Animals are eukaryotic, multicellular organisms that form the biological kingdom Animalia. With few exceptions, animals are motile (able to move), heterotrophic (consume organic material), reproduce sexually, and their embryonic development includes a blastula stage. The body plan of the animal derives from this blastula, differentiating specialized tissues and organs as it develops; this plan eventually becomes fixed, although some undergo metamorphosis at some stage in their lives.

Zoology is the study of animals. Currently there are over 66 thousand (less than 5% of all animals) vertebrate species, and over 1.3 million (over 95% of all animals) invertebrate species in existence. Classification of animals into groups (taxonomy) is accomplished using either the hierarchical Linnaean system; or cladistics, which displays diagrams (phylogenetic trees) called cladograms to show relationships based on the evolutionary principle of the most recent common ancestor. Some recent classifications based on modern cladistics have explicitly abandoned the term "kingdom", noting that the traditional Kingdoms are not monophyletic, i.e., do not consist of all the descendants of a common Ancestor

Animals are divided by body plan into vertebrates and invertebrates. Vertebrates—fishes, amphibians, reptiles, birds, and mammals —have a vertebral column (spine); invertebrates do not. All vertebrates and most invertebrates are bilaterally symmetrical (Bilateria ). These invertebrates include arthropods, molluscs, roundworms, ringed worms, flatworms, and other phyla in Ecdysozoa and Spiralia. Echinoderm larvae are initially bilaterally symmetrical, but later as adults develop radial symmetry; Cnidarians are radially symmetrical; ctenophores are biradially symmetrical; and sponges have no symmetry.

Animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. Animals emerged as a clade within Apoikozoa as the sister group to the choanoflagellates.

The word "animal" comes from the Latin animalis , meaning having breath, having soul or living being. The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects, and humans. In everyday non-scientific usage, the word often implies exclusion of humans – that is, "animal" is used to refer only to non-human members of the kingdom Animalia; sometimes, only closer relatives of humans such as mammals and other vertebrates, are meant. ^[119]

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

Animals are eukaryotic and multicellular, ^[124] which separates them from bacteria and most protists, which are prokaryotic and unicellular. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae, which are autotrophs. They lack rigid cell walls, which separates them from plants, algae, and fungi, all of which do have rigid cell walls. ^[150] All animals are motile, ^[158] if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals, and which allows for differentiation into specialized tissues and organs.

All animals are composed of eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules. During development, it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.

With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals. Typically, there is also an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.

Nearly all animals undergo some form of sexual reproduction. They produce haploid gametes by meiosis (see Origin and function of meiosis). The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. ^[112] These fuse to form zygotes, which develop via multiple successive mitoses and differentiation into new individuals ^[112] (see Allogamy).

Some animals are also capable of asexual reproduction. ^[112] This may take place through parthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation. ^[112]

During sexual reproduction, mating with a close relative (inbreeding) generally leads to inbreeding depression. For instance, inbreeding was found to increase juvenile mortality in 11 small animal species. ^[32] Inbreeding depression is considered to be largely due to expression of deleterious recessive mutations. Mating with unrelated or distantly related members of the same species is generally thought to provide the advantage of masking deleterious recessive mutations in progeny. (see Heterosis). Animals have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossing (see Inbreeding avoidance).

Chimpanzees have adopted dispersal as a way to separate close relatives and prevent inbreeding. Their spersal route is known as natal dispersal, whereby individuals move away from the area of birth.

In various species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality. Females that are pair bonded to a male of poor genetic quality, as is the case in inbreeding, are more likely to engage in extra-pair copulations in order to improve their reproductive success and the survivability of their offspring.

A zygote initially develops into a hollow sphere, called a blastula, ^[112] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers —an external ectoderm and an internal endoderm. In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs.

All animals are heterotrophs, meaning that they feed directly or indirectly on other living things. They are often further subdivided into groups such as carnivores, herbivores, omnivores, and parasites.

Predation is a biological interaction where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked). Predators may or may not kill their prey prior to feeding on them, but the act of predation almost always results in the death of the prey. The other main category of consumption is detritivory, the consumption of dead organic matter. It can at times be difficult to separate the two feeding behaviours, for example, where parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations.

Most animals indirectly use the energy of sunlight by eating plants or plant-eating animals. Most plants use light to convert inorganic molecules in their environment into carbohydrates, fats, proteins and other biomolecules, characteristically containing reduced carbon in the form of carbon-hydrogen bonds. Starting with carbon dioxide (CO 2 ) and water (H 2 O), photosynthesis converts the energy of sunlight into chemical energy in the form of simple sugars (e.g., glucose), with the release of molecular oxygen. These sugars are then used as the building blocks for plant growth, including the production of other biomolecules. When an animal eats plants (or eats other animals which have eaten plants), the reduced carbon compounds in the food become a source of energy and building materials for the animal. They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion.

