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 old ...

, 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. 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 in ...

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 exclud ...

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 from t ...

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 exclu ...

a, Archaea/Archaebacteria, and Bacteria/Eubacteria) while textbooks in Great Britain, India, Greece, Brazil and other countries use five kingdoms only (Animalia, Plantae, Fungi, Protista and

Monera Monera (/məˈnɪərə/) (Greek - μονήρης (monḗrēs), "single", "solitary") is a biological kingdom that is made up of prokaryotes. As such, it is composed of single-celled organisms that lack a nucleus. The taxon Monera was first ...

). 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 gro ...

, 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''. Zoo ...

'' (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 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 agr ...

into biology in 1735, the highest rank was given the name "kingdom" and was followed by four other main or principal ranks: class, order,

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

and

species In biology, a species is the basic unit of Taxonomy (biology), 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 ...

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

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. Ideall ...

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 exampl ...

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 (384–322 BC) classified animal species in his ''

History of Animals ''History of Animals'' ( grc-gre, Τῶν περὶ τὰ ζῷα ἱστοριῶν, ''Ton peri ta zoia historion'', "Inquiries on Animals"; la, Historia Animalium, "History of Animals") is one of the major texts on biology by the ancient G ...

'', 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'', Routledg ...

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

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 exclud ...

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. ( ...

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 become Advertising is the practice and techniques employed to bring attention to a product or service. Advertising aims to put a product or service in the spotlight in hopes of drawing it attention from consumers. It is typically used to promote a ...

rapidly blurred at its boundaries and outmoded". In 1860 John Hogg proposed the ''Protoctista'', a third kingdom of life composed of "all the lower creatures, or the primary organic beings"; he retained Regnum Lapideum as a fourth kingdom of minerals. In 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 sp ...

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 exclu ...

'', 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 mic ...

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 Nucle ...

(

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 & Connec ...

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. Bact ...

s). In 1937 Édouard Chatton 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

of prokaryotic organisms; as a revised phylum Monera of the Protista, it included organisms now classified as Bacteria and Archaea. Ernst Haeckel, in his 1904 book ''The Wonders of Life'', had placed the blue-green algae (or Phycochromacea) in Monera; this would gradually gain acceptance, and the blue-green algae would become classified as bacteria in the phylum Cyanobacteria. 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 contribut ...

and

C. B. van Niel Cornelis Bernardus van Niel (also known as Kees van Niel) (November 4, 1897 – March 10, 1985) was a Dutch-American microbiologist. He introduced the study of general microbiology to the United States and made key discoveries explaining the ...

promoted and popularized Édouard Chatton's earlier work, particularly in their paper of 1962, "The Concept of a Bacterium"; this created, for the first time, a rank above kingdom—a ''superkingdom'' or ''empire''—with the

two-empire system The two-empire system (two-superkingdom system) was the top-level biological classification system in general use before the establishment of the three-domain system. It classified cellular life into Prokaryota and Eukaryota as either "empires" or ...

of prokaryotes and eukaryotes. The two-empire system would later be expanded to the

three-domain system The three-domain system is a biological classification introduced by Carl Woese, 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 fr ...

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 from t ...

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 nu ...

; his Plantae were mostly multicellular autotrophs, 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

*Branch Myxomonera **Phylum Cyanophyta **Phylum Myxobacteriae *Branch Mastigomonera **Phylum Eubacteriae **Phylum Actinomycota **Phylum Spirochaetae , , Kingdom

*Phylum Euglenophyta *Phylum Chrysophyta *Phylum Pyrrophyta *Phylum Hyphochytridiomycota *Phylum Plasmodiophoromycota *Phylum

Sporozoa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. Th ...

*Phylum Cnidosporidia *Phylum Zoomastigina *Phylum

Sarcodina 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 pseudop ...

*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 majority ...

*Subkingdom Phaeophycophyta **Phylum Phaeophyta *Subkingdom Euchlorophyta **Branch Chlorophycophyta ***Phylum Chlorophyta ***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 Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...

