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LifeDomainKingdomPhylumClassOrderFamilyGenusSpecies
The hierarchy of biological classification's eight major taxonomic ranks. A kingdom contains one or more phyla. Intermediate minor rankings are not shown.

In biology, a phylum (/ˈfaɪləm/; pl.: phyla) is a level of classification, or taxonomic rank, that is below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent.[1][2][3] Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and the fungus kingdom Fungi contains about eight phyla. Current research in phylogenetics is uncovering the relationships among phyla within larger clades like Ecdysozoa and Embryophyta.

General description

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The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon (φῦλον, "race, stock"), related to phyle (φυλή, "tribe, clan").[4][5] Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as a group ("a self-contained unity"): "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., stock / tribe] (Phylon)."[a] In plant taxonomy, August W. Eichler (1883) classified plants into five groups named divisions, a term that remains in use today for groups of plants, algae and fungi.[1][6] The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier.[7]

Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan.[8] At its most basic, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).[9] Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were.[citation needed]

Definition based on genetic relation

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The most important objective measure in the above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group.[9] Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine the relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids, so the phyla were merged (the bearded worms are now an annelid family).[10] On the other hand, the highly parasitic phylum Mesozoa was divided into two phyla (Orthonectida and Rhombozoa) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes.[11]

This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size.[9]

Definition based on body plan

[edit]

A definition of a phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by a set of characters shared by all its living representatives.

This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on a subjective decision about which groups of organisms should be considered as phyla.

The approach is useful because it makes it easy to classify extinct organisms as "stem groups" to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities.[9] However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display a character unique to a sub-set of the crown group.[9] Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan.[12]

A classification using this definition may be strongly affected by the chance survival of rare groups, which can make a phylum much more diverse than it would be otherwise.[13]

Known phyla

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Animals

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Main article: Animal

Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species,[14] and some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.[15]

Protostome Bilateria Nephrozoa
Deuterostome
Basal/disputed
Vendobionta
Others
Phylum Meaning Common name Distinguishing characteristic Taxa described
Agmata Fragmented Calcareous conical shells 5 species, extinct
Annelida Little ring [16]: 306  Segmented worms, annelids Multiple circular segments 22,000+ extant
Arthropoda Jointed foot Arthropods Segmented bodies and jointed limbs, with Chitin exoskeleton 1,250,000+ extant;[14] 20,000+ extinct
Brachiopoda Arm foot[16]: 336  Lampshells[16]: 336  Lophophore and pedicle 300–500 extant; 12,000+ extinct
Bryozoa (Ectoprocta) Moss animals Moss animals, sea mats, ectoprocts[16]: 332  Lophophore, no pedicle, ciliated tentacles, anus outside ring of cilia 6,000 extant[14]
Chaetognatha Longhair jaw Arrow worms[16]: 342  Chitinous spines either side of head, fins approx. 100 extant
Chordata With a cord Chordates Hollow dorsal nerve cord, notochord, pharyngeal slits, endostyle, post-anal tail approx. 55,000+[14]
Cnidaria Stinging nettle Cnidarians Nematocysts (stinging cells) approx. 16,000[14]
Ctenophora Comb bearer Comb jellies[16]: 256  Eight "comb rows" of fused cilia approx. 100–150 extant
Cycliophora Wheel carrying Circular mouth surrounded by small cilia, sac-like bodies 3+
Dicyemida Lozenge animal Single anteroposterior axial celled endoparasites, surrounded by ciliated cells 100+
Echinodermata Spiny skin Echinoderms[16]: 348  Fivefold radial symmetry in living forms, mesodermal calcified spines approx. 7,500 extant;[14] approx. 13,000 extinct
Entoprocta Inside anus[16]: 292  Goblet worms Anus inside ring of cilia approx. 150
Gastrotricha Hairy stomach[16]: 288  Hairybellies Two terminal adhesive tubes approx. 690
Gnathostomulida Jaw orifice Jaw worms[16]: 260  Tiny worms related to rotifers with no body cavity approx. 100
Hemichordata Half cord[16]: 344  Acorn worms, hemichordates Stomochord in collar, pharyngeal slits approx. 130 extant
Kinorhyncha Motion snout Mud dragons Eleven segments, each with a dorsal plate approx. 150
Loricifera Armour bearer Brush heads Umbrella-like scales at each end approx. 122
Micrognathozoa Tiny jaw animals Accordion-like extensible thorax 2
Mollusca Soft[16]: 320  Mollusks/molluscs Muscular foot and mantle round shell 85,000+ extant;[14] 80,000+ extinct[17]
Monoblastozoa
(Nomen inquirendum) 		One sprout animals distinct anterior/posterior parts and being densely ciliated, especially around the "mouth" and "anus". 1
Nematoda Thread like Roundworms, threadworms, eelworms, nematodes[16]: 274  Round cross section, keratin cuticle 25,000[14]
Nematomorpha Thread form[16]: 276  Horsehair worms, Gordian worms[16]: 276  Long, thin parasitic worms closely related to nematodes approx. 320
Nemertea A sea nymph[16]: 270  Ribbon worms[16]: 270  Unsegmented worms, with a proboscis housed in a cavity derived from the coelom called the rhynchocoel approx. 1,200
Onychophora Claw bearer Velvet worms[16]: 328  Worm-like animal with legs tipped by chitinous claws approx. 200 extant
Orthonectida Straight swimmer Parasitic, microscopic, simple, wormlike organisms 20
Petalonamae Shaped like leaves An extinct phylum from the Ediacaran. They are bottom-dwelling and immobile, shaped like leaves (frondomorphs), feathers or spindles. 3 classes, extinct
Phoronida Zeus's mistress Horseshoe worms U-shaped gut 11
Placozoa Plate animals Trichoplaxes, placozoans[16]: 242  Differentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between 4+
Platyhelminthes Flat worm[16]: 262  Flatworms[16]: 262  Flattened worms with no body cavity. Many are parasitic. approx. 29,500[14]
Porifera Pore bearer Sponges[16]: 246  Perforated interior wall, simplest of all known animals 10,800 extant[14]
Priapulida Little Priapus Penis worms Penis-shaped worms approx. 20
Proarticulata Before articulates An extinct group of mattress-like organisms that display "glide symmetry." Found during the Ediacaran. 3 classes, extinct
Rotifera Wheel bearer Rotifers[16]: 282  Anterior crown of cilia approx. 3,500[14]
Saccorhytida Saccus : "pocket" and "wrinkle" Saccorhytus is only about 1 mm (1.3 mm) in size and is characterized by a spherical or hemispherical body with a prominent mouth. Its body is covered by a thick but flexible cuticle. It has a nodule above its mouth. Around its body are 8 openings in a truncated cone with radial folds. Considered to be a deuterostome[18] or an early ecdysozoan.[19] 2 species, extinct
Tardigrada Slow step Water bears, moss piglets Microscopic relatives of the arthropods, with a four segmented body and head 1,000
Trilobozoa Three-lobed animal Trilobozoans A taxon of mostly discoidal organisms exhibiting tricentric symmetry. All are Ediacaran-aged 18 genera, extinct
Vetulicolia Ancient dweller Vetulicolians Might possibly be a subphylum of the chordates. Their body consists of two parts: a large front part and covered with a large "mouth" and a hundred round objects on each side that have been interpreted as gills or openings near the pharynx. Their posterior pharynx consists of 7 segments. 15 species, extinct
Xenacoelomorpha Strange hollow form Xenacoelomorphs Small, simple animals. Bilaterian, but lacking typical bilaterian structures such as gut cavities, anuses, and circulatory systems[20] 400+
Total: 39 1,525,000[14]

