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Animals are multicellular, eukaryotic in the Biology kingdom Animalia (). With few exceptions, animals heterotroph, breathe oxygen, have and are motility, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals form a clade, meaning that they arose from a single common ancestor. Over 1.5 million extant taxon animal species have been described, of which around 1.05 million are , over 85,000 are , and around 65,000 are . It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from to . They have complex ecologies and interactions with each other and their environments, forming intricate . The scientific study of animals is known as zoology, and the study of animal behaviour is known as ethology.
The animal kingdom is divided into five major clades, namely Porifera, Ctenophora, Placozoa, Cnidaria and Bilateria. Most living animal species belong to the clade Bilateria, a highly proliferative clade whose members have a bilaterally symmetric and significantly cephalization body plan, and the vast majority of bilaterians belong to two large clades: the , which includes organisms such as , , , and ; and the , which include , and , the latter of which contains the . The much smaller basal phylum Xenacoelomorpha have an uncertain position within Bilateria.
Animals first appeared in the fossil record in the late Cryogenian period and diversified in the subsequent Ediacaran period in what is known as the Avalon explosion. Earlier evidence of animals is still controversial; the sponge-like organism Otavia has been dated back to the Tonian period at the start of the Neoproterozoic, but its identity as an animal is heavily contested. Nearly all modern animal phyla first appeared in the fossil record as marine species during the Cambrian explosion, which began around 539 million years ago (Mya), and most classes during the Ordovician radiation 485.4 Mya. Common to all living animals, 6,331 groups of have been identified that may have arisen from a single common ancestor that lived about 650 Mya during the Cryogenian period.
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 relationships between taxa.
make use of many other animal species for Human food (including meat, eggs, and ), for animal product (such as leather, fur, and wool), as and as for transportation, and services. , the first animal, have been used hunting dog, guard dog and in warfare, as have equestrianism, pigeon post and falconry; while other terrestrial and are for sports, trophies or profits. Non-human animals are also an important cultural element of human evolution, having appeared in and since the earliest times, and are frequently featured in mythology, religion, , literature, heraldry, politics, and .
Typically, there is an internal Digestion chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians). (1983). 9780521270281, Cambridge University Press Archive. ISBN 9780521270281
During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised into specialised tissues and organs, making the formation of complex structures possible, and allowing cells to be differentiated. The extracellular matrix may be calcified, forming structures such as Exoskeleton, , and spicules. 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.
Repeated instances of inbreeding during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. (1987). 9780120176243 ISBN 9780120176243 Animals have evolved numerous mechanisms for avoiding close inbreeding.
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 ; or parthenogenesis, where fertile eggs are produced without mating, such as in . (2002). 9780471489689, Wiley. ISBN 9780471489689
Most animals rely on biomass and bioenergy produced by and (collectively called producers) through photosynthesis. Herbivores, as primary consumers, eat the plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidise , , and other biomolecules, which allows the animal to grow and to sustain basal metabolism and fuel other biological processes such as locomotion. Some benthic animals living close to hydrothermal vents and on the dark sea floor consume organic matter produced through chemosynthesis (via oxidising inorganic compounds such as hydrogen sulfide) by archaea and bacteria. (2025). 9780077221249, McGraw Hill. ISBN 9780077221249
Animals evolved in the sea. Lineages of arthropods colonised land around the same time as , probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician. such as the lobe-finned fish Tiktaalik started to move on to land in the late Devonian, about 375 million years ago. Animals occupy virtually all of earth's and microhabitats, with adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms. Animals are however not particularly Thermophile; very few of them can survive at constant temperatures above or in the most extreme cold deserts of continental Antarctica.
The collective global geomorphic influence of animals on the processes shaping the Earth's surface remains largely understudied, with most studies limited to individual species and well-known exemplars.
The first body fossils of animals appear in the Ediacaran, represented by forms such as Charnia and Spriggina. It had long been doubted whether these fossils truly represented animals, but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature. 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 specialised for aerobic metabolism they became fully dependent on oxygen in their environments.
Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 539 million years ago, in beds such as the Burgess shale. Extant phyla in these rocks include , , , , , and , along with numerous now-extinct forms such as the 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. That view is supported by the discovery of Auroralumina attenboroughii, the earliest known Ediacaran crown-group cnidarian (557–562 mya, some 20 million years before the Cambrian explosion) from Charnwood Forest, England. It is thought to be one of the earliest , catching small prey with its as modern cnidarians do.
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago. (2025). 9780805371710, Pearson, Benjamin Cummings. ISBN 9780805371710 Early fossils that might represent animals appear for example in the 665-million-year-old rocks of the Trezona Formation of South Australia. These fossils are interpreted as most probably being early sponges.
such as tracks and burrows found in the Tonian period (from 1 gya) may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms. However, similar tracks are produced by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution. Around the same time, the layered mats of called decreased in diversity, perhaps due to grazing by newly evolved animals. 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.
The Porifera (sponges) have long been assumed to be sister to the rest of the animals, but there is evidence that the Ctenophora may be in that position. Molecular phylogenetics has supported both the sponge-sister and ctenophore-sister hypotheses. In 2017, Roberto Feuda and colleagues, using amino acid differences, presented both, with the following cladogram for the sponge-sister view that they supported (their ctenophore-sister tree simply interchanging the places of ctenophores and sponges):
Conversely, a 2023 study by Darrin Schultz and colleagues uses ancient to construct the following ctenophore-sister phylogeny:
The Ctenophora and Cnidaria are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus. Animals in both phyla have distinct tissues, but these are not organised into discrete organs. They are diploblastic, having only two main germ layers, ectoderm and endoderm.
