps (fruiting bodies) or (microscopically) a sacculate parenthesome. There are six orders, 17 families, and 39 genera in the Tremell ...

{{br Subclass Dacrymycetidae{{br Subclass Auromycetidae{{br Class Homobasidiomycetes{{br Subclass Clavomycetidae{{br Subclass Pileomycetidae , , {{nowrap, Kingdom

{{br Subkingdom Radiata{{br Infrakingdom

Spongiaria Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through th ...

{{br Phylum

{{br Subphylum

Hyalospongiae Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers conside ...

{{br Subphylum

Calcispongiae The calcareous sponges of class Calcarea are members of the animal phylum Porifera, the cellular sponges. They are characterized by spicules made of calcium carbonate in the form of calcite or aragonite. While the spicules in most species have ...

{{br Subphylum

{{extinct{{br Infrakingdom

Coelenterata Coelenterata is a term encompassing the animal phyla Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their relatives) and Ctenophora (comb jellies). The name comes , referring to the hollow body cavity common to these two phyla ...

{{br Phylum

{{br Subphylum

Anthozoa Anthozoa is a subphylum of marine invertebrates which includes the sea anemones, stony corals and soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plankton. The basic unit of ...

{{br Subphylum

Medusozoa Medusozoa is a clade in the phylum Cnidaria, and is often considered a subphylum. It includes the classes Hydrozoa, Scyphozoa, Staurozoa and Cubozoa, and possibly the parasitic Polypodiozoa. Medusozoans are distinguished by having a medusa sta ...

{{br Infrakingdom

Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical ''Trichoplax adhaerens'', '' Hoilungia hongkongensis'', a ...

{{br Phylum

{{br Subkingdom

Myxozoa Myxozoa (etymology: Greek: μύξα ''myxa'' "slime" or "mucus" + thematic vowel o + ζῷον ''zoon'' "animal") is a subphylum of aquatic cnidarian animals – all obligate parasites. It contains the smallest animals ever known to have lived. O ...

{{br Phylum Myxosporidia{{br Subkingdom

{{br Branch

{{br Infrakingdom Lophozoa{{br Superphylum Polyzoa{{br Phylum

{{br Subphylum

Gymnolaemata Gymnolaemata are a class of Bryozoans. Gymnolaemata are sessile, mostly marine organisms and grow on the surfaces of rocks, kelp, and in some cases on animals, like fish. Zooids are cylindrical or flattened. The lophophore is protruded by acti ...

{{br Subphylum Lophopoda{{br Phylum Kamptozoa{{br Subphylum

{{br Subphylum

Cycliophora ''Symbion'' is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-sta ...

{{br Superphylum Conchozoa{{br Phylum

{{br Subphylum

Bivalvia Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

{{br Subphylum Glossophora{{br Infraphylum Univalvia{{br Infraphylum Spiculata{{br Infraphylum

Cephalopoda A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

{{br Phylum

Brachiozoa Brachiozoa is a grouping of lophophorate animals including Brachiopoda and Phoronida. It also includes their ancestors, the extinct tommotiids. References Lophophorata Protostome unranked clades {{Protostome-stub ...

{{br Subphylum Brachiopoda{{br Subphylum Phoronida{{br Superphylum Sipuncula{{br Phylum Sipuncula{{br Superphylum Vermizoa{{br Phylum Annelida{{br Subphylum

Polychaeta Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are ...

{{br Infraphylum Operculata{{br Infraphylum Pharyngata{{br Subphylum

Clitellata The Clitellata are a class of annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycles. The clitellates comprise around 8,000 species. Unlike the class of Polychaeta, th ...

{{br Subphylum

Echiura The Echiura, or spoon worms, are a small group of marine animals. Once treated as a separate phylum, they are now considered to belong to Annelida. Annelids typically have their bodies divided into segments, but echiurans have secondarily los ...

{{br Subphylum Pogonophora{{br Phylum Nemertina{{br Infrakingdom Chaetognathi{{br Phylum

{{br Infrakingdom

Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropoda ( insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla. They were first defined by Aguinaldo ''et al.'' in 1997, based mainly on phylogenetic ...

{{br Superphylum Haemopoda{{br Phylum

{{br Subphylum Cheliceromorpha{{br Infraphylum

Pycnogonida Sea spiders are marine arthropods of the order Pantopoda ( ‘all feet’), belonging to the class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). They are cosmopolitan, foun ...