Plants

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Main article: Plant

The kingdom Plantae is defined in various ways by different biologists (see Current definitions of Plantae). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta, to form the clade Viridiplantae. The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida,[21] a group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions.

The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta,[22] while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes),[23] or for conifers alone as below.

Since the first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses.[24]

Archaeplastida Biliphyta[21] Other algae
Viridiplantae Green algae
Embryophyte (Land plants)
Division Meaning Common name Distinguishing characteristics Species described
Anthocerotophyta[25] Anthoceros-like plants Hornworts Horn-shaped sporophytes, no vascular system 100–300+
Bryophyta[25] Bryum-like plants, moss plants Mosses Persistent unbranched sporophytes, no vascular system approx. 12,000
Charophyta Chara-like plants Charophytes approx. 1,000
Chlorophyta (Yellow-)green plants[16]: 200  Chlorophytes approx. 7,000
Cycadophyta[26] Cycas-like plants, palm-like plants Cycads Seeds, crown of compound leaves approx. 100–200
Ginkgophyta[27] Ginkgo-like plants Ginkgophytes Seeds not protected by fruit only 1 extant; 50+ extinct
Glaucophyta Blue-green plants Glaucophytes 15
Gnetophyta[28] Gnetum-like plants Gnetophytes Seeds and woody vascular system with vessels approx. 70
Lycophyta[29] Lycopodium-like plants

Wolf plants

Clubmosses Microphyll leaves, vascular system 1,290 extant
Angiospermae Seed container Flowering plants, angiosperms Flowers and fruit, vascular system with vessels 300,000
Marchantiophyta,[30]

Hepatophyta[25]

Marchantia-like plants

Liver plants

Liverworts Ephemeral unbranched sporophytes, no vascular system approx. 9,000
Polypodiophyta Polypodium-like plants
 Ferns Megaphyll leaves, vascular system approx. 10,560
Picozoa Extremely small animals Picozoans, picobiliphytes 1
Pinophyta,[23]

Coniferophyta[31]

Pinus-like plants

Cone-bearing plant

Conifers Cones containing seeds and wood composed of tracheids 629 extant
Prasinodermophyta Prasinoderma-like plants Picozoans, picobiliphytes, biliphytes 8
Rhodophyta Red plants Red algae Use phycobiliproteins as accessory pigments. approx. 7,000
Total: 16

Fungi

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Main article: Fungi
Division Meaning Common name Distinguishing characteristics Species described
Ascomycota Bladder fungus[16]: 396  Ascomycetes,[16]: 396  sac fungi Tend to have fruiting bodies (ascocarp).[32] Filamentous, producing hyphae separated by septa. Can reproduce asexually.[33] 30,000
Basidiomycota Small base fungus[16]: 402  Basidiomycetes,[16]: 402  club fungi Bracket fungi, toadstools, smuts and rust. Sexual reproduction.[34] 31,515
Blastocladiomycota Offshoot branch fungus[35] Blastoclads Less than 200
Chytridiomycota Little cooking pot fungus[36] Chytrids Predominantly Aquatic saprotrophic or parasitic. Have a posterior flagellum. Tend to be single celled but can also be multicellular.[37][38][39] 1000+
Glomeromycota Ball of yarn fungus[16]: 394  Glomeromycetes, AM fungi[16]: 394  Mainly arbuscular mycorrhizae present, terrestrial with a small presence on wetlands. Reproduction is asexual but requires plant roots.[34] 284
Microsporidia Small seeds[40] Microsporans[16]: 390  1400
Neocallimastigomycota New beautiful whip fungus[41] Neocallimastigomycetes Predominantly located in digestive tract of herbivorous animals. Anaerobic, terrestrial and aquatic.[42] approx. 20 [43]
Zygomycota Pair fungus[16]: 392  Zygomycetes[16]: 392  Most are saprobes and reproduce sexually and asexually.[42] approx. 1060
Total: 8

