70-472: Centroneuralia is a proposed clade of animals with bilateral symmetry as an embryo, consisting of the Chordata and Protostomia , united by the presence of a central nervous system . An alternative to the traditional protostome- deuterostome dichotomy, it has found weak support in several studies. Under this hypothesis, Centroneuralia would be sister to Xenambulacraria ( Xenacoelomorpha + Ambulacraria ) at
140-402: A cartilaginous / bony axial endoskeleton ( spine ) and are cladistically and phylogenetically a subgroup of the clade Craniata (i.e. chordates with a skull ); Tunicata or Urochordata ( sea squirts , salps , and larvaceans ), which only retain the synapomorphies during their larval stage; and Cephalochordata ( lancelets ), which resemble jawless fish but have no gills or
210-399: A gastrula with a digestive chamber and two separate germ layers , an external ectoderm and an internal endoderm . In most cases, a third germ layer, the mesoderm , also develops between them. These germ layers then differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within
280-717: A notochord , a hollow dorsal nerve cord , an endostyle or thyroid , pharyngeal slits , and a post- anal tail . In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two conserved signature indels (CSIs) in their proteins: cyclophilin -like protein and inner mitochondrial membrane protease ATP23, which are exclusively shared by all vertebrates , tunicates and cephalochordates . These CSIs provide molecular means to reliably distinguish chordates from all other animals . Chordates are divided into three subphyla : Vertebrata ( fish , amphibians , reptiles , birds and mammals ), whose notochords are replaced by
350-547: 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. Eumetazoa , a clade which contains Ctenophora and ParaHoxozoa , has been proposed as a sister group to Porifera . A competing hypothesis
420-503: A detailed classification within the living chordates. Attempts to produce evolutionary " family trees " shows that many of the traditional classes are paraphyletic . Hemichordates [REDACTED] Echinoderms [REDACTED] Cephalochordates [REDACTED] Tunicates [REDACTED] Craniates ( vertebrates ) [REDACTED] While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in
490-429: A distinct head . The vertebrates and tunicates compose the clade Olfactores , which is sister to Cephalochordata (see diagram under Phylogeny ). Extinct taxa such as the conodonts are chordates, but their internal placement is less certain. Hemichordata (which includes the acorn worms ) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum which are now thought to be closer to
560-436: A long time regarded as larvae of the other two groups. The other two groups, the sea squirts and the salps, metamorphize into adult forms which lose the notochord, nerve cord, and post anal tail. Both are soft-bodied filter feeders with multiple gill slits. They feed on plankton which they collect in their mucus. Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus. Most attach firmly to
630-549: A new study have shown possible affinity of these Ediacaran organisms to the ascidians. Ausia and Burykhia lived in shallow coastal waters slightly more than 555 to 548 million years ago, and are believed to be the oldest evidence of the chordate lineage of metazoans. The Russian Precambrian fossil Yarnemia is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those of Ausia and Burykhia , so this identification has been questioned. Fossils of one major deuterostome group,
700-463: A poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before 900 million years ago and the earliest chordates around 896 million years ago . However, molecular estimates of dates often disagree with each other and with
770-1105: A population due to the increased prevalence of harmful recessive traits. 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 cnidarians ; or parthenogenesis , where fertile eggs are produced without mating , such as in aphids . Animals are categorised into ecological groups depending on their trophic levels and how they consume organic material . Such groupings include carnivores (further divided into subcategories such as piscivores , insectivores , ovivores , etc.), herbivores (subcategorized into folivores , graminivores , frugivores , granivores , nectarivores , algivores , etc.), omnivores , fungivores , scavengers / detritivores , and parasites . Interactions between animals of each biome form complex food webs within that ecosystem . In carnivorous or omnivorous species, predation
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#1732851866837840-675: 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. Chordate And see text A chordate ( / ˈ k ɔːr d eɪ t / KOR -dayt ) is a deuterostomal bilaterian animal belonging to the phylum Chordata ( / k ɔːr ˈ d eɪ t ə / kor- DAY -tə ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies ) that distinguish them from other taxa . These five synapomorphies are
910-480: A state of flux. The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes , the latter of which contains chordates. It seems very likely the 555 million-year-old Kimberella was a member of the protostomes. If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least 558 million years ago , and hence well before