{{br Infraphylum

Chelicerata The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...

{{br Subphylum

Trilobitomorpha The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilo ...

{{extinct{{br Subphylum

Mandibulata Mandibulata, termed "mandibulates", is a clade of arthropods that comprises the extant subphyla Myriapoda (millipedes and others), Crustacea and Hexapoda (insects and others). Mandibulata is currently believed to be the sister group of the clade ...

{{br Infraphylum

Crustacea Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group c ...

{{br Infraphylum

Myriapoda Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. The fossil record of myriapods reaches back into the late Silurian ...

{{br Infraphylum

Insecta Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs o ...

{{br Phylum

Lobopoda ''Lobopoda'' is a genus of comb-clawed beetles in the family Tenebrionidae. The type species is '' Lobopoda striata''. The following subgenera In biology, a subgenus (plural: subgenera) is a taxonomic rank directly below genus. In the Int ...

{{br Subphylum

Onychophora Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...

{{br Subphylum

Tardigrada Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...

{{br Superphylum Nemathelminthes{{br Phylum Nemathelminthes{{br Subphylum Scalidorhyncha{{br Infraphylum Priapozoa{{br Infraphylum

Kinorhyncha Kinorhyncha ( grc, κινέω, kīnéō, I move, ' "snout") is a phylum of small marine invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons. Modern species are or less, ...

{{br Subphylum

Nematoida Nematoida is a grouping of animals, including the roundworms The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known ...

{{br Infraphylum

Nematoda The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...

{{br Infraphylum

Nematomorpha Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes, or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. Most species range in size f ...

{{br Infrakingdom

Platyzoa The paraphyletic "Platyzoa" are a group of protostome unsegmented animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the phylum Platyhelminthes (or flatworms), and a new phylum, the Acanthognatha, into which ...

{{br Phylum

Acanthognatha Gnathifera (from the Greek '' gnáthos'', “jaw”, and the Latin '' -fera'', “bearing”) is a clade of generally small spiralians characterized by complex jaws made of chitin. It comprises the phyla Gnathostomulida, Rotifera, Micrognathozoa ...

{{br Subphylum Trochata{{br Infraphylum Rotifera{{br Infraphylum

{{br Subphylum Monokonta{{br Phylum

{{br Subphylum

Turbellaria The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to large freshwater forms mo ...

{{br Infraphylum Mucorhabda{{br Infraphylum

Rhabditophora Rhabditophora (from ''rhabdito''-, rhabdite + Greek -φορος ''phoros'' bearer, i.e., "rhabdite bearers") is a class of flatworms. It includes all parasitic flatworms (clade Neodermata) and most free-living species that were previously g ...

{{br Subphylum Neodermata{{br Infraphylum

Trematoda Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive h ...

{{br Infraphylum Cercomeromorpha{{br Branch

{{br Infrakingdom

Coelomopora Ambulacraria , or Coelomopora , is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around ...

{{br Phylum Hemichordata{{br Subphylum Pterobranchia{{br Subphylum Enteropneusta{{br Phylum

{{br Subphylum Homalozoa{{br Subphylum

Pelmatozoa Pelmatozoa was once a clade of Phylum Echinodermata. It included stalked and sedentary echinoderms. The main class of Pelmatozoa were the Crinoidea which includes sea lily and feather star Crinoids are marine animals that make up the class C ...

{{br Infraphylum Blastozoa{{br Infraphylum

Crinozoa Crinozoa is a subphylum of mostly sessile echinoderms, of which the crinoids, or sea lilies, are the only extant members. Crinozoans have an extremely extensive fossil history, which may or may not extend into the Precambrian (provided the enigma ...

{{br Subphylum

Eleutherozoa Eleutherozoa is a proposed subphylum of echinoderms. They are mobile animals with the mouth directed towards the substrate. They usually have a madreporite, tube feet, and moveable spines of some sort, and some have Tiedemann's bodies on the r ...

{{br Infraphylum

Asterozoa The Asterozoa are a subphylum in the phylum Echinodermata. Characteristics include a star-shaped body and radially divergent axes of symmetry. The subphylum includes the class Asteroidea (the starfish), the class Ophiuroidea (the brittle stars ...

{{br Infraphylum Echinozoa{{br Infrakingdom

Chordonia A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...

{{br Phylum Urochorda{{br Subphylum Tunicata{{br Infraphylum

Ascidiae Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tuni ...