Phylum Microsporidia is generally included in kingdom Fungi, though its exact relations remain uncertain,[44] and it is considered a protozoan by the International Society of Protistologists[45] (see Protista, below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor),[46] which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish the Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina, Kickxellomycotina, Mucoromycotina, and Zoopagomycotina.[44]

Protists

[edit]
Main article: Taxonomy of Protista

Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi.[16]: 120  Protista is a paraphyletic taxon,[47] which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system.[48]

Protist taxonomy has long been unstable,[49] with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by the Catalogue of Life,[50] and correspond to the Protozoa-Chromista scheme,[45] with updates from the latest (2022) publication by Cavalier-Smith.[51] Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP.[52]

Stramenopiles Diaphoretickes
Alveolata
Rhizaria
"Hacrobia"
Amorphea
Excavates
Orphan groups
Phylum Meaning Common name Distinguishing characteristics Species described Image
Amoebozoa Amorphous animals Amoebozoans Presence of pseudopodia for amoeboid movement, tubular cristae.[52] approx. 2,400[53]
Apicomplexa Apical infolds[54] Apicomplexans, sporozoans Mostly parasitic, at least one stage of the life cycle with flattened subpellicular vesicles and a complete apical complex, non-photosynthetic apicoplast.[52] over 6,000[54]
Apusozoa
(paraphyletic) 		Apusomonas-like animals Gliding biciliates with two or three connectors between centrioles 32
Bigyra Two rings Stramenopiles with a double helix in ciliary transition zone
Cercozoa Flagellated animal Cercozoans Defined by molecular phylogeny, lacking distinctive morphological or behavioural characters.[52]
Chromerida Chromera-like organisms Chrompodellids, chromerids, colpodellids[55] Biflagellates, chloroplasts with four membranes, incomplete apical complex, cortical alveoli, tubular cristae.[52] 8[56]
Choanozoa
(paraphyletic) 		Funnel animals[16] Opisthokont protists Filose pseudopods; some with a colar of microvilli surrounding a flagellum approx. 300[53]
Ciliophora Cilia bearers Ciliates Presence of multiple cilia and a cytostome. approx. 4,500[57]
Cryptista Hidden[16] Defined by molecular phylogeny, flat cristae.[52] 246[56][52]
Dinoflagellata Whirling flagellates[16] Dinoflagellates Biflagellates with a transverse ribbon-like flagellum with multiple waves beating to the cell's left and a longitudinal flagellum beating posteriorly with only one or few waves.[52] 2,957 extant
955 fossil[56]
Endomyxa Within mucus[16][58] Defined by molecular phylogeny,[52] typically plasmodial endoparasites of other eukaryotes.[58]
Eolouka
(paraphyletic) 		Early groove[59] Heterotrophic biflagellates with ventral feeding groove.[59] 23
Euglenozoa True eye animals Biflagellates, one of the two cilia inserted into an apical or subapical pocket, unique ciliary configuration.[52] 2,037 extant
20 fossil[56]
Haptista Fasten[16] Thin microtubule-based appendages for feeding (haptonema in haptophytes, axopodia in centrohelids), complex mineralized scales.[52] 517 extant
1,205 fossil[56]
Hemimastigophora Incomplete or atypical flagellates[60] Hemimastigotes[61] Ellipsoid or vermiform phagotrophs, two slightly spiraling rows of around 12 cilia each, thecal plates below the membrane supported by microtubules and rotationally symmetrical, tubular and saccular cristae.[52][60] 10[62]
Heterolobosea;[63]
Percolozoa 		Percolomonas-like animals Heteroloboseans, amoebomastigotes[16] Complex life cycle containing amoebae, flagellates and cysts.[52] Amoeboflagellates with an amoeba, a flagellate, and a cyst stage in their life cycles. Amoebae usually cylindrical, with a monopodial locomotive form, relatively fast-moving via eruptive lobopodia. Flagellates usually with two or four flagella that arise at the anterior end of a feeding groove. Golgi apparatus lacking a classic stacked form. Mitochondria with discoidal cristae, some species with acristate, hydrogen-producing mitochondrion-related organelles.[63] approx. 170[63]
Malawimonada Malawimonas-like organisms Malawimonads Small free-living bicilates with two kinetosomes, one or two vanes in posterior cilium. 3[64]
Metamonada Middle monads Metamonads Anaerobic or microaerophilic, some without mitochondria; four kinetosomes per kinetid
Ochrophyta;
Heterokontophyta 		Ochre plants, heterokont plants Heterokont algae, stramenochromes, ochrophytes, heterokontophytes Biflagellates with tripartite mastigonemes, chloroplasts with four membranes and chlorophylls a and c, tubular cristae.[52] 21,052 extant
2,262 fossil[56]
Opisthosporidia
(often considered fungi) 		Opisthokont spores[65] Parasites with chitinous spores and extrusive host-invasion apparatus
Perkinsozoa Perkinsus-like animals Perkinsozoans, perkinsids Parasitic biflagellates, incomplete apical complex, formation of zoosporangia or undifferentiated cells via a hypha-like tube.[52] 26
Provora Devouring voracious protists[66] Defined by molecular phylogeny, free-living eukaryovorous heterotrophic biflagellates with ventral groove and extrusomes.[66] 7[66]
Pseudofungi False fungi Defined by molecular phylogeny, phagotrophic heterokonts with a helical ciliary transition zone.[67] over 1,200[68]
Retaria Reticulopodia-bearing organisms[58] Feeding by reticulopodia (or axopodia) typically projected through various types of skeleton, closed mitosis.[69] 10,000 extant
50,000 fossil
Sulcozoa
(paraphyletic) 		Groove-bearing animals[59] Aerobic flagellates (none, 1, 2 or 4 flagella) with dorsal semi-rigid pellicle of one or two submembrane dense layers, ventral feeding groove, branching ventral pseudopodia, typically filose.[59] 40+
Telonemia Telonema-like organisms[70] Telonemids[71] Phagotrophic pyriform biflagellates with a unique complex cytoskeleton, tubular cristae, tripartite mastigonemes, cortical alveoli.[70][71] 7
Total: 26, but see below.