980-530: A taxon comprising tunicates, cephalochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under the ICZN code because of its subsequent latinization. Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: There are soft constraints that separate chordates from other biological lineages, but are not part of
1050-464: Is a consumer–resource interaction where the predator feeds on another organism, its prey , who often evolves anti-predator adaptations to avoid being fed upon. Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various antagonistic/ competitive coevolutions . Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in parasitoid wasps ,
1120-433: Is a stage in embryonic development that is unique to animals, allowing cells to be differentiated into specialised tissues and organs. All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins . During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making
1190-458: Is disputed, as they might be water-escape or other structures. Animals are monophyletic , meaning they are derived from a common ancestor. Animals are the sister group to the choanoflagellates , with which they form the Choanozoa . The dates on the phylogenetic tree indicate approximately how many millions of years ago ( mya ) the lineages split. Ros-Rocher and colleagues (2021) trace
1260-437: Is itself a chordate, and that craniates ' nearest relatives are tunicates. Recent identification of two conserved signature indels (CSIs) in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates , tunicates and cephalochordates also provide strong evidence of the monophyly of Chordata. All of the earliest chordate fossils have been found in
1330-450: Is known as zoology , and the study of animal behaviour is known as ethology . The animal kingdom is divided into five infrakingdoms / superphyla , namely Porifera , Ctenophora , Placozoa , Cnidaria and Bilateria . Most living animal species belong to the infrakingdom Bilateria, a highly proliferative clade whose members have a bilaterally symmetric body plan , and the vast majority of bilaterians belong to two large superphyla :
1400-423: Is not yet settled. A specific relationship between Vertebrates and Tunicates is also strongly supported by two CSIs found in the proteins predicted exosome complex RRP44 and serine palmitoyltransferase, that are exclusively shared by species from these two subphyla but not Cephalochordates , indicating Vertebrates are more closely related to Tunicates than Cephalochordates . Below is a phylogenetic tree of
1470-808: Is often used to refer only to nonhuman animals. The term metazoa is derived from Ancient Greek μετα ( meta ) 'after' (in biology, the prefix meta- stands for 'later') and ζῷᾰ ( zōia ) 'animals', plural of ζῷον zōion 'animal'. Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular . Unlike plants and algae , which produce their own nutrients , animals are heterotrophic , feeding on organic material and digesting it internally. With very few exceptions, animals respire aerobically . All animals are motile (able to spontaneously move their bodies) during at least part of their life cycle , but some animals, such as sponges , corals , mussels , and barnacles , later become sessile . The blastula
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#17328518668371540-422: Is only produced by sponges and pelagophyte algae. Its likely origin is from sponges based on molecular clock estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover a Phanerozoic origin, while analyses of sponges recover a Neoproterozoic origin, consistent with the appearance of 24-ipc in the fossil record. The first body fossils of animals appear in
1610-537: Is the Benthozoa clade, which would consist of Porifera and ParaHoxozoa as a sister group of Ctenophora . 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
1680-605: The Cambrian explosion , which began around 539 million years ago (Mya), and most classes during the Ordovician radiation 485.4 Mya. 6,331 groups of genes common to all living animals have been identified; these 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
1750-614: 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 specialized for aerobic metabolism they became fully dependent on oxygen in their environments. Many animal phyla first appear in
1820-482: The Wnt and TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for transcription factors including homeodomain proteins involved in the control of development . Giribet and Edgecombe (2020) provide what they consider to be a consensus internal phylogeny of the animals, embodying uncertainty about
1890-893: The biological kingdom Animalia ( / ˌ æ n ɪ ˈ m eɪ l i ə / ). With few exceptions, animals consume organic material , breathe oxygen , have myocytes and are able to move , 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 living animal species have been described , of which around 1.05 million are insects , over 85,000 are molluscs , and around 65,000 are vertebrates . It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex ecologies and interactions with each other and their environments, forming intricate food webs . The scientific study of animals