{{br Infraphylum Thaliae{{br Subphylum Appendicularia{{br Phylum

{{br Subphylum

Acraniata Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordat ...

{{br Infraphylum Cephalochordata{{br Infraphylum Conodonta {{extinct{{br Subphylum

Vertebrata Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

{{br Infraphylum

Agnatha Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclosto ...

{{br Infraphylum

Gnathostomata Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, livi ...

{{br Subkingdom Mesozoa{{br Phylum Mesozoa , , {{nowrap, Kingdom

{{br Subkingdom Biliphyta{{br Infrakingdom Glaucophyta{{br Phylum Glaucophyta{{br Infrakingdom

{{br Phylum

{{br Subphylum Rhodellophytina{{br Subphylum Macrorhodophytina{{br Subkingdom

Viridiplantae Viridiplantae (literally "green plants") are a clade of eukaryotic organisms that comprise approximately 450,000–500,000 species and play important roles in both terrestrial and aquatic ecosystems. They are made up of the green algae, which ar ...

{{br Infrakingdom

{{br Phylum

{{br Subphylum

Chlorophytina The Chlorophytina are a proposed basal Tetraphytina The Tetraphytina (Cavalier-Smith 2008) or 'core Chlorophyta' are a proposed derived Chlorophyta Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The ...

{{br Infraphylum Prasinophytae{{br Infraphylum Tetraphytae{{br Subphylum Phragmophytina{{br Infraphylum Charophytae{{br Infraphylum Rudophytae{{br Infrakingdom Cormophyta{{br Phylum Bryophyta{{br Subphylum

Hepaticae The Marchantiophyta () are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of ge ...

{{br Subphylum

Anthocerotae Hornworts are a group of non-vascular Embryophytes (land plants) constituting the division Anthocerotophyta (). The common name refers to the elongated horn-like structure, which is the sporophyte. As in mosses and liverworts, hornworts have a ...

{{br Subphylum Musci{{br Infraphylum Sphagneae{{br Infraphylum Bryatae{{br Phylum Tracheophyta{{br Subphylum Pteridophytina{{br Infraphylum Psilophytae{{br Infraphylum Lycophytae{{br Infraphylum Sphenophytae{{br Infraphylum Filices{{br Subphylum Spermatophytina{{br Infraphylum

Gymnospermae The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμνό ...

{{br Infraphylum

Angiospermae Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...

, , {{nowrap, Kingdom

{{br Subkingdom

Cryptista Cryptista is a clade of algae-like eukaryotes. It is most likely related to Archaeplastida which includes plants and many algae, within the larger group Diaphoretickes. Although it has sometimes placed along with Haptista in the group Hacrob ...

{{br Phylum

Cryptophyta The cryptophyceae are a class of algae, most of which have plastids. About 220 species are known, and they are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10–50 μm in size and flattened in shape ...

{{br Subkingdom Chromobiota{{br Infrakingdom

{{br Superphylum

Sagenista Sagenista is a group of heterokonts containing the labyrinthulids and Eogyrea, a class of yet uncultured protists. Originally, it contained the Labyrinthulids and bicosoecids. However at present the bicosoecids have been removed, and Eogyrea wer ...

{{br Phylum

{{br Subphylum Bicoecia{{br Subphylum Labyrinthista{{br Phylum

Ochrophyta The ochrophytes, subphylum Ochrophytina, is a group of mostly photosynthetic heterokonts. Their plastid is of red algal origin. The classification of the group is still being worked out. Originally, the ochrophytes were regarded as a phylum ...

{{br Subphylum

Phaeista The ochrophytes, subphylum Ochrophytina, is a group of mostly photosynthetic heterokonts. Their plastid is of red algal origin. The classification of the group is still being worked out. Originally, the ochrophytes were regarded as a phylum de ...

{{br Infraphylum Hypogyrista{{br Infraphylum

Chrysista The ochrophytes, subphylum Ochrophytina, is a group of mostly photosynthetic heterokonts. Their plastid is of red algal origin. The classification of the group is still being worked out. Originally, the ochrophytes were regarded as a phylum ...

{{br Subphylum

Diatomeae A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

{{br Phylum

Bigyra Bigyra is a grouping of heterokont organisms. It includes Bicosoecida, Blastocystis and Labyrinthulida. It has also been described as containing Opalozoa, Bicoecia, and Sagenista. Phylogeny The cladogram below shows the internal relationships w ...