The number of protist phyla varies greatly from one classification to the next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae,[50] but other systems consider these phyla part of Protista.[72] In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista, and all alveolates except ciliates in one phylum Myzozoa, later lowered in rank and included in a paraphyletic phylum Miozoa.[51] Even within a phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta. These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete.[52]

Bacteria

[edit]
Main article: Bacterial phyla

Currently there are 41 bacterial phyla (not including "Cyanobacteria") that have been validly published according to the Bacteriological Code[73]

  1. Abditibacteriota
  2. Acidobacteriota, phenotypically diverse and mostly uncultured
  3. Actinomycetota, High-G+C Gram positive species
  4. Aquificota, deep-branching
  5. Armatimonadota
  6. Atribacterota
  7. Bacillota, Low-G+C Gram positive species, such as the spore-formers Bacilli (aerobic) and Clostridia (anaerobic)
  8. Bacteroidota
  9. Balneolota
  10. Bdellovibrionota
  11. Caldisericota, formerly candidate division OP5, Caldisericum exile is the sole representative
  12. Calditrichota
  13. Campylobacterota
  14. Chlamydiota
  15. Chlorobiota, green sulphur bacteria
  16. Chloroflexota, green non-sulphur bacteria
  17. Chrysiogenota, only 3 genera (Chrysiogenes arsenatis, Desulfurispira natronophila, Desulfurispirillum alkaliphilum)
  18. Coprothermobacterota
  19. Deferribacterota
  20. Deinococcota, Deinococcus radiodurans and Thermus aquaticus are "commonly known" species of this phyla
  21. Dictyoglomota
  22. Elusimicrobiota, formerly candidate division Thermite Group 1
  23. Fibrobacterota
  24. Fusobacteriota
  25. Gemmatimonadota
  26. Ignavibacteriota
  27. Kiritimatiellota
  28. Lentisphaerota, formerly clade VadinBE97
  29. Mycoplasmatota, notable genus: Mycoplasma
  30. Myxococcota
  31. Nitrospinota
  32. Nitrospirota
  33. Planctomycetota
  34. Pseudomonadota, the most well-known phylum, containing species such as Escherichia coli or Pseudomonas aeruginosa
  35. Rhodothermota
  36. Spirochaetota, species include Borrelia burgdorferi, which causes Lyme disease
  37. Synergistota
  38. Thermodesulfobacteriota
  39. Thermomicrobiota
  40. Thermotogota, deep-branching
  41. Verrucomicrobiota

Archaea

[edit]
Main article: Archaea

Currently there are 2 phyla that have been validly published according to the Bacteriological Code[73]

  1. Nitrososphaerota
  2. Thermoproteota, second most common archaeal phylum

Other phyla that have been proposed, but not validly named, include:

  1. "Euryarchaeota", most common archaeal phylum
  2. "Korarchaeota"
  3. "Nanoarchaeota", ultra-small symbiotes, single known species

See also

[edit]

Notes

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Phylum

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In biology, a phylum (plural: phyla) is a principal taxonomic rank in the hierarchical classification system, positioned below kingdom and above class, that groups organisms sharing fundamental body plans, developmental patterns, or organizational features derived from common ancestry. This rank reflects hypothesized evolutionary relationships and is essential for organizing biodiversity into nested categories that facilitate scientific communication and study. The taxonomic hierarchy, formalized by in works like (1758), originally included ranks such as kingdom, class, order, , and , but the phylum rank emerged in the early as classifications expanded to accommodate broader patterns of similarity among diverse organisms. Linnaeus's system emphasized for , but higher ranks like phylum were later incorporated to address larger-scale groupings, evolving with advances in , , and, more recently, . In modern usage, the full typically encompasses domain (the highest level, introduced in 1990 to distinguish prokaryotic and eukaryotic life), kingdom, , class, order, , , and , though intermediate and super-ranks may be added for precision. For animals and most other groups, the term "phylum" is standard, while in and , the equivalent rank is often called "division" to denote major plant or fungal lineages based on reproductive or structural traits. Phyla represent diverse evolutionary branches; for example, the animal kingdom (Animalia) comprises over 30 recognized phyla, including Chordata (vertebrates and relatives) and Arthropoda (, crustaceans, and allies), with Arthropoda encompassing over a million . Taxonomic assignments at the phylum level are governed by codes such as the International Code of Zoological Nomenclature (ICZN) for animals and the International Code of Nomenclature for algae, fungi, and plants (ICN) for others, ensuring stability through principles of priority and typification. Contemporary revisions, informed by genomic data, continue to refine phyla boundaries, as seen in the NCBI Taxonomy's 2021 inclusion of formal phylum ranks for prokaryotes to align with bacterial and archaeal diversity, with further updates in 2024 introducing kingdom ranks and in 2025 adding the realm rank while discontinuing superkingdom. This ongoing evolution underscores the phylum's role in bridging broad evolutionary history with detailed organismal classification.