1960-574: The echinoderms (whose modern members include starfish , sea urchins and crinoids ), are quite common from the start of the Cambrian, 542 million years ago . The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate. Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon , from the earlier Cambrian, was a hemichordate or chordate. Another fossil, Haikouella lanceolata , also from
2030-412: The echinoderms , and together they form the clade Ambulacraria , the sister phylum of the chordates. Chordata, Ambulacraria, and possibly Xenacoelomorpha are believed to form the superphylum Deuterostomia , although this has recently been called into doubt. Chordata is the third-largest phylum of the animal kingdom (behind only the protostomal phyla Arthropoda and Mollusca ) and is also one of
2100-665: The evolutionary relationships between taxa . Humans make use of many other animal species for food (including meat , eggs , and dairy products ), for materials (such as leather , fur , and wool ), as pets and as working animals for transportation , and services . Dogs , the first domesticated animal, have been used in hunting , in security and in warfare , as have horses , pigeons and birds of prey ; while other terrestrial and aquatic animals are hunted for sports, trophies or profits. Non-human animals are also an important cultural element of human evolution , having appeared in cave arts and totems since
2170-468: 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 molluscs , brachiopods , onychophorans , tardigrades , arthropods , echinoderms and hemichordates , along with numerous now-extinct forms such as the predatory Anomalocaris . The apparent suddenness of the event may however be an artifact of
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2240-497: 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 habitats and microhabitats, with faunas 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 heat tolerant ; very few of them can survive at constant temperatures above 50 °C (122 °F) or in
2310-459: The notochord is replaced by the vertebral column . It consists of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal cord , and with projections that link the vertebrae. Hagfishes have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates, but they are members of the craniates, the group within which vertebrates are thought to have evolved . However
2380-495: The protostomes , which includes organisms such as arthropods , molluscs , flatworms , annelids and nematodes ; and the deuterostomes , which include echinoderms , hemichordates and chordates , the latter of which contains the vertebrates . The much smaller basal phylum Xenacoelomorpha have an uncertain position within Bilateria. Animals first appear in the fossil record in the late Cryogenian period and diversified in
2450-647: The Cambrian explosion, possibly as early as 1 billion years ago. 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 . Trace fossils 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
2520-463: The Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes—although it also had short tentacles round its mouth. Haikouichthys and Myllokunmingia , also from the Chengjiang fauna, are regarded as fish . Pikaia , discovered much earlier (1911) but from
2590-712: The Cnidaria) never grow larger than 20 μm , and one of the smallest species ( Myxobolus shekel ) is no more than 8.5 μm when fully grown. The following table lists estimated numbers of described extant species for the major animal phyla, along with their principal habitats (terrestrial, fresh water, and marine), and free-living or parasitic ways of life. 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
2660-483: The Early Cambrian Chengjiang fauna , and include two species that are regarded as fish , which implies that they are vertebrates. Because the fossil record of early chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial. It has also proved difficult to produce
2730-647: The Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate. On the other hand, fossils of early chordates are very rare, since invertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian. The best known and earliest unequivocally identified Tunicate is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale at Shankou village, Anning, near Kunming ( South China ). The evolutionary relationships between
2800-564: The base of Bilateria . Centroneuralia, as a proposed clade, originates in phylogenomics . More precisely, recent studies correlate support for Deuterostomia with simpler, site-homogeneous models, while more sophisticated and site-heterogeneous models recover Centroneuralia more often. Xenacoelomorpha [REDACTED] Ambulacraria [REDACTED] Chordata [REDACTED] Ecdysozoa [REDACTED] Spiralia [REDACTED] † Kimberella [REDACTED] Animal Animals are multicellular , eukaryotic organisms in
2870-435: The chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, embryological , and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected. Combining such analyses with data from a small set of ribosome RNA genes eliminated some older ideas, but opened up