{{br Subphylum Bigyromonada{{br Subphylum

Pseudofungi Pseudofungi is a grouping of heterokonts, also known as the Heterokontimycotina. It consists of the Oomycota and Hyphochytridiomycetes. Although numerous biochemical, ultrastructural, and genetic traits clearly place them in the heterokonts, th ...

{{br Subphylum

Opalinata Opalinata is a superclass of non- phagotrophic heterokonts that unites the classes Opalinea and Blastocystea, and is the sister group to Opalomonadea. Description When Opalinata was first erected as a taxon in 1926, it was placed as the sole c ...

{{br Infrakingdom

{{br Phylum

Seven kingdoms

Cavalier-Smith and his collaborators revised their classification in 2015. In this scheme they introduced two superkingdoms of Prokaryota and Eukaryota and seven kingdoms. Prokaryota have two kingdoms:

and

. (This was based on the consensus in the Taxonomic Outline of Bacteria and Archaea, and the

Catalogue of Life The Catalogue of Life is an online database that provides an index of known species of animals, plants, fungi, and microorganisms. It was created in 2001 as a partnership between the global Species 2000 and the American Integrated Taxonomic In ...

). The Eukaryota have five kingdoms: Protozoa, Chromista, Plantae, Fungi, and Animalia. In this classification a

protist 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 e ...

is any of the eukaryotic

unicellular organism A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms a ...

s.{{cite journal , last1=Ruggiero , first1=Michael A. , last2=Gordon , first2=Dennis P. , last3=Orrell , first3=Thomas M. , last4=Bailly , first4=Nicolas , last5=Bourgoin , first5=Thierry , last6=Brusca , first6=Richard C. , last7=Cavalier-Smith , first7=Thomas , last8=Guiry , first8=Michael D. , last9=Kirk , first9=Paul M. , last10=Thuesen , first10=Erik V. , title=A higher level classification of all living organisms , journal= PLOS ONE , date=2015 , volume=10 , issue=4 , pages=e0119248 , doi=10.1371/journal.pone.0119248 , pmid=25923521 , pmc=4418965 , bibcode=2015PLoSO..1019248R , doi-access=free {{clade , label1= Life , 1={{clade , 1={{clade , label1=Superkingdom

, 1={{clade , 1=Kingdom

, 2=Kingdom

, 2={{clade , label1=Superkingdom

, 1={{clade , 1=Kingdom

— e.g.

, 2=Kingdom

— e.g. Alveolata, cryptophytes,

(

etc.),

, 3=Kingdom

ae — e.g.

s, red and

, 4=Kingdom

, 5=Kingdom

Summary

{{Full biological kingdom classification 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

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

s, and so are rejected by

phylogenetic classification Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...

systems. * The most recent research does not support the classification of the eukaryotes into any of the standard systems. {{As of, 2010, April, 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

{{anchor, Modern view 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 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 shared derived char ...

classification, where there is emphasis in arranging organisms into natural groups.

Three domains of life

{{main, Three-domain system, Domain (biology) {{PhylomapB, , caption=A

phylogenetic tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

based on

rRNA 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 ribos ...

data showing Woese's

. All smaller branches can be considered kingdoms., size = 440px 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 cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to for ...

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

(''all'' descendants of a common ancestor) or

paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

(''only some'' descendants of a common ancestor).{{citation needed, date=January 2017 Based on such RNA studies,

thought life could be divided into three large divisions and referred to them as the "three primary kingdom" model or "urkingdom" model.{{cite journal , last1=Balch , first1=W.E. , last2=Magrum , first2=L.J. , last3=Fox , first3=G.E. , last4=Wolfe , first4=C.R. , last5=Woese , first5=C.R. , name-list-style=amp , date=August 1977 , title=An ancient divergence among the bacteria , journal=

Journal of Molecular Evolution The ''Journal of Molecular Evolution'' is a monthly peer-reviewed scientific journal that covers molecular evolution. It is published by Springer Science+Business Media and was established in 1971. The founding editor was Emile Zuckerkandl, who rem ...