Animals living close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight. Instead chemosynthetic archaea and bacteria form the base of the food chain.

Taxonomy classifies organisms into groups. There are two taxonomic approaches: the Linnaean system classifies life according to an eight level hierarchy based on features other than phylogenomics (cladistics).

The three-domain system is an addition to the Linnaean system biological classification introduced by Carl Woese et al. in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called Eubacteria (now Bacteria) and Archaebacteria (now Archaea). Woese argued that, on the basis of differences in 16S rRNA genes, these two groups and the eukaryotes each arose separately from an ancestor with poorly developed genetic machinery, often called a progenote. To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdoms. The term "domain" was adopted in 1990.

Animals are thus classified under the domain Eukaryota.

The Linnaean hierarchy below the kingdom Animalia consists of these groups: phyla, classes, orders, families, genera, and species.

All the groups, from domain to species, are called taxa. There are occasional intermediate levels, such as superphyla and subphyla, in special situations. The International Commission on Zoological Nomenclature (ICZN) determines what names are valid for any taxon in the family, genus, and species group. It has additional but more limited provisions on names in higher ranks.

Animals are generally considered to have emerged within flagellated eukaryota. Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges. ^[119] Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists. ^[119] The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella. ^[119]

The first fossils that might represent animals appear in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia. These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.

The next oldest possible animal fossils are found towards the end of the Precambrian, around 610 million years ago, and are known as the Ediacaran or Vendian biota. ^[119] These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all. ^[119]

Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago. ^[119] It is still disputed whether this event, called the Cambrian explosion, is due to a rapid divergence between different groups or due to a change in conditions that made fossilization possible.

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

^[119] Trace fossils such as tracks and burrows found in the Tonian period indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as earthworms. During the beginning of the Tonian period around 1 billion years ago, there was a decrease in Stromatolite diversity, which may indicate the appearance of grazing animals, since stromatolite diversity increased when grazing animals became extinct at the End Permian and End Ordovician extinction events, and decreased shortly after the grazer populations recovered. However the discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist Gromia sphaerica casts doubt on their interpretation as evidence of early animal evolution.

Animals can be divided into two broad groups: vertebrates (animals with a backbone) and invertebrates (animals without a backbone). Half of all described vertebrate species are fishes and three-quarters of all described invertebrate species are insects. Over 95% of the described animal species in the world are invertebrates.

The following table lists the number of described living species for each major animal subgroup as estimated for the IUCN Red List of Threatened Species , 2014.3. [6]

Cladistics embraces the concept of the "most recent common ancestor", and assigning similar animals to groups called clades.

Animals are thought to be a basal Apoikozoan clade as sister of the Choanoflagellata. The "bilaterian" animals (Bilateria), whose body display bilateral symmetry, are thought to form a monophyletic group. The Bilaterians are further classified based on a major division between Deuterostomes and Protostomes. More basal animals lack a bilaterally symmetric body plan (Ctenophora, Porifera, Cnidaria and Placozoa), with their relationships still disputed. In 2017, the Ctenophora are found as basalmost animals with "full" support. However, ever since the first finding in 2008, such proposals are strongly contested, with Porifera as the alternative. Some of the issues are the rapid evolutionary rate within Ctenophora, insufficient sampling, and the recent internal divergence date of Ctenophora. It is indicated approximately how many million years ago (Mya) the clades diverged into newer clades.

Several animal phyla are recognized for their lack of bilateral symmetry, and are thought to have diverged from other animals early in evolution. Among these, the sponges (Porifera) were long thought to have diverged first, representing the oldest animal phylum. They lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges typically feed by drawing in water through pores. However, a series of phylogenomic studies from 2008–2015 have found support for Ctenophora, or comb jellies, as the basal lineage of animals. ^[153] This result has been controversial, since it would imply that sponges may not be so primitive, but may instead be secondarily simplified. Other researchers have argued that the placement of Ctenophora as the earliest-diverging animal phylum is a statistical anomaly caused by the high rate of evolution in ctenophore genomes.

The Ctenophora and the sponges are unique among the animals in lacking true hox genes. The presence of a Hox/Parahox gene in the Placozoa suggests that either the Porifera or the Ctenophora are the most basal animal clades. Another DNA based study suggests that the Ctenophora are the earliest branching animals. Another study also suggests that this group are a sister group to other animals.

Among the other phyla, the Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. Both have distinct tissues, but they are not organized into organs. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic. The tiny placozoans are similar, but they do not have a permanent digestive chamber.

The Myxozoa, microscopic parasites that were originally considered Protozoa, are now believed to have evolved within Cnidaria.

The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however—for instance adult echinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures.

Genetic studies have considerably changed our understanding of the relationships within the Bilateria.