, , Kingdom

*Subkingdom Gymnomycota **Phylum Myxomycota **Phylum

Acrasiomycota The family Acrasidae ( ICZN, or Acrasiomycota, ICBN) is a family of slime molds which belongs to the excavate group Percolozoa. The name element - comes from the Greek ''akrasia'', meaning "acting against one's judgement". This group consis ...

**Phylum

Labyrinthulomycota The Labyrinthulomycetes (ICBN) or Labyrinthulea ( ICZN) are a class of protists that produce a network of filaments or tubes, which serve as tracks for the cells to glide along and absorb nutrients for them. The two main groups are the labyri ...

*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 r ...

*Subkingdom

Eumycota 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 from t ...

**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 ***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 i ...

***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, Basi ...

, , Kingdom Animalia *Subkingdom Agnotozoa **Phylum

Mesozoa The Mesozoa are minuscule, worm-like parasites of marine invertebrates. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. A recent study recovered Mes ...

*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 as a ...

**Branch

Radiata Radiata or Radiates is a historical taxonomic rank that was used to classify animals with radially symmetric body plans. The term Radiata is no longer accepted, as it united several different groupings of animals that do not form a monophylet ...

***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 th ...

***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 The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it ...

****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, unsegment ...

****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 ****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 ****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 d ...

or Kamptozoa ***Grade

Coelomata The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it r ...

****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 l ...

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 ...

*****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 estim ...

*****Phylum Sipunculoidea *****Phylum Echiuroidea *****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, oft ...

****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 sea l ...

*****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 five ...

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 ribosomal ...

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 , 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'', suc ...

. {{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 among ...

(Bacteria) , 2=Kingdom

Archaebacteria 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 Archaebact ...

(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. Bact ...

, 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

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 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 (

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 bacte ...

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 mitochondrio ...

. 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, it was thought that these amitochondriate eukaryotes were primitively so, marking an important step in eukaryogenesis. As a result, these amitochondriate protists were separated from the protist kingdom, giving rise to the, at the same time, superkingdom and kingdom

Archezoa 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 cate ...

. This superkingdom was opposed to the Metakaryota superkingdom, grouping together the five other eukaryotic kingdoms ( Animalia,

and

). This was known as the Archezoa hypothesis, 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

, 2={{clade , label1=Superkingdom

‡ , 1={{clade , 1=Kingdom

‡ , 3={{clade , label1=Superkingdom Metakaryota‡ , 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 The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major clade and superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with tubulocristate mitochond ...

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 foramin ...

ns have been moved from Kingdom Protozoa to Kingdom Chromista. Cavalier-Smith no longer accepted the importance of the fundamental Eubacteria–Archaebacteria divide put forward by Woese and others and supported by recent research. The kingdom Bacteria (sole kingdom of empire

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

and Posibacteria; the bimembranous-unimembranous transition was thought to be far more fundamental than the long branch of genetic distance of Archaebacteria, viewed as having no particular biological significance. Cavalier-Smith does not accept the requirement for taxa to be

("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, th ...

). 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 cate ...

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 prot ...

, 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 c ...

, 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 Bacteria — includes

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

, 1={{clade , 1=Kingdom

— e.g. Amoebozoa, Choanozoa,

, 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 se ...

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 foramin ...

, 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 spec ...

red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondary ...

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 a ...

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 sist ...

, 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 Bacteria{{br Subkingdom Negibacteria{{br Infrakingdom Lipobacteria{{br Superphylum Eobacteria{{br Phylum

Heliobacteria Heliobacteria are a unique subset of prokaryotic bacteria that process light for energy. Distinguishable from other phototrophic bacteria, they utilize a unique photosynthetic pigment, bacteriochlorophyll ''g'' and are the only known Gram-posit ...

