Animal

Animals Temporal range: Cryogenian – present, 665–0 Ma Pha. Proterozoic Archean Had.
Scientific classification
Domain:	Eukaryota
Clade:	Amorphea
Clade:	Obazoa
(unranked):	Opisthokonta
(unranked):	Holozoa
(unranked):	Filozoa
Kingdom:	Animalia Linnaeus, 1758
Subdivisions
Bilateria (~30 phyla) Cnidaria Ctenophora Placozoa Porifera
Synonyms
Metazoa Haeckel 1874[1] Choanoblastaea Nielsen 2008[2] Gastrobionta Rothm. 1948[3] Zooaea Barkley 1939[3] Euanimalia Barkley 1939[3]

Animals are

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), 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 since the earliest times and are featured in mythology and religion.

Etymology

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]

Characteristics

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

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

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]

Ecology

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

The

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	Sea	Freshwater	Free-living	Parasitic
Arthropoda		1,257,000[66]	1,000,000 (insects)[74]	>40,000 (Malac- ostraca)[75]	94,000[67]	Yes[68]	>45,000[c][69]
Mollusca		85,000[66] 107,000[76]	35,000[76]	60,000[76]	5,000[67] 12,000[76]	Yes[68]	>5,600[69]
Chordata		>70,000[66][77]	23,000[78]	13,000[78]	18,000[67] 9,000[78]	Yes	40 (catfish)[79][69]
Platyhelminthes		29,500[66]	Yes[80]	Yes[68]	1,300[67]	Yes[68] 3,000–6,500[81]	>40,000[69] 4,000–25,000[81]
Nematoda		25,000[66]	Yes (soil)[68]	4,000[70]	2,000[67]	11,000[70]	14,000[70]
Annelida		17,000[66]	Yes (soil)[68]	Yes[68]	1,750[67]	Yes	400[69]
Cnidaria		16,000[66]		Yes[68]	Yes (few)[68]	Yes[68]	>1,350 (Myxozoa)[69]
Porifera		10,800[66]		Yes[68]	200–300[67]	Yes	Yes[82]
Echinodermata		7,500[66]		7,500[66]		Yes[68]
Bryozoa		6,000[66]		Yes[68]	60–80[67]	Yes
Rotifera		2,000[66]		>400[83]	2,000[67]	Yes
Nemertea		1,350[84][85]		Yes	Yes	Yes
Tardigrada		1,335[66]	Yes[86] (moist plants)	Yes	Yes	Yes
Total number of described extant species as of 2013[update]: 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

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.

sponges.^[99]

triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms.^[100] However, similar tracks are produced 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]

• 

  Dickinsonia costata from the Ediacaran biota (c. 635–542 mya) is one of the earliest animal species known.^[90]

• 

  Auroralumina attenboroughii, an Ediacaran predator (c. 560 mya)^[97]

• 

  Burgess shale

  .

Phylogeny

Further information: Lists of animals

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]

Opisthokonta	Holomycota (inc. fungi) Holozoa Ichthyosporea Pluriformea Filozoa Filasterea Choanozoa Choanoflagellatea Animalia 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

homeodomain proteins involved in the control of development.^[119][120]

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]

Animalia	Porifera Ctenophora ParaHoxozoa Placozoa Cnidaria Bilateria Xenacoelomorpha Nephrozoa Deuterostomia Ambulacraria Chordata Protostomia Ecdysozoa Spiralia blastopore  mouth symm.  embryo hox genes
multicellular

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]

Non-bilateria

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]

Bilateria

Main articles:

Symmetry (biology) § Bilateral symmetry

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

Further information: Embryological origins of the mouth and anus

Main articles: Protostome and Deuterostome

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]

Ecdysozoa

Main article: Ecdysozoa

arthropods, its body is divided into segments

.

The Ecdysozoa are protostomes, named after their shared

Tardigrada, are close relatives of the arthropods and share these traits. The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water;^[153] some are important parasites.^[154] Smaller phyla related to them are the Nematomorpha or horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.^[155]

Spiralia

Main article: Spiralia

Spiral cleavage

in a sea snail embryo

The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo.