, volume=9 , issue=4 , pages=305–311 , doi=10.1007/BF01796092 , pmid=408502 , bibcode=1977JMolE...9..305B , s2cid=27788891 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.{{cite journal , last=Moore , first=R.T. , year=1974 , title=Proposal for the recognition of super ranks , journal=Taxon , volume=23 , issue=4 , pages=650–652 , doi=10.2307/1218807 , jstor=1218807 , url=http://www.iapt-taxon.org/historic/Congress/IBC_1975/Prop034bis-037.pdf 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.{{cite journal , last=Luketa , first=S. , year=2012 , title = New views on the megaclassification of life , journal=

Protistology Protistology is a scientific discipline devoted to the study of protists, a highly diverse group of eukaryotic organisms. All eukaryotes apart from animals, plants and fungi are considered protists. Its field of study therefore overlaps with the ...

, volume=7 , issue=4 , pages=218–237 , url=http://protistology.ifmo.ru/num7_4/luketa_protistology_7-4.pdf Woese divided the prokaryotes (previously classified as the Kingdom Monera) into two groups, called

and Archaebacteria, stressing that there was as much genetic difference between these two groups as between either of them and all eukaryotes. {{clade , label1=

Life Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energy ...

, 1={{clade , 1=Domain

(

) , 2=Domain

( Archaebacteria) , 3=Domain Eukarya (

) 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.{{cite journal , last1=Dagan , first1=T. , last2=Roettger , first2=M. , last3=Bryant , last4=Martin , first4=W. , year=2010 , title=Genome Networks Root the Tree of Life between Prokaryotic Domains , journal= Genome Biology and Evolution , volume=2 , pages=379–92 , doi=10.1093/gbe/evq025 , pmid=20624742 , pmc=2997548 , name-list-style=amp There is no consensus on how many kingdoms exist in the classification scheme proposed by Woese.

Eukaryotic supergroups

{{anchor, Kingdoms of the Eukaryota {{Main, Supergroup (biology) Tree of Living Organisms 2

In 2004, a review article by Simpson and Roger noted that the Protista were "a grab-bag for all

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".{{cite journal , title=The real 'kingdoms' of eukaryotes , last1=Simpson , first1=Alastair G.B. , last2=Roger , first2=Andrew J. , journal=

Current Biology ''Current Biology'' is a biweekly peer-reviewed scientific journal that covers all areas of biology, especially molecular biology, cell biology, genetics, neurobiology, ecology, and evolutionary biology. The journal includes research arti ...

, volume=14 , issue=17 , pages=R693–R696 , doi=10.1016/j.cub.2004.08.038 , pmid=15341755, year=2004 , s2cid=207051421 , doi-access=free 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".{{cite journal , vauthors=Adl SM, ((Simpson AGB)), Farmer MA, Andersen RA, Anderson OR, Barta JR, Bowser SS, Brugerolle G, Fensome RA, Fredericq S, James TY, Karpov S, Kugrens P, Krug J, Lane CE, Lewis LA, Lodge J, Lynn DH, Mann DG, Mccourt RM, Mendoza L, Moestrup Ø, Mozley-Standridge SE, Nerad TA, Shearer CA, Smirnov AV, Spiegel FW, Taylor MF , display-authors=6 , title=The new higher-level classification of eukaryotes with emphasis on the taxonomy of protists , journal=

Journal of Eukaryotic Microbiology The ''Journal of Eukaryotic Microbiology'' is a peer-reviewed scientific journal covering all aspects of eukaryotic microbiology. The journal publishes research on protists, including lower algae and fungi. According to the '' Journal Citation Rep ...

, year=2005 , volume=52 , issue=5 , pages=399–451 , doi=10.1111/j.1550-7408.2005.00053.x , pmid=16248873 , s2cid=8060916 , doi-access=free The published classification deliberately did not use formal taxonomic ranks, including that of "kingdom". {{clade , label1=

, 1={{clade , 1={{clade , label1=Domain

, 1={{clade , 1= prokaryotic

, 2={{clade , label1=Domain

, 1={{clade , 1= prokaryotic

ns , 3={{clade , label1=Domain

, 1={{clade , label1=

, 1= various

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and thei ...

protozoa , label2=

, 2= most lobose

amoeboid An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopo ...

s and slime moulds , label3= Opisthokonta , 3=

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 interconne ...

s, etc. , label4=

, 4=

, and various other

protozoa , label5=

Chromalveolata Chromalveolata was a eukaryote supergroup present in a major classification of 2005, then regarded as one of the six major groups within the eukaryotes. It was a refinement of the kingdom Chromista, first proposed by Thomas Cavalier-Smith in ...

, 5=

Stramenopiles Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...