The tiny placozoans have no permanent digestive chamber and no symmetry; they superficially resemble amoebae. Their phylogeny is poorly defined, and under active research.
Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with and a mouth. Many bilaterians have a combination of circular that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis. They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary which swim with cilia and have an apical organ containing sensory cells. However, over evolutionary time, descendant spaces have evolved which have lost one or more of each of these characteristics. For example, adult echinoderms are radially symmetric (unlike their larvae), while some parasitic worms have extremely simplified body structures.
Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, the protostomes and the deuterostomes. It is often suggested that the basalmost bilaterians are the Xenacoelomorpha, with all other bilaterians belonging to the subclade Nephrozoa. However, this suggestion has been contested, with other studies finding that xenacoelomorphs are more closely related to Ambulacraria than to other bilaterians.
The main deuterostome phyla are the Ambulacraria and the Chordata. Ambulacraria are exclusively marine and include , starfish, , and . The chordates are dominated by the vertebrates (animals with Vertebral column), which consist of , , , , and mammals.
The protostomes include the Ecdysozoa, named after their shared Phenotypic trait of ecdysis, growth by moulting, Among the largest ecdysozoan phyla are the and the . The rest of the protostomes are in the Spiralia, named for their pattern of developing by spiral cleavage in the early embryo. Major spiralian phyla include the and Mollusca.
In 1758, Carl Linnaeus created the first hierarchical classification in his Systema Naturae. In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then, the last four have all been subsumed into a single phylum, the Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Jean-Baptiste de Lamarck, who called the Vermes une espèce de chaos ('a chaotic mess') and split the group into three new phyla: worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique, Lamarck had created nine phyla apart from vertebrates (where he still had four phyla: mammals, birds, reptiles, and fish) and molluscs, namely , annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and .
In his 1817 Le Règne Animal, Georges Cuvier used comparative anatomy to group the animals into four embranchements ('branches' with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and zoophytes (echinoderms, cnidaria and other forms).
In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals, with five phyla: coelenterates, echinoderms, articulates, molluscs, and vertebrates) and Protozoa (single-celled animals), including a sixth animal phylum, sponges. The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia.
Invertebrates including , , —principally bees and silkworms—and bivalve or gastropod molluscs are hunted or farmed for food, fibres. , cattle, sheep, , and other animals are raised as livestock for meat across the world. Animal fibres such as wool and silk are used to make textiles, while animal have been used as lashings and bindings, and leather is widely used to make shoes and other items. Animals have been hunted and farmed for their fur to make items such as coats and hats. Dyestuffs including carmine (cochineal), shellac, and kermes have been made from the bodies of insects. Working animals including cattle and horses have been used for work and transport from the first days of agriculture.
Animals such as the fruit fly Drosophila melanogaster serve a major role in science as model organism. Animals have been used to create since their discovery in the 18th century. Some medicines such as the cancer drug trabectedin are based on or other molecules of animal origin.
People have used to help chase down and retrieve animals, and birds of prey to catch birds and mammals, while tethered have been used to catch fish. Poison dart frogs have been used to poison the tips of blowdart.
A wide variety of animals are kept as pets, from invertebrates such as tarantulas, octopuses, and , reptiles such as and , and birds including Domestic canary, , and all finding a place. However, the most kept pet species are mammals, namely , , and . There is a tension between the role of animals as companions to humans, and their existence as animal rights of their own.
A wide variety of terrestrial and aquatic animals are hunted for sport.
Animals have been the Animal style from the earliest times, both historical, as in ancient Egypt, and prehistoric, as in the cave paintings at Lascaux. Major animal paintings include Albrecht Dürer's 1515 The Rhinoceros, and George Stubbs's horse portrait Whistlejacket. Insects, birds and mammals play roles in literature and film, such as in giant bug movies.
Animals including insects and mammals feature in mythology and religion. The scarab beetle was sacred in ancient Egypt, and the cow is sacred in Hinduism. Among other mammals, deer, Horse worship, lions, bats, bear worship, and wolves are the subjects of myths and worship.
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.
Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the archenteron develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily. (2025). 9780521837620, Cambridge University Press. ISBN 9780521837620 Most protostomes have Schizocoely, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by Enterocoely, through invagination of the endoderm.
(2025). 9780534406530, Cengage. ISBN 9780534406530
History of classification
In the classical era, Aristotle divided animals, based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then Scala naturae from man (with blood, two legs, rational soul) down through the live-bearing tetrapods (with blood, four 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.
Hendrik C. D. de Wit (1994). 9782880742645, Presses polytechniques et universitaires Romandes. ISBN 9782880742645 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.
In human culture
Practical uses
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. Marine fish of many species are fishing for food. A smaller number of species are fish farming. Humans and their livestock make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined.
Symbolic uses
The signs of the Western and are based on animals. In China and Japan, the butterfly has been seen as the personification of a person's soul, and in classical representation the butterfly is also the symbol of the soul.Hutchins, M., Arthur V. Evans, Rosser W. Garrison and Neil Schlager (Eds) (2003), Grzimek's Animal Life Encyclopedia, 2nd edition. Volume 3, Insects. Gale, 2003.
(2025). 9781137496393, Springer. ISBN 9781137496393 (2025). 9781476625058, McFarland & Company. ISBN 9781476625058
See also
Notes
External links
target="_blank" rel="nofollow"> Wildscreen Arkive – multimedia database of endangered/protected species
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