Introduction

Etymology and Origin

The term "phylum" in biological taxonomy originates from the Greek word phylon (φῦλον), meaning "tribe," "race," "stock," or "clan," which Haeckel Latinized as phylum to denote major evolutionary lineages of organisms. Ernst Haeckel introduced the term in his 1866 work Generelle Morphologie der Organismen, where he equated it with the German Stamm (stem or tribe) to represent the highest category in his proposed phylogenetic hierarchy, grouping organisms based on shared ancestry and fundamental body plan similarities. Haeckel's intent was to formalize a natural classification system reflecting evolutionary descent, extending Darwinian principles by using comparative morphology to infer monophyletic groups united by common developmental patterns and structural homologies. Before Haeckel's formal establishment of the phylum rank, early 19th-century extensions of the Linnaean system employed informal higher-level groupings to accommodate broader organizational patterns beyond classes and orders. For instance, introduced the concept of embranchements in , dividing the animal kingdom into four major branches—Vertebrata, , Articulata, and Radiata—based on distinct anatomical and functional types, which served as precursors to the phylum by emphasizing irreducible organizational plans. These pre-Haeckelian approaches, while not phylogenetically oriented, laid groundwork for recognizing large-scale divisions in without rigid rank . Haeckel's initial application of the phylum concept in 1866 included specific proposals for the animal kingdom, such as the phylum Protozoa for unicellular forms, Porifera for sponges, and Coelenterata for radially symmetric, tissue-level organized invertebrates like jellyfish and corals, reflecting his emphasis on progressive complexity from primitive to advanced body plans. These examples drew partly from Cuvier's embranchements and Karl Ernst von Baer's animal types, adapting them into an evolutionary framework where phyla represented ancient stems (phyla) diverging from common ancestors.

Historical Development

The concept of phylum as a major was formally introduced by in his 1866 work Generelle Morphologie der Organismen, where he proposed it as a primary division in his phylogenetic classification system to reflect evolutionary relationships among organisms. In the late 19th century, zoologists such as E. Ray Lankester further refined the phylum rank by emphasizing fundamental body plans and over strict genealogical descent, as seen in his 1877 reclassification of Vertebrata to include craniates, cephalochordates, and urochordates based on shared structural features like the and . This approach allowed for a more systematic grouping of diverse animal forms, prioritizing morphological coherence in higher taxa amid the expanding knowledge of invertebrate diversity. Charles Darwin's (1859) profoundly influenced this development by advocating a natural classification system rooted in , transforming phyla from static, artificial categories into dynamic evolutionary lineages that captured branching patterns of divergence. The 20th century brought debates over phylum boundaries, particularly with the rise of in the 1960s, pioneered by Robert Sokal and Peter Sneath in their 1963 book Principles of Numerical Taxonomy, which applied quantitative phenetic methods to assess overall similarity across characters, challenging subjective morphological judgments and prompting reevaluations of traditional phylum groupings. These methods highlighted inconsistencies in higher-level classifications by automating character scoring and clustering, though they faced criticism for underemphasizing evolutionary history in favor of observable traits. By the 1990s, the cladistic revolution, building on Willi Hennig's earlier principles, increasingly integrated molecular data into phylogenetic analyses, leading to significant phylum reclassifications; for instance, studies using 18S rRNA sequences demonstrated the of the traditional Aschelminthes group, necessitating its division into distinct clades such as . This shift marked a transition toward evidence-based boundaries for phyla, resolving longstanding ambiguities in pseudocoelomate relationships through parsimony-based trees that prioritized shared derived characters.

Taxonomic Framework

Position in Biological Classification

In biological classification, the phylum represents a major taxonomic rank positioned immediately below the kingdom and above the class within the standard Linnaean hierarchy. This hierarchy organizes organisms in a nested sequence: domain, kingdom, phylum, class, order, family, genus, and species, providing a structured framework for cataloging biodiversity based on shared characteristics. The rank was formally incorporated into taxonomic systems during the 19th century to accommodate increasing knowledge of organismal diversity. A phylum functions as an intermediate grouping that unites multiple classes of organisms exhibiting fundamental similarities in or pivotal evolutionary innovations, such as bilateral or segmentation. This level of classification emphasizes broad structural and developmental patterns that distinguish major evolutionary lineages while allowing for variation at lower ranks. In practice, the phylum rank facilitates the systematic arrangement of life's diversity, enabling scientists to identify and study large-scale patterns in and . Usage of the term varies across biological disciplines; in and , "division" serves as the equivalent to phylum, a convention established in the 19th century to align with plant and fungal classification traditions. This nomenclature difference reflects historical separations between zoological and botanical taxonomy, though modern codes of permit both terms interchangeably. Phyla play a vital role in biodiversity organization, with the animal kingdom (Animalia) encompassing approximately 30-40 recognized phyla, each representing distinct body plans like those of arthropods or chordates. In contrast, other groups exhibit fewer phyla; for example, the plant kingdom (Plantae) includes about 12 divisions, highlighting the relative simplicity in vascular and non-vascular plant lineages compared to animal diversity.

Relation to Other Taxonomic Ranks

The phylum rank occupies an intermediate position in the taxonomic hierarchy, situated below the kingdom and above the class. Kingdoms represent broader groupings primarily based on fundamental cellular structure and organization, such as the Animalia kingdom encompassing multicellular, heterotrophic eukaryotes with distinct body plans, while phyla delineate major evolutionary innovations like segmentation or radial symmetry within those kingdoms. In contrast, classes are narrower subdivisions within phyla, often emphasizing variations in organ systems or developmental patterns, such as the Mammalia class within the Chordata phylum, which highlights traits like hair and mammary glands. This positioning allows phyla to capture broad morphological and phylogenetic divergences that kingdoms overlook and classes refine. Phyla exhibit greater stability as "natural" taxonomic groups compared to lower ranks like orders and families, which tend to evolve and revise more rapidly due to finer-scale genetic and morphological variations. Higher ranks such as phylum rely on conserved, fundamental characters like architecture, rendering them less susceptible to frequent reclassification, whereas orders and families often shift with emerging of adaptive radiations or hybridizations. This relative stability has made phyla enduring anchors in biological , though not immune to change. Instances of rank inflation and deflation illustrate the phylum rank's flexibility, particularly in 20th-century revisions driven by improved anatomical and molecular . For example, in worm classifications, the group—thorny-headed parasites initially treated as a class or order within Platyhelminthes—were elevated to full phylum status by the mid-20th century based on unique structures and life cycle differences, reflecting a recognition of their distinct evolutionary lineage. Such elevations contributed to broader inflation at the phylum level, with the number of recognized animal phyla rising from about five in the to around 35 today, often through splitting previously lumped groups. Conversely, has occurred, as seen in annelid worms where separate phyla like and were downgraded to subgroups within in early 21st-century analyses. In cladistic systems, taxonomic ranks including phylum are non-mandatory, as classifications prioritize monophyletic clades defined by shared ancestry rather than rigid hierarchies. Phyla may align with major clades but can vary in scope without enforced ranking, allowing for more dynamic representations of evolutionary relationships as per the framework. This approach underscores the artificiality of ranks while preserving phyla's utility for summarizing deep divergences.