( Brown Algae, Diatoms etc.),

(or cryptomonads), and Alveolata , label6=

Archaeplastida The Archaeplastida (or kingdom Plantae '' sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor gr ...

(or

Primoplantae The Archaeplastida (or kingdom Plantae ''sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group ...

) , 6=

Land plants The Embryophyta (), or land plants, are the most familiar group of green plants that comprise vegetation on Earth. Embryophytes () have a common ancestor with green algae, having emerged within the Phragmoplastophyta clade of green algae as siste ...

red alga Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

e, and

s In this system the multicellular animals (

Metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

) are descended from the same ancestor as both the unicellular

s and the fungi which form the Opisthokonta. Plants are thought to be more distantly related to animals and fungi. Eukaryote Phylogeny

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.{{cite journal , last1=Parfrey , first1=Laura W. , last2=Barbero , first2=Erika , last3=Lasser , first3=Elyse , last4=Dunthorn , first4=Micah , year=2006 , last5=Bhattacharya , first5=Debashish , last6=Patterson , first6=David J. , last7=Katz , first7=Laura A. , name-list-style=amp , title=Evaluating support for the current classification of eukaryotic diversity , journal=

PLOS Genetics ''PLOS Genetics'' is a peer-reviewed open access scientific journal established in 2005 and published by the Public Library of Science. The founding editor-in-chief was Wayne N. Frankel ( Columbia University Medical Center). The current editors- ...

, volume=2 , issue=12 , pages=e220 , doi=10.1371/journal.pgen.0020220 , pmid=17194223 , pmc=1713255 {{As of, 2010, there is widespread agreement that the Rhizaria belong with the Stramenopiles and the Alveolata, in a

dubbed the

SAR supergroup The SAR supergroup, also just SAR or Harosa, is a clade that includes stramenopiles ( heterokonts), alveolates, and Rhizaria. The name is an acronym derived from the first letters of each of these clades; it has been alternatively spelled "RAS". ...

,{{Harvnb, Burki, Shalchian-Tabrizi, Minge, Skjæveland , 2007, p=4 so that Rhizaria is not one of the main eukaryote groups.{{cite journal , last1=Burki , first1=Fabien , last2=Shalchian-Tabrizi , first2=Kamran , last3=Pawlowski , first3=Jan , year=2008 , title=Phylogenomics reveals a new 'megagroup' including most photosynthetic eukaryotes , journal=

Biology Letters ''Biology Letters'' is a peer-reviewed, biological, scientific journal published by the Royal Society. It focuses on the rapid publication of short high quality research articles, reviews and opinion pieces across the biological sciences. ''Biolog ...

, pmid=18522922 , volume=4 , issue=4 , pmc=2610160 , pages=366–369 , doi=10.1098/rsbl.2008.0224 , name-list-style=amp{{cite journal , last1=Burki , first1=F. , last2=Inagaki , year=2009 , first2=Y. , last3=Brate , first3=J. , last4=Archibald , first4=J. M. , last5=Keeling , first5=P. J. , last6=Cavalier-Smith , first6=T. , last7=Sakaguchi , first7=M. , last8=Hashimoto , first8=T. , last9=Horak , first9=A. , last10=Kumar , first10=S. , last11=Klaveness , first11=D., author-link11=Dag Klaveness (limnologist) , last12=Jakobsen , first12=K. S. , last13=Pawlowski , first13=J. , last14=Shalchian-Tabrizi , first14=K. , title=Large-scale phylogenomic analyses reveal that two enigmatic protist lineages, Telonemia and Centroheliozoa, are related to photosynthetic Chromalveolates , journal= Genome Biology and Evolution , volume=1 , pages=231–238 , doi=10.1093/gbe/evp022 , pmc=2817417 , pmid=20333193 , display-authors=8{{cite journal , last1=Hackett , first1=J.D. , last2=Yoon , first2=H.S. , last3=Li , first3=S. , last4=Reyes-Prieto , first4=A., last5=Rummele , first5=S.E. , last6=Bhattacharya , first6=D. , name-list-style=amp , year=2007 , title=Phylogenomic analysis supports the monophyly of cryptophytes and haptophytes and the association of Rhizaria with chromalveolates , journal=

Molecular Biology and Evolution ''Molecular Biology and Evolution'' is a monthly peer-reviewed scientific journal published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. It publishes work in the intersection of molecular biology and evo ...