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2940-568: The cladistic exclusion of hagfish from the vertebrates is controversial, as they may instead be degenerate vertebrates who have secondarily lost their vertebral columns. The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish . However, molecular phylogenetics , which uses biochemical features to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish. If lampreys are more closely related to
3010-470: The classification of chordates. Some chordate lineages may only be found by DNA analysis, when there is no physical trace of any chordate-like structures. Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are monophyletic , meaning that the Chordata include all and only the descendants of a single common ancestor, which
3080-456: The dark sea floor consume organic matter produced through chemosynthesis (via oxidizing inorganic compounds such as hydrogen sulfide ) by archaea and bacteria . Animals evolved in the sea. Lineages of arthropods colonised land around the same time as land plants , probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician . Vertebrates such as
3150-450: The earliest times, and are frequently featured in mythology , religion , arts , literature , heraldry , politics , and sports . The word animal comes from the Latin noun animal of the same meaning, which is itself derived from Latin animalis 'having breath or soul'. The biological definition includes all members of the kingdom Animalia. In colloquial usage, the term animal
3220-534: The earliest-branching chordate subphylum. The tunicates have three distinct adult shapes. Each is a member of one of three monophylitic clades. All tunicate larvae have the standard chordate features, including long, tadpole -like tails. Their larva also have rudimentary brains, light sensors and tilt sensors. The smallest of the three groups of tunicates is the Appendicularia . They retain tadpole-like shapes and active swimming all their lives, and were for
3290-565: 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
3360-408: The first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric , have a coelom , possess a closed circulatory system , and exhibit metameric segmentation . Although the name Chordata is attributed to William Bateson (1885), it was already in prevalent use by 1880. Ernst Haeckel described
3430-651: The formal definition: The following schema is from the 2015 edition of Vertebrate Palaeontology . The invertebrate chordate classes are from Fishes of the World . While it is structured so as to reflect evolutionary relationships (similar to a cladogram ), it also retains the traditional ranks used in Linnaean taxonomy . Cephalochordates , one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs. These burrowing filter-feeders compose
3500-448: The formation of complex structures possible. This may be calcified, forming structures such as shells , bones , 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. Animal cells uniquely possess the cell junctions called tight junctions , gap junctions , and desmosomes . With few exceptions—in particular,
3570-426: The fossil record, and their assumption that the molecular clock runs at a known constant rate has been challenged. Traditionally, Cephalochordata and Craniata were grouped into the proposed clade "Euchordata", which would have been the sister group to Tunicata/Urochordata. More recently, Cephalochordata has been thought of as a sister group to the "Olfactores", which includes the craniates and tunicates. The matter
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#17328518668373640-520: 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 predators , catching small prey with its nematocysts as modern cnidarians do. Some palaeontologists have suggested that animals appeared much earlier than
3710-721: 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 microorganisms called stromatolites 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
3780-462: The hagfish than the other vertebrates, this would suggest that they form a clade , which has been named the Cyclostomata . There is still much ongoing differential (DNA sequence based) comparison research that is trying to separate out the simplest forms of chordates. As some lineages of the 90% of species that lack a backbone or notochord might have lost these structures over time, this complicates
3850-441: The larvae feed on the hosts' living tissues, killing them in the process, but the adults primarily consume nectar from flowers. Other animals may have very specific feeding behaviours , such as hawksbill sea turtles which mainly eat sponges . Most animals rely on biomass and bioenergy produced by plants and phytoplanktons (collectively called producers ) through photosynthesis . Herbivores, as primary consumers , eat
3920-435: The most ancient taxons. Chordate fossils have been found from as early as the Cambrian explosion over 539 million years ago. Of the more than 81,000 living species of chordates, about half are ray-finned fishes ( class Actinopterygii ) and the vast majority of the rest are tetrapods , a terrestrial clade of lobe-finned fishes ( Sarcopterygii ) who evolved air-breathing using lungs . The name "chordate" comes from