{{br Phylum Hadobacteria{{br Subphylum Chlorobacteria{{br Subphylum Deinobacteria{{br Superphylum Endoflagellata{{br Phylum Spirochaetae{{br Subphylum Euspirochaetae{{br Subphylum Leptospirae{{br Infrakingdom Glycobacteria{{br Superphylum Pimelobacteria{{br Phylum Sphingobacteria{{br Subphylum Chlorobibacteria{{br Subphylum

Flavobacteria The class Flavobacteriia is composed of a single order of environmental bacteria. According to Bernardet ''et al''., Flavobacteriia are Gram-negative aerobic rods, 2–5 μm long, 0.3–0.5 μm wide, with rounded or tapered end ...

{{br Phylum

Eurybacteria Eurybacteria is a taxon created by Cavalier-Smith, which includes several groups of Gram-negative bacteria. In this model, it is the ancestor of gram positive bacteria. Their endospores are characterized by producing and presenting external fl ...

{{br Subphylum Sclenobacteria{{br Subphylum Fusobacteria{{br Subphylum Fibrobacteria{{br Phylum Cyanobacteria{{br Subphylum Gloeobacteria{{br Subphylum Phycobacteria{{br Phylum

Proteobacteria 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 ...

{{br Subphylum Rhodobacteria{{br Infraphylum Alphabacteria{{br Infraphylum Chromatibacteria{{br Subphylum Thiobacteria{{br Superphylum Planctobacteria{{br Phylum Planctobacteria{{br Subkingdom Unibacteria{{br Infrakingdom Posibacteria{{br Phylum Posibacteria{{br Subphylum Teichobacteria{{br Infraphylum Endobacteria{{br Infraphylum

Actinobacteria 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 ...

{{br Subphylum Togobacteria{{br Infrakingdom

{{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{{br Subphylum Anaeromonada{{br Subphylum Parabasala{{br Subkingdom Neozoa{{br Infrakingdom Sarcomastigota{{br Phylum Neomonada{{br Subphylum Apusozoa{{br Subphylum Isomita{{br Subphylum Choanozoa{{br Phylum Cercozoa{{br Subphylum Phytomyxa{{br Subphylum Reticulofilosa{{br Subphylum

Monadofilosa Monadofilosa is a grouping of Cercozoa. (It is sometimes considered one of three, the other two being Phytomyxa and Reticulofilosa.) These organisms are single-celled amoeboid protists. Classification Monadofilosa includes the testaceans, whi ...

{{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 Amoebozoa{{br Subphylum

Lobosa Lobosa is a taxonomic group of amoebae in the phylum Amoebozoa. Most lobosans possess broad, bluntly rounded pseudopods, although one genus in the group, the recently discovered ''Sapocribrum,'' has slender and threadlike (filose) pseudopodia. I ...

{{br Subphylum

Conosa Conosa is a grouping of Amoebozoa. It is subdivided into three groups – Archamoebae, Variosea (paraphyletic) and Mycetozoa (polyphyletic). In some classifications, the mycetozoan Myxogastria and Dictyostelia are united in Macromycetozo ...

{{br Infraphylum Archamoebae{{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{{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 occasional ...

{{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{{br Phylum Dinozoa{{br Subphylum Protalveolata{{br Subphylum Dinoflagellata{{br Phylum

{{br Subphylum Gregarinae{{br Subphylum Coccidiomorpha{{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 fro ...

{{br Phylum Radiozoa{{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 ela ...

{{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 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 i ...

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

{{br Subphylum Hemiascomycotina{{br Class Taphrinomycetes{{br Class Geomycetes{{br Class Endomycetes{{br Subclass Dipomycetidae{{br Subclass Saccharomycetidae{{br Subphylum Euascomycotina{{br Class Discomycetes{{br Subclass Calycomycetidae{{br Subclass Lecomycetidae{{br Subclass Pezomycetidae{{br Class

Pyrenomycetes Sordariomycetes is a class of fungi in the subdivision Pezizomycotina (Ascomycota), consisting of 28 orders, 90 families, 1344 genera. Sordariomycetes is from the Latin sordes (filth) because some species grow in animal feces, though growth ha ...