Classification Criteria

Morphological and Anatomical Basis

The morphological and anatomical basis for defining phyla varies by major biological group, relying on shared fundamental body plans or structural features that reflect evolutionary . In and fungi, the equivalent rank is often termed "division," emphasizing reproductive and organizational traits. For prokaryotes, morphological criteria are limited due to cellular simplicity. In animals, this basis emphasizes developmental and architectural features conserved across members of a phylum, distinguishing them from other groups. In plants, divisions are delineated by traits such as the presence of ( and for water/nutrient transport), reproductive structures (e.g., spores in bryophytes vs. seeds in angiosperms), and life cycle patterns including . Non-vascular plants like mosses form one division (Bryophyta), while vascular seedless plants (e.g., ferns, Pteridophyta) differ from seed plants (e.g., gymnosperms, angiosperms) based on ovule enclosure and flower development. In fungi, phyla are defined primarily by reproductive morphology, including spore-producing structures and hyphal organization. For example, feature asci for ascospore production, produce basidiospores on basidia, and earlier groups like rely on motile zoospores, reflecting aquatic adaptations. These traits, combined with septal pore types, unify phyla despite convergent forms. For prokaryotes ( and ), morphological criteria play a secondary role in phylum definition, focusing on basic cellular features like shape (e.g., cocci, rods), cell wall composition (Gram-positive vs. Gram-negative staining), and motility (flagella presence). However, these are insufficient for deep phylogenetic splits and are integrated with molecular data in polyphasic taxonomy. In animals, key features include , tissue layering, and segmentation. Bilateral , where the body divides into mirror-image halves along a single plane, contrasts with radial and enables directed movement and in many phyla. Tissue organization refines boundaries: diploblastic animals possess two primary germ layers ( and ), forming simple body walls, while triploblastic forms add a layer, enabling complex internal structures. Key anatomical innovations serve as phylum-defining traits in animals, such as the presence or absence of a —a fluid-filled lined by providing support, organ movement, and function. Acoelomate plans lack this cavity, yielding solid bodies; pseudocoelomates have a partial, unlined cavity; true coelomates a complete one. Other innovations include circulatory systems—from open ( bathing organs) to closed (vessels)—and skeletal types like chitinous exoskeletons or hydrostatic mechanisms. Segmentation, repeating body units for modularity, delineates phyla as in annelids. These form the bauplan unifying members via . Historically, this approach for animals originated with , whose 1817 Le Règne Animal proposed four archetypes—Vertebrata, , Articulata, Radiata—based on integration and , not superficial traits. Cuvier's method linked structures to lifestyles, establishing phyla as natural groups. This influenced 19th-century by viewing archetypes as adapted forms. Morphological classification limitations include , yielding polyphyletic groups from similar selective pressures. Worm-like forms were once lumped, obscuring relationships in pre-molecular schemes. Challenges integrate phylogenetic , though morphology aids fossils and diversity interpretation.

Phylogenetic and Molecular Basis

In contemporary , phyla are primarily delineated using , defining them as monophyletic clades from a common ancestor and descendants, united by synapomorphies. Pioneered by Willi Hennig, this prioritizes evolutionary relationships over similarities, refining boundaries to exclude paraphyletic groups and align with life's tree. Molecular data revolutionize by quantifying divergences; 16S rRNA sequencing is key for prokaryotes due to universality, conserved structure, and variable regions capturing deep splits. Carl Woese's work revealed phylum-level lineages morphology missed. For eukaryotes, multi-gene phylogenies using protein-coding genes provide resolution, confirming structural synapomorphies genomically. Woese and Fox's 1977 16S rRNA analysis split prokaryotes into and domains, founding phyla and challenging Linnaean views. From the 2010s onward, initiatives reshaped eukaryotes, with Adl et al. revisions (2019, 2024) consolidating protists into monophyletic phyla/supergroups like Opisthokonta via phylogenomics. These highlight ancient divergences predating morphology, stressing integrative criteria. Phylogenies use models like Jukes-Cantor for distances, correcting multiple substitutions: d=−34ln⁡(1−43p)d = -\frac{3}{4} \ln \left(1 - \frac{4}{3} p \right) where pp is differing proportion, assuming equal rates. This is vital for accurate phylum inferences in deep molecular data.

