Animals first appeared in the fossil record in the lateCryogenian period and diversified in the subsequentEdiacaran period in what is known as theAvalon explosion. Earlier evidence of animals is still controversial; thesponge-like organismOtavia has been dated back to theTonian period at the start of theNeoproterozoic, but its identity as an animal is heavily contested.[5] Nearly all modern animal phyla first appeared in the fossil record asmarine species during theCambrian explosion, which began around 539 million years ago (Mya), and mostclasses during theOrdovician radiation 485.4 Mya. Common to all living animals, 6,331 groups ofgenes have been identified that may have arisen from a singlecommon ancestor that lived about 650 Mya during theCryogenian period.
The wordanimal comes from the Latin nounanimal of the same meaning, which is itself derived from Latinanimalis 'having breath or soul'.[6] The biological definition includes all members of the kingdom Animalia.[7] In colloquial usage, the termanimal is often used to refer only to nonhuman animals.[8][9][10][11] The termmetazoa is derived from Ancient Greekμεταmeta 'after' (in biology, the prefixmeta- stands for 'later') andζῷᾰzōia 'animals', plural ofζῷονzōion 'animal'.[12][13]
Characteristics
Animals are unique in having the ball of cells of the earlyembryo (1) develop into a hollow ball orblastula (2).
nerve tissues, which transmit signals and coordinate the body.
Typically, there is an internaldigestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).[26]
Development
Animal development is controlled byHox genes, which signal the times and places to develop structures such as body segments and limbs.[27][28]
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 allowingcells to be differentiated.[29] The extracellular matrix may be calcified, forming structures such asshells,bones, andspicules.[30] In contrast, the cells of other multicellular organisms (primarily algae, plants, andfungi) are held in place by cell walls, and so develop by progressive growth.[31]
Nearly all animals make use of some form of sexual reproduction.[32] They producehaploidgametes bymeiosis; the smaller, motile gametes arespermatozoa and the larger, non-motile gametes areova.[33] These fuse to formzygotes,[34] which develop viamitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.[35] In most other groups, the blastula undergoes more complicated rearrangement.[36] It firstinvaginates to form agastrula with a digestive chamber and two separategerm layers, an externalectoderm and an internalendoderm.[37] In most cases, a third germ layer, themesoderm, also develops between them.[38] These germ layers then differentiate to form tissues and organs.[39]
Animals evolved in the sea. Lineages of arthropods colonised land around the same time asland plants, probably between 510 and 471 million years ago during theLate Cambrian or EarlyOrdovician.[57]Vertebrates such as thelobe-finned fishTiktaalik started to move on to land in the lateDevonian, about 375 million years ago.[58][59] Animals occupy virtually all of earth'shabitats and microhabitats, withfaunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms.[60] Animals are however not particularlyheat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F)[61] or in the most extreme cold deserts of continentalAntarctica.[62]
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.[63]
Theblue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to 190tonnes and measuring up to 33.6 metres (110 ft) long.[64][65] The largest extant terrestrial animal is theAfrican bush elephant (Loxodonta africana), weighing up to 12.25 tonnes[64] and measuring up to 10.67 metres (35.0 ft) long.[64] The largest terrestrial animals that ever lived weretitanosaursauropod dinosaurs such asArgentinosaurus, which may have weighed as much as 73 tonnes, andSupersaurus which may have reached 39 metres.[66][67] Several animals are microscopic; someMyxozoa (obligate parasites within the Cnidaria) never grow larger than 20μm,[68] and one of the smallest species (Myxobolus shekel) is no more than 8.5 μm when fully grown.[69]
Theblue whale is the largest animal that has ever lived; it can be up to 33.6 metres (110 ft) long.
The following table lists estimated numbers of described extant species for the major animal phyla,[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 thetaxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.[75][76][a]
Evidence of animals is found as long ago as theCryogenian period.24-Isopropylcholestane (24-ipc) has been found in rocks from roughly 650 million years ago; it is only produced by sponges andpelagophyte algae. Its likely origin is from sponges based onmolecular clock estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover aPhanerozoic origin, while analyses of sponges recover aNeoproterozoic origin, consistent with the appearance of 24-ipc in the fossil record.[92][93]
The first body fossils of animals appear in theEdiacaran, represented by forms such asCharnia andSpriggina. It had long been doubted whether these fossils truly represented animals,[94][95][96] but the discovery of the animal lipidcholesterol in fossils ofDickinsonia establishes their nature.[97] Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely byanaerobic respiration, but as they became specialised for aerobic metabolism they became fully dependent on oxygen in their environments.[98]
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.[106] Early fossils that might represent animals appear for example in the 665-million-year-old rocks of theTrezona Formation ofSouth Australia. These fossils are interpreted as most probably being earlysponges.[107]Trace fossils such as tracks and burrows found in theTonian period (from 1 gya) may indicate the presence oftriploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms.[108] However, similar tracks are produced by the giant single-celled protistGromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution.[109][110] Around the same time, the layered mats ofmicroorganisms calledstromatolites decreased in diversity, perhaps due to grazing by newly evolved animals.[111] 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.[112][113]
Animals aremonophyletic, meaning they are derived from a common ancestor. Animals are the sister group to thechoanoflagellates, with which they form theChoanozoa.[114] 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. The animal clade had certainly originated by 650 mya, and may have come into being as much as 800 mya, based onmolecular clock evidence for different phyla.[115]
The relationships at the base of the animal tree have been debated.[116][117] Other than Ctenophora, the Bilateria and Cnidaria are the only groups with symmetry, and other evidence shows they are closely related.[118] In addition to sponges, Placozoa has no symmetry and was often considered a "missing link" between protists and multicellular animals. The presence ofhox genes in Placozoa shows that they were once more complex.[119]
ThePorifera (sponges) have long been assumed to be sister to the rest of the animals, but there is evidence that theCtenophora may be in that position. Molecular phylogenetics has supported both the sponge-sister and ctenophore-sister hypotheses. In 2017, Roberto Feuda and colleagues, usingamino 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):[120]
Non-bilaterians include sponges (centre) and corals (background).
Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing the oldest animal phylum and forming asister clade to all other animals.[122] 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.[123][124] Sponges lack the complex organisation found in most other animal phyla;[125] their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals.[126] They typically feed by drawing in water through pores, filtering out small particles of food.[127]
The Ctenophora and Cnidaria are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus.[128] Animals in both phyla have distinct tissues, but these are not organised into discreteorgans.[129] They arediploblastic, having only two main germ layers, ectoderm and endoderm.[130]
The tiny placozoans have no permanent digestive chamber and no symmetry; they superficially resemble amoebae.[131][132] Their phylogeny is poorly defined, and under active research.[123][133]
The remaining animals, the great majority—comprising some 29 phyla and over a million species—form theBilateriaclade, which have a bilaterally symmetricbody plan. The Bilateria aretriploblastic, with three well-developed germ layers, and their tissuesform distinct organs. The digestive chamber has two openings, a mouth and an anus, and in theNephrozoa there is an internal body cavity, acoelom 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.[134][135] A modern consensusphylogenetic tree for the Bilateria is shown below.[136]
Idealisednephrozoan body plan.[c] With an elongated body and a direction of movement the animal has head and tail ends. Sense organs and mouth form thebasis of the head. Opposed circular and longitudinal muscles enableperistaltic motion.
Having a front end means that this part of the body encounters stimuli, such as food, favouringcephalisation, the development of a head withsense organs and a mouth. Many bilaterians have a combination of circularmuscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;[135] these enable soft-bodied animals with ahydrostatic skeleton to move byperistalsis.[137] They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primarylarvae which swim withcilia 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 someparasitic worms have extremely simplified body structures.[134][135]
Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, theprotostomes and thedeuterostomes.[138] It is often suggested that the basalmost bilaterians are theXenacoelomorpha, with all other bilaterians belonging to the subcladeNephrozoa.[139][140][141] However, this suggestion has been contested, with other studies finding that xenacoelomorphs are more closely related toAmbulacraria than to other bilaterians.[142]
The bilaterian gut develops in two ways. In manyprotostomes, the blastopore develops into the mouth, while indeuterostomes it becomes the anus.
Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radialcleavage during cell division, while many protostomes (theSpiralia) undergo spiral cleavage.[143]Animals from both groups possess a complete digestive tract, but in protostomes the first opening of theembryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily.[144][145] Most protostomes haveschizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms byenterocoelic pouching, through invagination of the endoderm.[146]
The protostomes include theEcdysozoa, named after their sharedtrait ofecdysis, growth by moulting,[153] Among the largest ecdysozoan phyla are thearthropods and thenematodes.[154] The rest of the protostomes are in theSpiralia, named for their pattern of developing by spiral cleavage in the early embryo. Major spiralian phyla include theannelids andmolluscs.[155]
In theclassical era, Aristotledivided animals,[d] based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were thenarranged on a scale 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.[157]
In 1758,Carl Linnaeus created the firsthierarchical classification in hisSystema Naturae.[158] In his original scheme, the animals were one of three kingdoms, divided into the classes ofVermes,Insecta,Pisces,Amphibia,Aves, andMammalia. Since then, the last four have all been subsumed into a single phylum, theChordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 byJean-Baptiste de Lamarck, who called the Vermesune espèce de chaos ('a chaotic mess')[e] and split the group into three new phyla: worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in hisPhilosophie Zoologique, Lamarck had created nine phyla apart from vertebrates (where he still had four phyla: mammals, birds, reptiles, and fish) and molluscs, namelycirripedes, annelids, crustaceans, arachnids, insects, worms,radiates, polyps, andinfusorians.[156]
In his 1817Le Règne Animal,Georges Cuvier usedcomparative anatomy to group the animals into fourembranchements ('branches' with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), andzoophytes (radiata) (echinoderms, cnidaria and other forms).[160] This division into four was followed by the embryologistKarl Ernst von Baer in 1828, the zoologistLouis Agassiz in 1857, and the comparative anatomistRichard Owen in 1860.[161]
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.[162][161] The protozoa were later moved to the former kingdomProtista, leaving only the Metazoa as a synonym of Animalia.[163]
The human population exploits a large number of other animal species for food, both ofdomesticated livestock species inanimal husbandry and, mainly at sea, by hunting wild species.[164][165] Marine fish of many species arecaught commercially for food. A smaller number of species arefarmed commercially.[164][166][167] Humans and theirlivestock make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined.[168]
^The application ofDNA barcoding to taxonomy further complicates this; a 2016 barcoding analysis estimated a total count of nearly 100,000insect species forCanada 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).[77]
^The French prefixune espèce de is pejorative.[159]
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![Dickinsonia costata from the Ediacaran biota (c. 635–542 mya) is one of the earliest animal species known.[97]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aDickinsoniaCostata.jpg)
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