{{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{{br Superclass Hemibasidiomycetia{{br Class Ustomycetes{{br Superclass Hymenomycetia{{br Class Gelimycetes{{br Subclass Tremellomycetidae{{br Subclass Dacrymycetidae{{br Subclass Auromycetidae{{br Class Homobasidiomycetes{{br Subclass Clavomycetidae{{br Subclass Pileomycetidae , , {{nowrap, Kingdom Animalia{{br Subkingdom

{{br Infrakingdom Spongiaria{{br Phylum

{{br Subphylum Hyalospongiae{{br Subphylum Calcispongiae{{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 ph ...

{{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 ...

{{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'', an ...

{{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. ...

{{br Phylum

Myxosporidia Myxosporea is a class of microscopic parasites, belonging to the Myxozoa clade within Cnidaria. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate (generally an annelid but sometimes a bryozoan ...

{{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 action ...

{{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, biv ...

{{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, a ...

{{br Phylum

Brachiozoa Brachiozoa is a grouping of lophophorate animals including Brachiopoda and Phoronida. It also includes their ancestors, the extinct tommotiid Tommotiids are an extinct group of Cambrian invertebrates thought to be early lophophorates (the g ...

{{br Subphylum Brachiopoda{{br Subphylum Phoronida{{br Superphylum

Sipuncula The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of unsegmented marine annelid worms. The name ''Sipuncula'' is from the genus name '' Sipunculus'', and comes from the Latin ...

{{br Phylum

{{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 m ...

{{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 lo ...

{{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 tr ...

{{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, ...

{{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 mit ...

{{br Subphylum Trilobitomorpha {{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 clad ...

{{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 ...

{{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, a ...

{{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{{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 Wasserbär ...

{{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{{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 fr ...

{{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{{br Subphylum Trochata{{br Infraphylum

Rotifera The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Ha ...

{{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 more ...

{{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 group ...

{{br Subphylum

Neodermata Neodermata is a clade of rhabditophoran flatworms containing the parasitic groups Trematoda, Monogenea and Cestoda. Description All neodermatans are parasites, in many groups having a free-swimming larval stage. The most striking feature un ...

{{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 ...

{{br Infraphylum Cercomeromorpha{{br Branch

{{br Infrakingdom Coelomopora{{br Phylum Hemichordata{{br Subphylum

Pterobranchia Pterobranchia is a class of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. Ther ...

{{br Subphylum

Enteropneusta The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. The closest non-hemichordate relatives of the Enteropneusta are the echinoderms. There are 111 known species of acorn worm in the ...

{{br Phylum

{{br Subphylum

Homalozoa Homalozoa is an obsolete extinct subphylum of Paleozoic era echinoderms, prehistoric marine invertebrates. They are also referred to as carpoids. Description The Homalozoa lacked the typical pentamer body form of other echinoderms, but all were ...

{{br Subphylum Pelmatozoa{{br Infraphylum

Blastozoa Blastozoa is a subphylum of extinct animals belonging to Phylum Echinodermata. This subphylum is characterized by the presence of specialized respiratory structures and brachiole plates used for feeding. This subphylum ranged from the Cambrian to ...

{{br Infraphylum Crinozoa{{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 ...

{{br Infraphylum Asterozoa{{br Infraphylum

Echinozoa Echinozoa is a subphylum of free-living echinoderms in which the body is or originally was a modified globe with meridional symmetry. Echinozoans lack arms, brachioles, or other appendages, and do not at any time exhibit pinnate structure. Their t ...

{{br Infrakingdom Chordonia{{br Phylum Urochorda{{br Subphylum Tunicata{{br Infraphylum Ascidiae{{br Infraphylum Thaliae{{br Subphylum Appendicularia{{br Phylum

{{br Subphylum Acraniata{{br Infraphylum

Cephalochordata A cephalochordate (from Greek: κεφαλή ''kephalé'', "head" and χορδή ''khordé'', "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called lancelets. Cephalochordates possess 5 synapomorphies, or prim ...

{{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, with c ...

{{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, cyclostom ...