Rouphozoa which includes the gastrotrichs and the flatworms. All of these are grouped as the Platytrochozoa, which has a sister group, the Gnathifera, which includes the rotifers.^[157][158]

The Lophotrochozoa includes the

lugworms, and leeches. These two groups have long been considered close relatives because they share trochophore larvae.^[161][162]

History of classification

Further information:

History of zoology since 1859

Jean-Baptiste de Lamarck led the creation of a modern classification of invertebrates, breaking up Linnaeus's "Vermes" into 9 phyla by 1809.^[163]

In the

arranged on a scale from man (with blood, 2 legs, rational soul) down through the live-bearing tetrapods (with blood, 4 legs, sensitive soul) and other groups such as crustaceans (no blood, many legs, sensitive soul) down to spontaneously generating creatures like sponges (no blood, no legs, vegetable soul). Aristotle was uncertain whether sponges were animals, which in his system ought to have sensation, appetite, and locomotion, or plants, which did not: he knew that sponges could sense touch, and would contract if about to be pulled off their rocks, but that they were rooted like plants and never moved about.^[164]

In 1758,

cirripedes, annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and infusorians.^[163]

In his 1817

zoophytes (radiata) (echinoderms, cnidaria and other forms).^[167] This division into four was followed by the embryologist Karl Ernst von Baer in 1828, the zoologist Louis Agassiz in 1857, and the comparative anatomist Richard Owen in 1860.^[168]

In 1874,

Protista, leaving only the Metazoa as a synonym of Animalia.^[170]

In human culture

Practical uses

Main article:

Animals in culture

The human population exploits a large number of other animal species for food, both of

domesticated livestock species in animal husbandry and, mainly at sea, by hunting wild species.^[171][172] Marine fish of many species are caught commercially for food. A smaller number of species are farmed commercially.^{[171][173][174]} Humans and their livestock make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined.^[175]

Invertebrates including

Working animals including cattle and horses have been used for work and transport from the first days of agriculture.^[186]

Animals such as the fruit fly Drosophila melanogaster serve a major role in science as experimental models.^{[187][188][189][190]} Animals have been used to create vaccines since their discovery in the 18th century.^[191] Some medicines such as the cancer drug trabectedin are based on toxins or other molecules of animal origin.^[192]

People have used

blowpipe darts.^[196][197]

A wide variety of animals are kept as pets, from invertebrates such as tarantulas and octopuses, insects including

praying mantises,^[198] reptiles such as snakes and chameleons,^[199] and birds including canaries, parakeets, and parrots^[200] all finding a place. However, the most kept pet species are mammals, namely dogs, cats, and rabbits.^{[201][202][203]} There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own.^[204]

A wide variety of terrestrial and aquatic animals are hunted for sport.^[205]

Symbolic uses

Still Life with Lobster and Oysters by Alexander Coosemans

, c. 1660

Animals have been the

giant bug movies.^{[208][209][210]}

Animals including

signs of the Western and Chinese zodiacs are based on animals.^[222][223]

See also

• Animal coloration
• Ethology
• Fauna
• List of animal names
• Lists of organisms by population

Notes

1. ^ Henneguya zschokkei does not have mitochondrial DNA or utilize aerobic respiration.^[17]
2. ^ The application of DNA barcoding to taxonomy further complicates this; a 2016 barcoding analysis estimated a total count of nearly 100,000 insect species for Canada alone, and extrapolated that the global insect fauna must be in excess of 10 million species, of which nearly 2 million are in a single fly family known as gall midges (Cecidomyiidae).^[73]
3. ^ Not including parasitoids.^[69]
4. ^ Compare File:Annelid redone w white background.svg for a more specific and detailed model of a particular phylum with this general body plan.
5. ^ In his History of Animals and Parts of Animals.
6. ^ The prefix une espèce de is pejorative.^[166]

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External links

Wikimedia Commons has media related to Animals.

Wikispecies has information related to Animalia.

• Tree of Life Project Archived 12 June 2011 at the Wayback Machine
• Animal Diversity Web – University of Michigan's database of animals
• Wildscreen Arkive – multimedia database of endangered/protected species