Animal
| Animals Temporal range: Cryogenian – present,
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|---|---|
  | |
Scientific classification 
| |
| Domain: | Eukaryota |
| (unranked): | Amorphea |
| (unranked): | Obazoa |
| (unranked): | Opisthokonta |
| (unranked): | Holozoa |
| (unranked): | Filozoa |
| Kingdom: | Animalia Linnaeus, 1758 |
| Phyla (in bold)[4] | |
| |
| Synonyms | |
Animals are
Most living animal species are in
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 for 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.
Etymology
The word "animal" comes from the Latin animalis, meaning 'having breath', 'having soul' or 'living being'.[8] The biological definition includes all members of the kingdom Animalia.[9] In colloquial usage, the term animal is often used to refer only to nonhuman animals.[10][11][12][13] The term "metazoa" is derived from the Ancient Greek μετα (meta, meaning "later") and ζῷᾰ (zōia, plural of ζῷον zōion, meaning animal).[14][15]
Characteristics
Animals have several characteristics that set them apart from other living things. Animals are
Structure
All animals are composed of cells, surrounded by a characteristic
With few exceptions—in particular, the sponges and
Reproduction and development
.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.[39][40] Animals have evolved numerous mechanisms for avoiding close inbreeding.[41]
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.[42][43]
Ecology
Animals are categorised into
mussels and shrimpsMost 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.[53][54][55] 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).[56]
Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as
Diversity
Size
The
Numbers and habitats
The following table lists estimated numbers of described extant species for all the animal groups,[70] along with their principal habitats (terrestrial, fresh water,[71] and marine),[72] and free-living or parasitic ways of life.[73] 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.[74] 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.[75][76][b]
| Phylum | Example | Described species | Land | Sea
|
Freshwater | Free-living | Parasitic
|
|---|---|---|---|---|---|---|---|
Arthropoda
|
 |
1,257,000[70] | 1,000,000 (insects)[78] |
>40,000 (Malac- ostraca)[79] |
94,000[71] | Yes[72] | >45,000[c][73] |
| Mollusca |  |
85,000[70] 107,000[80] |
35,000[80] | 60,000[80] | 5,000[71] 12,000[80] |
Yes[72] | >5,600[73] |
Chordata
|
 |
>70,000[70][81] | 23,000[82] | 13,000[82] | 18,000[71] 9,000[82] |
Yes | 40 (catfish)[83][73] |
Platyhelminthes
|
 |
29,500[70] | Yes[84] | Yes[72] | 1,300[71] | Yes[72] 3,000–6,500[85] |
>40,000[73] 4,000–25,000[85] |
Nematoda
|
 |
25,000[70] | Yes (soil)[72] | 4,000[74] | 2,000[71] | 11,000[74] | 14,000[74] |
Annelida
|
 |
17,000[70] | Yes (soil)[72] | Yes[72] | 1,750[71] | Yes | 400[73] |
| Cnidaria |  |
16,000[70] | Yes[72] | Yes (few)[72] | Yes[72] | >1,350 (Myxozoa)[73] | |
Porifera
|
 |
10,800[70] | Yes[72] | 200–300[71] | Yes | Yes[86] | |
Echinodermata
|
 |
7,500[70] | 7,500[70] | Yes[72] | |||
| Bryozoa |  |
6,000[70] | Yes[72] | 60–80[71] | Yes | ||
Rotifera
|
 |
2,000[70] | >400[87] | 2,000[71] | Yes | ||
| Nemertea |  |
1,350[88][89] | Yes | Yes | Yes | ||
Tardigrada
|
 |
1,335[70] | Yes[90] (moist plants) |
Yes | Yes | Yes | |
Gastrotricha
|
 |
794[70] | Yes[90] | Yes | Yes | ||
| Xenacoelomorpha |  |
430[70] | Yes[90] | Yes | |||
| Nematomorpha |  |
354[70] | Yes (moist places)[90] |
Yes (one genus)[91] |
Yes | Yes (as adults)[90] |
Yes (as juveniles)[90] |
Brachiopoda
|
 |
396[70] (30,000 extinct)[90] |
Yes[90] | Yes | |||
| Kinorhyncha |  |
196[70] | Yes (mud)[90] | Yes | |||
| Ctenophora |  |