{{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, living ...

{{br Subkingdom

{{br Phylum

, , {{nowrap, Kingdom

{{br Subkingdom Biliphyta{{br Infrakingdom

Glaucophyta 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 ...

{{br Phylum

{{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 Chlorophyta{{br Phylum Chlorophyta{{br Subphylum Chlorophytina{{br Infraphylum Prasinophytae{{br Infraphylum Tetraphytae{{br Subphylum Phragmophytina{{br Infraphylum Charophytae{{br Infraphylum Rudophytae{{br Infrakingdom Cormophyta{{br Phylum Bryophyta{{br Subphylum Hepaticae{{br Subphylum Anthocerotae{{br Subphylum

Musci Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) ''sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hornw ...

{{br Infraphylum Sphagneae{{br Infraphylum Bryatae{{br Phylum

{{br Subphylum Pteridophytina{{br Infraphylum Psilophytae{{br Infraphylum Lycophytae{{br Infraphylum Sphenophytae{{br Infraphylum

Filices A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except th ...

{{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, γυμνόσ ...

{{br Infraphylum Angiospermae , , {{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 Hacrobia, ...

{{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 w ...

{{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 de ...

{{br Subphylum Phaeista{{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 de ...

{{br Subphylum Diatomeae{{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 relationshi ...

{{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 cl ...

{{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: Bacteria and Archaea. (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 Info ...

). 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 exclu ...

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 and ...

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 Bacteria , 2=Kingdom Archaea , 2={{clade , label1=Superkingdom

, 1={{clade , 1=Kingdom

— e.g. Amoebozoa, Choanozoa,

, 2=Kingdom

— e.g. Alveolata, cryptophytes,

(

etc.),

, 3=Kingdom

ae — e.g.

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 clades, and so are rejected by phylogenetic classification systems. * The most recent research does not support the classification of the eukaryotes into any of the standard systems. {{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 cha ...

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 ribosomal ...

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 fo ...

RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

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 co ...

(''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 , 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

, 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 trans ...

, 1={{clade , 1=Domain Bacteria (

) , 2=Domain Archaea (

) , 3=Domain

Eukarya 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 ...

(

) 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 In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gen ...

, 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 , 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 , 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 Bacteria , 1={{clade , 1=

prokaryotic 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 ...

Bacteria , 2={{clade , label1=Domain Archaea , 1={{clade , 1=

Archaeans , 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= Amoebozoa , 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 (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...

s and

slime mould Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic mu ...

s , label3= Opisthokonta , 3=

, choanoflagellates, 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 group ...

(or Primoplantae) , 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 sist ...

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 i ...

) are descended from the same ancestor as both the unicellular choanoflagellates and the fungi which form the Opisthokonta. Plants are thought to be more distantly related to animals and fungi. 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-in- ...

, 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 clade 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 , 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=

, 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 ...

, 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=

, 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" 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 The eocyte hypothesis in evolutionary biology proposes the origin of eukaryotes from a group of prokaryotes called eocytes (later classified as Thermoproteota, a group of archaea). After his team at the University of California, Los Angeles dis ...

proposes that the

s emerged from a phylum within the archaea 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 Scien ...

, 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 a ...

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. Etym ...

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 has c ...

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 cel ...

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 ''Nature Reviews Microbiology'' is a monthly peer-reviewed review journal published by Nature Portfolio. It was established in 2003. The journal publishes reviews and perspectives on microbiology, bridging fundamental research and its clinical, in ...

, 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=

, 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 ''Mimivirus'' is a genus of giant viruses, in the family ''Mimiviridae''. Amoeba serve as their natural hosts. This genus contains a single identified species named ''Acanthamoeba polyphaga mimivirus'' (APMV). It also refers to a group of phyl ...

, 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 for ...

, 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 Kingdom commonly refers to: * A monarchy ruled by a king or queen * Kingdom (biology), a category in biological taxonomy Kingdom may also refer to: Arts and media Television * ''Kingdom'' (British TV series), a 2007 British television drama s ...

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