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Temporal range: Cryogenian – present,
EchinodermCnidariaTardigradeCrustaceanArachnidSpongeInsectBryozoaAcanthocephalaFlatwormMolluscaAnnelidVertebrateTunicatePhoronidaAnimal diversity b.png
About this image
Scientific classification e
Domain: Eukaryota
Clade: Amorphea
Clade: Obazoa
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Linnaeus, 1758
  • Metazoa Haeckel 1874[1]
  • Choanoblastaea Nielsen 2008[2]
  • Gastrobionta Rothm. 1948[3]
  • Zooaea Barkley 1939[3]
  • Euanimalia Barkley 1939[3]

Animals are

insects, over 85,000 are molluscs, and around 65,000 are vertebrates—but it has been estimated there are around 7.77 million animal species in total. Animals range in length from 8.5 micrometres (0.00033 in) to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The scientific study of animals is known as zoology

Most living animal species are in

fossil record as marine species during the Cambrian explosion, which began around 539 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago

Historically, Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between taxa.

Humans make use of many animal species, such as for food (including meat, milk, and eggs), for materials (such as leather and wool), as pets, and as working animals including for transport. Dogs have been used in hunting, as have birds of prey, while many terrestrial and aquatic animals were hunted for sports. Nonhuman animals have appeared in art from the earliest times and are featured in mythology and religion.


The word "animal" comes from the Latin animalis, meaning 'having breath', 'having soul' or 'living being'.[4] The biological definition includes all members of the kingdom Animalia.[5] In colloquial usage, the term animal is often used to refer only to nonhuman animals.[6][7][8][9] The term "metazoa" is derived from the Ancient Greek μετα (meta, meaning "later") and ζῷᾰ (zōia, plural of ζῷον zōion, meaning animal).[10][11]



Animals have several characteristics that set them apart from other living things. Animals are

embryonic development that is unique to animals, allowing cells to be differentiated into specialised tissues and organs.[20]


All animals are composed of cells, surrounded by a characteristic

spicules.[22] In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth.[23] Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.[24]

With few exceptions—in particular, the sponges and

nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).[26]

Reproduction and development

Sexual reproduction is nearly universal in animals, such as these dragonflies

Nearly all animals make use of some form of sexual reproduction.

gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm.[32] In most cases, a third germ layer, the mesoderm, also develops between them.[33] These germ layers then differentiate to form tissues and organs.[34]

Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits.[35][36] Animals have evolved numerous mechanisms for avoiding close inbreeding.[37]

Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in Hydra and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.[38][39]


Animals are categorised into

Most animals rely on the biomass and energy produced by plants through photosynthesis. Herbivores eat plant material directly, while carnivores, and other animals on higher trophic levels typically acquire it indirectly by eating other animals. Animals oxidize carbohydrates, lipids, proteins, and other biomolecules, which allows the animal to grow and to sustain biological processes such as locomotion.[49][50][51] Animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter of archaea and bacteria produced in these locations through chemosynthesis (by oxidizing inorganic compounds, such as hydrogen sulfide).[52]

Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as

nematodes) inhabit the most extreme cold deserts of continental Antarctica.[58]




titanosaur sauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 73 tonnes, and Supersaurus which may have reached 39 meters.[62][63] Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 µm,[64] and one of the smallest species (Myxobolus shekel) is no more than 8.5 µm when fully grown.[65]

Numbers and habitats of major phyla

The following table lists estimated numbers of described extant species for the major animal phyla,[66] along with their principal habitats (terrestrial, fresh water,[67] and marine),[68] and free-living or parasitic ways of life.[69] Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. 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.[70] Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.[71][72][b]

Phylum Example Described species Land
Freshwater Free-living
1,257,000[66] 1,000,000
94,000[67] Yes[68] >45,000[c][69]
35,000[76] 60,000[76] 5,000[67]
Yes[68] >5,600[69]
green spotted frog facing right
>70,000[66][77] 23,000[78] 13,000[78] 18,000[67]
Yes 40
Pseudoceros dimidiatus.jpg
29,500[66] Yes[80] Yes[68] 1,300[67] Yes[68]




25,000[66] Yes (soil)[68] 4,000[70] 2,000[67] 11,000[70] 14,000[70]
17,000[66] Yes (soil)[68] Yes[68] 1,750[67] Yes 400[69]
Table coral
16,000[66] Yes[68] Yes (few)[68] Yes[68] >1,350
A colourful Sponge on the Fathom.jpg
10,800[66] Yes[68] 200–300[67] Yes Yes[82]
Starfish, Caswell Bay - - 409413.jpg
7,500[66] 7,500[66] Yes[68]
Bryozoan at Ponta do Ouro, Mozambique (6654415783).jpg
6,000[66] Yes[68] 60–80[67] Yes
20090730 020239 Rotifer.jpg
2,000[66] >400[83] 2,000[67] Yes
1,350[84][85] Yes Yes Yes
Tardigrade (50594282802).jpg
1,335[66] Yes[86]
(moist plants)
Yes Yes Yes
Total number of described extant species as of 2013: 1,525,728[66]