, volume=24 , issue=8 , pages=1702–1713 , doi=10.1093/molbev/msm089 , pmid=17488740 , doi-access=free 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."{{cite journal , last1=Rogozin , first1=I.B. , last2=Basu , first2=M.K. , last3=Csürös , first3=M., last4=Koonin , first4=E.V. , year=2009 , title=Analysis of rare genomic changes does not support the unikont–bikont phylogeny, and suggests cyanobacterial symbiosis as the point of primary radiation of eukaryotes , journal= Genome Biology and Evolution , volume=1 , pages=99–113 , doi=10.1093/gbe/evp011 , pmc=2817406 , name-list-style=amp , pmid=20333181 {{As of, 2010, December, 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.{{cite journal , last1=Burki , first1=Fabien , last2=Shalchian-Tabrizi , first2=Kamran , last3=Minge , first3=Marianne , last4=Skjæveland , first4=Åsmund , last5=Nikolaev , first5=Sergey I. , last6=Jakobsen , first6=Kjetill S. , last7=Pawlowski , first7=Jan , name-list-style=amp , editor1-last=Butler , year=2007 , editor1-first=Geraldine , title=Phylogenomics reshuffles the eukaryotic supergroups , journal= PLOS ONE , volume=2 , issue=8 , pages=e790 , doi=10.1371/journal.pone.0000790 , pmid=17726520 , pmc=1949142 , bibcode=2007PLoSO...2..790B , doi-access=free{{cite journal , last1=Kim , first1=E. , last2=Graham , first2=L. E. , year=2008 , title=EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata , journal= PLOS ONE , volume=3 , issue=7 , pages=e2621 , editor1-last=Redfield , editor1-first=Rosemary Jeanne , doi=10.1371/journal.pone.0002621 , pmid=18612431 , pmc=2440802 , name-list-style=amp , last3=Redfield , first3=Rosemary Jeanne , bibcode=2008PLoSO...3.2621K , author3-link=Rosemary Redfield , doi-access=free{{clear

Comparison of top level classification

{{further, Tree of life (biology)#Proposals for top levels Some authors have added

non-cellular life Non-cellular life, or acellular life is life that exists without a cellular structure for at least part of its life cycle. Historically, most (descriptive) definitions of life postulated that an organism must be composed of one or more cells, ...

to their classifications. This can create a "superdomain" called "Acytota", also called "Aphanobionta", of non-cellular life; with the other superdomain being "

cytota The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a membrane, and contains many biomolecules such as proteins, DNA and RNA, as well as many small molecules of nutrients and ...

" or cellular life.{{cite journal , author=Trifonov EN, Kejnovsky E , title=Acytota - associated kingdom of neglected life. , journal=J Biomol Struct Dyn , year=2016 , volume= 34 , issue= 8 , pages=1641–8 , pmid=26305806 , doi=10.1080/07391102.2015.1086959 , pmc= , s2cid=38178747 , url=https://pubmed.ncbi.nlm.nih.gov/26305806/{{cite book , title=Biological systematics: The state of the art , publication-place=London , isbn=0-412-36440-9 , oclc=27895507 , page= , last1=Minelli , first1=Alessandro , year=1993 The eocyte hypothesis proposes that the

s emerged from a phylum within the

archaea Archaea ( ; singular archaeon ) is 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 Archaeba ...

called the

Thermoproteota The Thermoproteota (also known as crenarchaea) are archaea that have been classified as a phylum of the Archaea domain. Initially, the Thermoproteota were thought to be sulfur-dependent extremophiles but recent studies have identified characteris ...

(formerly known as eocytes or Crenarchaeota).{{cite journal , first1=John M. , last1=Archibald , title=The eocyte hypothesis and the origin of eukaryotic cells , journal=

PNAS ''Proceedings of the National Academy of Sciences of the United States of America'' (often abbreviated ''PNAS'' or ''PNAS USA'') is a peer-reviewed multidisciplinary scientific journal. It is the official journal of the National Academy of Scie ...