Most appear to belong to two major lineages: the deuterostomes and the protostomes, the latter of which includes the Ecdysozoa, and Lophotrochozoa. The Chaetognatha or arrow worms have been traditionally classified as deuterostomes, though recent molecular studies have identified this group as a basal protostome lineage.

In addition, there are a few small groups of bilaterians with relatively cryptic morphology whose relationships with other animals are not well-established.

For example, recent molecular studies have identified Acoelomorpha and Xenoturbella as forming a monophyletic group, but studies disagree as to whether this group evolved from within deuterostomes, or whether it represents the sister group to all other bilaterian animals (Nephrozoa). Other groups of uncertain affinity include the Rhombozoa and Orthonectida. One phyla, the Monoblastozoa, was described by a scientist in 1892, but so far there have been no evidence of its existence.

Deuterostomes differ from protostomes in several ways. Animals from both groups possess a complete digestive tract. However, in protostomes, the first opening of the gut to appear in embryological development (the archenteron) develops into the mouth, with the anus forming secondarily. In deuterostomes the anus forms first, with the mouth developing secondarily. In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes, it forms through invagination of the endoderm, called enterocoelic pouching. Deuterostome embryos undergo radial cleavage during cell division, while protostomes undergo spiral cleavage.

All this suggests the deuterostomes and protostomes are separate, monophyletic lineages.

The main phyla of deuterostomes are the Echinodermata and Chordata. The former are radially symmetric and exclusively marine, such as starfish, sea urchins, and sea cucumbers. The latter are dominated by the vertebrates, animals with backbones. These include fish, amphibians, reptiles, birds, and mammals.

In addition to these, the deuterostomes also include the Hemichordata, or acorn worms, which are thought to be closely related to Echinodermata forming a group known as Ambulacraria. Although they are not especially prominent today, the important fossil graptolites may belong to this group.

The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis. The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and 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. A number are important parasites. 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.

The Lophotrochozoa, evolved within Protostomia, include two of the most successful animal phyla, the Mollusca and Annelida. The former, which is the second-largest animal phylum by number of described species, includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented. Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla. Lophotrochozoa also includes the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a ring of ciliated tentacles around the mouth, called a lophophore. These were traditionally grouped together as the lophophorates. but it now appears that the lophophorate group may be paraphyletic, with some closer to the nemerteans and some to the molluscs and annelids. They include the Brachiopoda or lamp shells, which are prominent in the fossil record, the Entoprocta, the Phoronida, and possibly the Bryozoa or moss animals.

The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors. A number of parasites are included in this group, such as the flukes and tapeworms. Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha. The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly the Cycliophora. These groups share the presence of complex jaws, from which they are called the Gnathifera.

A relationship between the Brachiopoda and Nemertea has been suggested by molecular data.

A second study has also suggested this relationship.

This latter study also suggested that Annelida and Mollusca may be sister clades.

Another study has suggested that Annelida and Mollusca are sister clades.

^[116] This clade has been termed the Neotrochozoa.

Aristotle divided the living world between animals and plants, and this was followed by Carl Linnaeus, in the first hierarchical classification. In Linnaeus's 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, whereas the various other forms have been separated out.

In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals) and Protozoa (single-celled animals). The protozoa were later moved to the kingdom Protista, leaving only the metazoa. Thus Metazoa is now considered a synonym of Animalia.

A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms.

Model organisms are in vivo models and are widely used to research human disease when human experimentation would be unfeasible or unethical. This strategy is made possible by the common descent of all living organisms, and the conservation of metabolic and developmental pathways and genetic material over the course of evolution. Studying model organisms can be informative, but care must be taken when extrapolating from one organism to another.

In researching human disease, model organisms allow for better understanding the disease process without the added risk of harming a human.

The species chosen will usually meet a determined taxonomic equivalency to humans, so as to react to disease or its treatment in a way that resembles human physiology as needed. Although biological activity in a model organism does not ensure an effect in humans, many drugs, treatments and cures for human diseases are developed in part with the guidance of animal models. ^[156] There are three main types of disease models: homologous, isomorphic and predictive. Homologous animals have the same causes, symptoms and treatment options as would humans who have the same disease. Isomorphic animals share the same symptoms and treatments. Predictive models are similar to a particular human disease in only a couple of aspects, but are useful in isolating and making predictions about mechanisms of a set of disease features.

Because they are easy to keep and breed, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have long been the most intensively studied metazoan model organisms, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their genomes, but as many genes, introns, and linkages lost, these ecdysozoans can teach us little about the origins of animals in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean, annelid, and molluscan genome projects currently in progress. Analysis of the starlet sea anemone genome has emphasized the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa. An analysis of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed. Other model organisms belonging to the animal kingdom include the house mouse (Mus musculus ), laboratory rat (Rattus norvegicus ) and zebrafish (Danio rerio