Evolutionary origin

Animals are found as long ago as the Ediacaran biota, towards the end of the Precambrian, and possibly somewhat earlier. It had long been doubted whether these life-forms included animals,[87][88][89] but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature.[90] Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration, but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments.[91]

Many animal phyla first appear in the

predatory Anomalocaris. The apparent suddenness of the event may however be an artefact of the fossil record, rather than showing that all these animals appeared simultaneously.[93][94][95][96]
That view is supported by the discovery of
nematocysts as modern cnidarians do.[97]

Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.

triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms.[100] However, similar tracks are produced today by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution.[101][102] Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals.[103] Objects such as sediment-filled tubes that resemble trace fossils of the burrows of wormlike animals have been found in 1.2 gya rocks in North America, in 1.5 gya rocks in Australia and North America, and in 1.7 gya rocks in Australia. Their interpretation as having an animal origin is disputed, as they might be water-escape or other structures.[104][105]


External phylogeny

Animals are

Choanoflagellata, with which they form the Choanozoa.[106]
The dates on the phylogenetic tree indicate approximately how many millions of years ago (mya) the lineages split.[107][108][109][110][111]

Ros-Rocher and colleagues (2021) trace the origins of animals to unicellular ancestors, providing the external phylogeny shown in the cladogram. Uncertainty of relationships is indicated with dashed lines.[112]




Ichthyosporea Abeoforma whisleri-2.jpg

Corallochytrium limacisporum.png


Ministeria vibrans.jpeg


Desmarella moniliformis.jpg


Polychaeta (no) 2.jpg

760 mya
950 mya
1100 mya
1300 mya

Internal phylogeny

The most

Porifera, Ctenophora, Cnidaria, and Placozoa, have body plans that lack bilateral symmetry. Their relationships are still disputed; the sister group to all other animals could be the Porifera or the Ctenophora,[113] both of which lack hox genes, important in body plan development.[114]

These genes are found in the Placozoa

Giribet and Edgecombe (2020) provide what they consider to be a consensus internal phylogeny of the animals, embodying uncertainty about the structure at the base of the tree (dashed lines).[121]


Porifera Estonian Museum of Natural History - Sponge.png

Mnemiopsis leidyi 247259012.png


Trichoplax adhaerens photograph (no background).png

Jellyfish, Shaw Ocean Discovery Centre (7201323966).png


Proporus sp. (no background).png


Echinaster serpentarius (USNM E28192) 001.png

Chordata Cyprin carpi 090613-0329 tdp.png


Aptostichus simus Monterey County.jpg

Grapevinesnail 01a.jpg

hox genes

An alternative phylogeny, from Kapli and colleagues (2021), proposes a clade Xenambulacraria for the Xenacoelamorpha + Ambulacraria; this is either within Deuterostomia, as sister to Chordata, or the Deuterostomia are recovered as paraphyletic, and Xenambulacraria is sister to the proposed clade Centroneuralia, consisting of Chordata + Protostomia.[122]


Several animal phyla lack bilateral symmetry. These are the

Porifera (sea sponges), Placozoa, Cnidaria (which includes jellyfish, sea anemones, and corals), and Ctenophora
(comb jellies).

Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing the oldest animal phylum and forming a

sister clade to all other animals.[123] Despite their morphological dissimilarity with all other animals, genetic evidence suggests sponges may be more closely related to other animals than the comb jellies are.[124][125] Sponges lack the complex organization found in most other animal phyla;[126] their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals.[127] They typically feed by drawing in water through pores, filtering out food and nutrients.[128]

The comb jellies and Cnidaria are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus.

diploblastic, having only two main germ layers, ectoderm and endoderm.[131]

The tiny placozoans have no permanent digestive chamber and no symmetry; they superficially resemble amoebae.[132][133] Their phylogeny is poorly defined, and under active research.[124][134]


basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion

The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a clade, the Bilateria, which have a bilaterally symmetric body plan. The Bilateria are triploblastic, with three well-developed germ layers, and their tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is an internal body cavity, a coelom or pseudocoelom. These animals have a head end (anterior) and a tail end (posterior), a back (dorsal) surface and a belly (ventral) surface, and a left and a right side.[135][136]

Having a front end means that this part of the body encounters stimuli, such as food, favouring

parasitic worms have extremely simplified body structures.[135][136]

Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, the

deuterostomes.[138] It is often suggested that the basalmost bilaterians are the Xenacoelomorpha, with all other bilaterians belonging to the subclade Nephrozoa[139][140][141] However, this suggestion has been contested, with other studies finding that xenacoelomorphs are more closely related to Ambulacraria than to other bilaterians.[122]

Protostomes and deuterostomes

The bilaterian gut develops in two ways. In many protostomes, the blastopore develops into the mouth, while in deuterostomes
it becomes the anus.

Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage.[142] Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily.[143][144] Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm.[145]

The main deuterostome phyla are the Echinodermata and the Chordata.

Hemichordata (acorn worms).[150][151]

arthropods, its body is divided into segments