3990-748: The most extreme cold deserts of continental Antarctica . The blue whale ( Balaenoptera musculus ) is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long. The largest extant terrestrial animal is the African bush elephant ( Loxodonta africana ), weighing up to 12.25 tonnes and measuring up to 10.67 metres (35.0 ft) long. The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus , which may have weighed as much as 73 tonnes, and Supersaurus which may have reached 39 meters. Several animals are microscopic; some Myxozoa ( obligate parasites within
4060-620: The oldest animal phylum and forming a sister clade to all other animals. 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. Sponges lack the complex organization found in most other animal phyla; their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals. They typically feed by drawing in water through pores, filtering out small particles of food. The comb jellies and Cnidaria are radially symmetric and have digestive chambers with
4130-525: The origins of animals to unicellular ancestors, providing the external phylogeny shown in the cladogram. Uncertainty of relationships is indicated with dashed lines. Holomycota (inc. fungi) [REDACTED] Ichthyosporea [REDACTED] Pluriformea [REDACTED] Filasterea [REDACTED] [REDACTED] [REDACTED] The most basal animals, the Porifera , Ctenophora , Cnidaria , and Placozoa , have body plans that lack bilateral symmetry . Their relationships are still disputed;
4200-519: The plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher trophic levels indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidize carbohydrates , lipids , proteins 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 cold seeps on
4270-409: The possibility that tunicates (urochordates) are "basal deuterostomes", surviving members of the group from which echinoderms, hemichordates and chordates evolved. Some researchers believe that, within the chordates, craniates are most closely related to cephalochordates, but there are also reasons for regarding tunicates (urochordates) as craniates' closest relatives. Since early chordates have left
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#17328518668374340-410: The sea floor, where they remain in one place for life, feeding on plankton. The salps float in mid-water, feeding on plankton , and have a two-generation cycle in which one generation is solitary and the next forms chain-like colonies . The etymology of the term Urochordata (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in
4410-731: The sister group to all other animals could be the Porifera or the Ctenophora, both of which lack hox genes , which are important for body plan development . Hox genes are found in the Placozoa, Cnidaria, and Bilateria. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian . 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of
4480-407: The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova . These fuse to form zygotes , which develop via mitosis 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. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form
4550-474: The sponges and placozoans —animal bodies are differentiated into tissues . These include muscles , which enable locomotion, and nerve tissues , which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians). Nearly all animals make use of some form of sexual reproduction. They produce haploid gametes by meiosis ;
4620-527: The start of the Cambrian 538.8 million years ago . Three enigmatic species that are possible very early tunicates, and therefor deuterostomes, were also found from the Ediacaran period – Ausia fenestrata from the Nama Group of Namibia , the sac-like Yarnemia ascidiformis , and one from a second new Ausia -like genus from the Onega Peninsula of northern Russia , Burykhia hunti . Results of
4690-411: The structure at the base of the tree (dashed lines). Porifera [REDACTED] Ctenophora [REDACTED] Placozoa [REDACTED] Cnidaria [REDACTED] Xenacoelomorpha [REDACTED] Ambulacraria [REDACTED] Chordata [REDACTED] Ecdysozoa [REDACTED] Spiralia [REDACTED] An alternative phylogeny, from Kapli and colleagues (2021), proposes
4760-511: 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 became clearly established in the fossil record as marine species during
4830-408: The tail. The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used. Craniates all have distinct skulls . They include the hagfish , which have no vertebrae . Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes , are by their tails". Most craniates are vertebrates , in which
4900-467: The total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. 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. 3,000–6,500 4,000–25,000 Evidence of animals is found as long ago as the Cryogenian period. 24-Isopropylcholestane (24-ipc) has been found in rocks from roughly 650 million years ago; it
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