187[70] | Yes[90] | Yes | |||
| Onychophora |  |
187[70] | Yes[90] | Yes | |||
| Chaetognatha |  |
186[70] | Yes[90] | Yes | |||
| Entoprocta |  |
172[70] | Yes[90] | Yes |
Yes | ||
Hemichordata
|
 |
126[70] | Yes[90] | Yes | |||
Rhombozoa
|
 |
107[70] | Yes | ||||
Gnathostomulida
|
 |
97[70] | Yes (sand)[90] | Yes | |||
| Loricifera |  |
30[70] | Yes (sand)[90] | Yes | |||
| Orthonectida |  |
29[70] | Yes | ||||
| Priapulida |  |
20[70] | Yes[90] | Yes | |||
Phoronida
|
 |
16[70] | Yes[90] | Yes | |||
| Placozoa |  |
4[92] | Yes[90] | Yes | |||
Cycliophora
|
 |
2[93] | Yes[93] | Yes[94][93] | |||
Micrognathozoa
|
 |
One[90] | Yes (sand)[90] | Yes | |||
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,[95][96][97] but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature.[98] 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.[99]
Many animal phyla first appear in the
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.
![Dickinsonia costata from the Ediacaran biota (c. 635–542 mya) is one of the earliest animal species known.[98]](//upload.wikimedia.org/wikipedia/commons/thumb/f/fb/DickinsoniaCostata.jpg/320px-DickinsoniaCostata.jpg)
Dickinsonia costata from the Ediacaran biota (c. 635–542 mya) is one of the earliest animal species known.[98]
![Auroralumina attenboroughii, an Ediacaran predator (c. 560 mya)[105]](//upload.wikimedia.org/wikipedia/commons/thumb/6/63/Auroralumina_attenboroughii_reconstruction.jpg/209px-Auroralumina_attenboroughii_reconstruction.jpg)
Auroralumina attenboroughii, an Ediacaran predator (c. 560 mya)[105]
Burgess shale.
![The Francevillian biota is thought to be one of the earliest forms of Eukaryote known (2.1 ga). These representatives of some of the earliest life on earth are found within the Francevillian B Formation within Gabon, the formation being a Black shale.[114][115]](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Gabonionta_I.jpg/195px-Gabonionta_I.jpg)
![Dickinsonia costata from the Ediacaran biota (c. 635–542 mya) is one of the earliest animal species known.[98]](http://upload.wikimedia.org/wikipedia/commons/thumb/f/fb/DickinsoniaCostata.jpg/320px-DickinsoniaCostata.jpg)
![Auroralumina attenboroughii, an Ediacaran predator (c. 560 mya)[105]](http://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Auroralumina_attenboroughii_reconstruction.jpg/209px-Auroralumina_attenboroughii_reconstruction.jpg)
Phylogeny
Animals are
These genes are found in the Placozoa
The phylogenetic tree indicates approximately how many millions of years ago (mya) the lineages split.[125][126][127][128][129][4]
| Choanozoa |
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| 950 mya |
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Non-bilateria
Several animal phyla lack bilateral symmetry. Among these, the sponges (Porifera) probably diverged first, representing the oldest animal phylum.[130] Sponges lack the complex organization found in most other animal phyla;[131] their cells are differentiated, but in most cases not organised into distinct tissues.[132] They typically feed by drawing in water through pores.[133]
The Ctenophora (comb jellies) and Cnidaria (which includes
Bilateria
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.[139][140]
Having a front end means that this part of the body encounters stimuli, such as food, favouring
Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, the
Protostomes and deuterostomes
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.[146] 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.[147][148] 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.[149]
The main deuterostome phyla are the Echinodermata and the Chordata.
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