Phyla in Eukaryotic Domains

Animal Phyla

The kingdom Animalia encompasses approximately 35 recognized phyla, reflecting the immense diversity of multicellular, heterotrophic eukaryotes that exhibit and specialized sensory systems. While these phyla span a wide array of body plans and ecological roles, more than 99% of all described animal species—estimated at over 1.5 million—are concentrated in just nine major phyla, including Arthropoda, , and Chordata, which dominate terrestrial, marine, and freshwater habitats due to their adaptive radiations. This uneven distribution underscores how evolutionary innovations in a few lineages have driven the proliferation of animal life, with smaller phyla often representing or specialized groups. The origins of most animal phyla trace back to the , a rapid diversification event approximately 540 million years ago that marked the emergence of complex body plans in the fossil record. During this period, environmental changes such as rising oxygen levels and the evolution of predation likely facilitated the appearance of key morphological traits, including bilateral symmetry and segmentation, setting the stage for the metazoan radiation. Among the basal phyla, Porifera (sponges) stands out as the most primitive, lacking true tissues and organs; instead, they rely on a porous body structure with choanocytes for filter-feeding, comprising about 9,000 primarily in marine environments. Cnidaria, including and corals, represents an early divergence with radial symmetry and cnidocytes—specialized stinging cells for prey capture and defense—organizing cells into two true tissue layers separated by , with around 10,000 mostly aquatic. Higher phyla exhibit more advanced features aligned with bilaterian ancestry. , with over 85,000 species, features soft-bodied animals typically equipped with a muscular foot for locomotion and a mantle for respiration or shell , encompassing diverse forms from snails to octopuses. Arthropoda, the largest phylum with more than 1 million species accounting for over 85% of all known animals, is defined by a chitinous that provides protection and support, paired with jointed appendages enabling versatile movement, feeding, and sensing across , crustaceans, and arachnids. Finally, Chordata, including vertebrates and basal forms like , is characterized by a for structural support, a dorsal hollow cord for centralized nervous control, pharyngeal slits, and a post-anal at some life stage, totaling about 65,000 species with profound ecological and evolutionary impact. Smaller phyla like Phoronida (horseshoe worms), with about 20 species, remain understudied with limited genomic data, highlighting gaps in our understanding of lophotrochozoan diversity, while Bryozoa (moss animals), with approximately 6,000 species, also warrants further study. Recent discoveries, such as the enigmatic , continue to challenge phylum boundaries; this simple worm-like , once misclassified, now anchors the phylum as a basal lineage, prompting revisions to early phylogeny based on its lack of complex organs and contested affinities. These findings emphasize the ongoing refinement of through molecular and evidence.

Plant and Algal Phyla

In botanical taxonomy, the term "division" serves as a synonym for "phylum," reflecting the hierarchical classification of plants and algae within the kingdom Plantae. This kingdom encompasses approximately 12 major divisions, which include both algal lineages and the embryophytes (land plants), characterized by their photosynthetic capabilities and structural adaptations to diverse environments. These divisions are delineated based on reproductive strategies, vascular tissue presence, and pigmentation, with embryophytes representing a monophyletic clade derived from green algal ancestors. Among the key algal divisions, , or , stands out as ancestral to land plants, featuring and b pigments that enable oxygenic similar to that in higher plants. This division includes unicellular, colonial, and multicellular forms, many of which inhabit freshwater and marine habitats, serving as a bridge to terrestrial colonization. Within embryophytes, non-vascular divisions such as Bryophyta (mosses) lack specialized conductive tissues, relying on for water and nutrient transport, and dominate moist environments with simple, upright structures up to several centimeters tall. Vascular but seedless divisions, like Pteridophyta (ferns and allies), introduced and for efficient transport, allowing larger sizes and broader ecological distribution, though reproduction still depends on water for . Seed-producing divisions include gymnosperms, such as Pinophyta (), which bear naked seeds on cones and dominate boreal forests with habits, and angiosperms (flowering plants), the most diverse group with over 300,000 species, featuring enclosed seeds in fruits and for enhanced reproductive efficiency. The evolutionary history of plant and algal phyla traces back to the transition from aquatic around 1 billion years ago, marking the diversification of the streptophyte lineage that eventually gave rise to embryophytes. A pivotal milestone occurred approximately 420 million years ago in the period, when terrestrial vascular emerged, adapting to land through developments like cuticles for water retention and stomata for . This shift enabled the conquest of terrestrial habitats, transforming global ecosystems by increasing oxygen levels and stabilizing soils. A hallmark of embryophyte phyla is the alternation of generations life cycle, where a multicellular diploid phase alternates with a haploid phase, an innovation absent in most algal ancestors and defining the clade's reproductive strategy. This diplohaplontic cycle supports adaptation to variable conditions, with the becoming dominant in vascular . Complementing this, algal phyla like (red algae) exhibit unique pigmentation, including , a red that absorbs blue-green light, allowing in deeper marine waters up to 200 meters. , primarily marine and multicellular, contribute significantly to formation and global carbon fixation through their calcifying structures.

Fungal and Protist Phyla

The kingdom Fungi encompasses more than 15 recognized phyla, reflecting its evolutionary diversity as a monophyletic group of eukaryotic organisms primarily characterized by , chitinous cell walls, and filamentous or yeast-like growth forms. Major phyla include , known as sac fungi for their reproductive structures and encompassing yeasts like used in , as well as lichens and plant pathogens; , or club fungi, featuring basidia and including macroscopic mushrooms such as and rusts that impact agriculture; Mucoromycota, conjugation fungi that reproduce via zygospores, exemplified by bread molds like ; and Glomeromycota, which form arbuscular mycorrhizal symbioses with plant , enhancing uptake in over 80% of land . These phyla play crucial ecological roles as decomposers of , mutualistic partners in ecosystems, and opportunistic pathogens affecting humans, animals, and crops. A basal lineage within Fungi is Chytridiomycota, distinguished by its production of flagellated zoospores, a primitive trait linking it to early fungal evolution and contrasting with the non-motile spores of higher fungi. Fungi as a whole belong to the supergroup Opisthokonta, positioned phylogenetically as the sister clade to animals (Metazoa), sharing a common ancestor with posterior flagella in some life stages and supporting their divergence around 1 billion years ago based on molecular clock estimates. Protists represent a paraphyletic assemblage of predominantly unicellular or colonial eukaryotic microorganisms that do not fit into the kingdoms Animalia, Plantae, or Fungi, accounting for the majority of eukaryotic phylogenetic diversity through their polyphyletic nature and inclusion of lineages like , , and . This group comprises roughly 50-60 phyla or major clades, often classified into supergroups based on molecular and ultrastructural traits such as mechanisms (e.g., , flagella, or cilia) and specialized organelles; notable examples include , featuring amoeba-like cells with lobe-shaped such as and the social Dictyostelium discoideum; , a diverse supergroup with excavated feeding grooves and including parasitic flagellates like Giardia lamblia causing ; and the SAR clade (Stramenopiles, , ), which encompasses Chromalveolata subgroups such as diatoms (Bacillariophyta) with silica frustules that form vast blooms contributing to global carbon cycling, and , non-motile parasites like responsible for , infecting over 200 million people annually. Protists exhibit varied ecological roles, from primary producers in aquatic food webs to symbionts and pathogens influencing disease dynamics and . As a "junk drawer" category in traditional , protists highlight the challenges of eukaryotic , with ongoing phylogenomic studies refining their boundaries.