, volume=105 , issue=51 , pages=20049–20050 , date=23 December 2008 , doi=10.1073/pnas.0811118106, pmid=19091952 , bibcode=2008PNAS..10520049A , pmc=2629348 , doi-access=free {{cite journal , first1=James A. , last1= Lake , first2=Eric , last2=Henderson , first3=Melanie , last3=Oakes , first4=Michael W. , last4=Clark , title=Eocytes: A new ribosome structure indicates a kingdom with a close relationship to eukaryotes , journal=

, volume=81 , pages=3786–3790 , date=June 1984 , issue=12 , doi=10.1073/pnas.81.12.3786 , pmid=6587394 , pmc=345305 , bibcode=1984PNAS...81.3786L , doi-access=free {{biological classification with acellular

Viruses

The

International Committee on Taxonomy of Viruses The International Committee on Taxonomy of Viruses (ICTV) authorizes and organizes the taxonomic classification of and the nomenclatures for viruses. The ICTV has developed a universal taxonomic scheme for viruses, and thus has the means to ap ...

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 rules. The term is commonly used to describe a monarchical or dynastic state. A realm may also be a subdivision within an empire, if it has its own monarch, e.g. the German Empire. Et ...

and subrealm.{{cite web , url=https://talk.ictvonline.org/information/w/ictv-information/383/ictv-code , title=ICTV Code , website=talk.ictvonline.org , publisher=International Committee on Taxonomy of Viruses , access-date=26 April 2020 There is ongoing debate as to whether

viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's ...

can be included in the tree of life. The arguments against include the fact that they are obligate intracellular

parasites Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

that lack

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...

and are not capable of replication outside of a host cell.{{cite journal , last=Moreira , first=David , author2=Purificación López-García , title=Ten reasons to exclude viruses from the tree of life , journal= Nature Reviews Microbiology , year=2009 , volume=7 , pages=306–311 , issue=4 , doi=10.1038/nrmicro2108 , pmid=19270719 , s2cid=3907750{{cite journal , title=New views on the megaclassification of life , journal=

, date=2012 , first=Stefan , last=Luketa , volume=7 , issue=4 , pages=218–237 , url=http://protistology.ifmo.ru/num7_4/luketa_protistology_7-4.pdf Another argument is that their placement in the tree would be problematic, since it is suspected that viruses have arisen multiple times{{citation needed, date=October 2019, and they have a penchant for harvesting nucleotide sequences from their hosts. On the other hand, arguments favor their inclusion.{{cite journal , last1=Hegde , first1=Nagendra , first2=Mohan S. , last2=Maddur , first3=Srini V. , last3=Kaveri , first4=Jagadeesh , last4=Bayry , name-list-style=amp , title=Reasons to include viruses in the tree of life , journal= Nature Reviews Microbiology , year=2009 , volume=7 , pages=615 , issue=8 , doi=10.1038/nrmicro2108-c1 , pmid=19561628 , doi-access=free One comes from the discovery of unusually large and complex viruses, such as Mimivirus, that possess typical cellular genes.{{cite journal , last1=Raoult , first1=Didier , first2=Stéphane , last2=Audic , first3=Catherine , last3=Robert , first4=Chantal , last4=Abergel , first5=Patricia , last5=Renesto , first6=Hiroyuki , last6=Ogata , first7=Bernard , last7=La Scola , first8=Marie , last8=Suzan , first9=Jean-Michel , last9=Claverie , title=The 1.2 megabase genome sequence of Mimivirus , journal=

Science Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earliest archeological evidence ...

, year=2004 , volume=306 , pages=1344–1350 , doi=10.1126/science.1101485 , bibcode=2004Sci...306.1344R , pmid=15486256 , issue=5700, s2cid=84298461

Notes

{{notelist

References

{{reflist, refs= {{cite journal , last1=Woese , first1=C.R. , last2=Kandler , first2=O. , last3=Wheelis , first3=M.L. , year=1990 , title=Towards a natural systs: proposal for the domains Archaea, Bacteria, and Eucarya , journal=Proceedings of the National Academy of Sciences of the United States of America , volume=87 , issue=12 , pages=4576–9 , pmid=2112744 , pmc=54159 , doi=10.1073/pnas.87.12.4576 , bibcode=1990PNAS...87.4576W , doi-access=free

External links

A Brief History of the Kingdoms of Life
at Earthling Nature

{{Taxonomic ranks {{DEFAULTSORT:Rank01 Kingdom

Definition and associated terms

History

Two kingdoms of life

Three kingdoms of life

Four kingdoms

Five kingdoms

Six kingdoms

Eight kingdoms

Six kingdoms (1998)

Seven kingdoms

Summary

Beyond traditional kingdoms

Three domains of life

Eukaryotic supergroups

Comparison of top level classification

Viruses

See also

Notes

References

Further reading

External links