Phyla in Prokaryotic Domains

Bacterial Phyla

The domain Bacteria encompasses a vast array of prokaryotic microorganisms, with approximately 30 to 50 formally recognized phyla based on cultivated and validated taxa, though metagenomic analyses suggest up to 169 distinct phyla when including uncultured lineages. Among these, the phyla Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes dominate in terms of described species diversity and ecological prevalence, accounting for the majority of known bacterial taxa across diverse habitats from to microbiomes. These phyla exhibit remarkable metabolic versatility, enabling to thrive in extreme environments and drive global biogeochemical cycles. A defining feature of many is the distinction between Gram-positive and Gram-negative cell wall architectures, which influences their interactions with environments and hosts. Gram-positive phyla such as Firmicutes and Actinobacteria possess thick layers, providing structural rigidity and resistance to certain stresses, while Gram-negative phyla like Proteobacteria and Bacteroidetes feature outer membranes rich in lipopolysaccharides that confer protection against antibiotics and host defenses. Metabolic diversity further underscores bacterial adaptations: for instance, the phylum specializes in oxygenic phototrophy, using chlorophyll-based to produce oxygen and , a process central to Earth's oxygenation history. In contrast, Nitrospirae exemplify chemolithotrophy, oxidizing to nitrate in a key step of the , often in aquatic and soil ecosystems. Pathogenic potential is evident in phyla like Spirochaetes, where genera such as (e.g., ) cause diseases including through tissue invasion and immune evasion strategies. Bacterial phyla trace their evolutionary roots to approximately 3.5 billion years ago, with fossil evidence of microbial mats indicating early diversification in environments. This ancient lineage has facilitated critical symbioses, such as those involving the order Rhizobiales within Proteobacteria, which form root nodules in to fix atmospheric , enhancing growth and worldwide. Such interactions highlight bacteria's role in stability and . Advancements in have revealed candidate phyla, including Candidatus Omnitrophica (now classified under Omnitrophota), which are uncultured lineages detected in anaerobic habitats and exhibit dependencies on host interactions for nutrient acquisition. Additionally, unique staining properties distinguish certain phyla; for example, many Actinobacteria, including species, display due to mycolic acids in their cell walls, aiding identification in clinical diagnostics for . These features collectively illustrate the of , shaping microbial contributions to and disease.

Archaeal Phyla

The domain encompasses approximately 20 recognized phyla, with recent classifications in the Genome Taxonomy Database (GTDB) reporting between 18 and 20 phyla based on genome phylogenies. These phyla are dominated by Halobacteriota (traditionally Euryarchaeota), which includes methanogenic and halophilic lineages adapted to anaerobic and hypersaline environments; (traditionally Crenarchaeota), comprising hyperthermophilic species prevalent in geothermal settings; and emerging groups such as Asgardarchaeota, notable for encoding genes resembling those involved in eukaryotic cellular processes like remodeling and . The vast majority of archaeal lineages remain uncultured—part of the over 99% of environmental prokaryotic diversity that is uncultured—and are characterized primarily through and single-amplified genomes, revealing their ecological roles in uncultivable niches. Archaea possess distinct biochemical traits that set them apart from , including ether-linked membrane lipids formed by isoprenoid chains bound to a glycerol-1-phosphate backbone, which confer enhanced stability against extreme temperatures, , and compared to the ester-linked fatty acids in other domains. Their multisubunit exhibits structural and functional homology to eukaryotic , sharing core subunits and promoter recognition mechanisms that enable complex transcription regulation akin to eukaryotes, rather than the simpler bacterial sigma-factor system. These features underscore archaea's intermediate position in prokaryotic-eukaryotic , with phyla delineated via 16S rRNA sequences and concatenated protein phylogenies for robust taxonomic assignment. Metabolic diversity among archaeal phyla supports their adaptation to niche environments, with —a pathway exclusive to —prevalent in Halobacteriota, where CO₂ and H₂ are converted to via unique cofactors like coenzyme M and methanofuran, contributing to global carbon cycling in anaerobic habitats. For instance, the class Methanobacteria within Halobacteriota performs hydrogenotrophic , generating energy through this catabolic process in sediments and guts. Halophiles in the same phylum accumulate compatible solutes like to maintain cellular integrity in salt-saturated conditions, while members, such as those in the order Sulfolobales, oxidize compounds at temperatures exceeding 80°C in volcanic pools. Archaea represent the closest prokaryotic relatives to eukaryotes, with Asgardarchaeota emerging as a key phylum harboring over 70 eukaryotic signature proteins involved in vesicle trafficking and cytoskeletal dynamics, suggesting an archaeal ancestor for the eukaryotic host cell. The 2015 discovery of Lokiarchaeota, integrated into Asgardarchaeota, provided genomic evidence for this proximity, including actin-like and ubiquitin-related genes that bolster endosymbiotic models for , where an archaeal cell engulfed an alphaproteobacterium to form the . Notably, Nitrososphaerota (formerly Thaumarchaeota) uniquely drive aerobic ammonia oxidation in oceanic water columns, influencing marine nitrogen budgets through the enzyme ammonia monooxygenase.

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