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93-418: Animals are multicellular , eukaryotic organisms in 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

186-416: 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 a population due to

279-559: 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

372-457: A motile single-celled propagule ; this single cell asexually reproduces by undergoing 2–5 rounds of mitosis as a small clump of non-motile cells, then all cells become single-celled propagules and the clump dissolves. With a few generations under Paramecium predation, the "clump" becomes a persistent structure: only some cells become propagules. Some populations go further and evolved multi-celled propagules: instead of peeling off single cells from

465-418: A separation between a sterile somatic cell line and a germ cell line evolved. However, Weismannist development is relatively rare (e.g., vertebrates, arthropods, Volvox ), as a great part of species have the capacity for somatic embryogenesis (e.g., land plants, most algae, many invertebrates). One hypothesis for the origin of multicellularity is that a group of function-specific cells aggregated into

558-531: 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

651-410: A single cell organism to one of many cells. Genes borrowed from viruses and mobile genetic elements (MGEs) have recently been identified as playing a crucial role in the differentiation of multicellular tissues and organs and even in sexual reproduction, in the fusion of egg cells and sperm. Such fused cells are also involved in metazoan membranes such as those that prevent chemicals from crossing

744-404: 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. Multicellular Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells. Colonial organisms are

837-414: A single species. Although such symbiosis is theorized to have occurred (e.g., mitochondria and chloroplasts in animal and plant cells— endosymbiosis ), it has happened only extremely rarely and, even then, the genomes of the endosymbionts have retained an element of distinction, separately replicating their DNA during mitosis of the host species. For instance, the two or three symbiotic organisms forming

930-407: A slug-like mass called a grex , which moved as a multicellular unit. This is essentially what slime molds do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte . A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected cells in one organism (this mechanism

1023-634: A species of human-like creatures with exaggerated or non-human characteristics, despite finding limited evidence. He divided Homo sapiens into four varieties , corresponding with the four known continents and four temperaments (some editions also classify Ferus wild children and Monstrosus monstrous to accommodate adaptations to extreme environments ). The first edition included Europæus albescens (whitish Europeans), Americanus rubescens (reddish Americans), Asiaticus fuscus (tawny Asians), and Africanus nigriculus (blackish Africans). The tenth edition solidified these descriptions by removing

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1116-460: 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 ,

1209-431: 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

1302-413: Is also considered probable in some green algae (e.g., Chlorella vulgaris and some Ulvophyceae ). In other groups, generally parasites, a reduction of multicellularity occurred, in the number or types of cells (e.g., the myxozoans , multicellular organisms, earlier thought to be unicellular, are probably extremely reduced cnidarians ). Multicellular organisms, especially long-living animals, face

1395-565: Is also typically considered to involve cellular differentiation . The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently in 16 different protoctistan phyla. For instance, during food shortages the amoeba Dictyostelium groups together in a colony that moves as one to a new location. Some of these amoeba then slightly differentiate from each other. Other examples of colonial organisation in protista are Volvocaceae , such as Eudorina and Volvox ,

1488-520: Is considered the starting point of zoological nomenclature . In 1766–1768 Linnaeus published the much enhanced 12th edition , the last under his authorship. Another again enhanced work in the same style titled " Systema Naturae " was published by Johann Friedrich Gmelin between 1788 and 1793. Since at least the early 20th century, zoologists have commonly recognized this as the last edition belonging to this series. Linnaeus (later known as "Carl von Linné", after his ennoblement in 1761) published

1581-448: Is debatable: The vast majority of living organisms are single celled, and even in terms of biomass, single celled organisms are far more successful than animals, although not plants. Rather than seeing traits such as longer lifespans and greater size as an advantage, many biologists see these only as examples of diversity, with associated tradeoffs. During the evolutionary transition from unicellular organisms to multicellular organisms,

1674-455: 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

1767-670: Is inexact, as living multicellular organisms such as animals and plants are more than 500 million years removed from their single-cell ancestors. Such a passage of time allows both divergent and convergent evolution time to mimic similarities and accumulate differences between groups of modern and extinct ancestral species. Modern phylogenetics uses sophisticated techniques such as alloenzymes , satellite DNA and other molecular markers to describe traits that are shared between distantly related lineages. The evolution of multicellularity could have occurred in several different ways, some of which are described below: This theory suggests that

1860-450: 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 :

1953-539: Is observable in Drosophila ). A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues. This is what plant and animal embryos do as well as colonial choanoflagellates . Because the first multicellular organisms were simple, soft organisms lacking bone, shell, or other hard body parts, they are not well preserved in

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2046-794: 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

2139-420: 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

2232-425: Is presumed land-evolved - multicellularity occurs by cells separating and then rejoining (e.g., cellular slime molds ) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division. The mechanism of this latter colony formation can be as simple as incomplete cytokinesis , though multicellularity

2325-545: 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

2418-873: The Systema Vegetabilium , rather confusingly labelled the 13th edition. Meanwhile, a 13th edition of the entire Systema appeared in parts between 1788 and 1793. It was as the Systema Vegetabilium that Linnaeus' work became widely known in England following translation from the Latin by the Lichfield Botanical Society , as A System of Vegetables (1783–1785). In his Imperium Naturæ , Linnaeus established three kingdoms, namely Regnum Animale , Regnum Vegetabile and Regnum Lapideum . This approach,

2511-516: The Cambrian explosion shortly after the Marinoan. The predation hypothesis suggests that to avoid being eaten by predators, simple single-celled organisms evolved multicellularity to make it harder to be consumed as prey. Herron et al.  performed laboratory evolution experiments on the single-celled green alga, Chlamydomonas reinhardtii , using paramecium as a predator. They found that in

2604-567: 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

2697-481: The Cryogenian period and consisted of two global glaciation events known as the Sturtian and Marinoan glaciations. Xiao et al . suggest that between the period of time known as the " Boring Billion " and the snowball Earth, simple life could have had time to innovate and evolve, which could later lead to the evolution of multicellularity. The snowball Earth hypothesis in regards to multicellularity proposes that

2790-586: The Ediacaran is not necessary for complex life and therefore is unlikely to have been the driving factor for the origin of multicellularity. A snowball Earth is a geological event where the entire surface of the Earth is covered in snow and ice. The term can either refer to individual events (of which there were at least two) or to the larger geologic period during which all the known total glaciations occurred. The most recent snowball Earth took place during

2883-558: 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

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2976-481: The Systema Naturæ (1758), are accepted to be among the starting points of nomenclature. Most of his names for species and genera were published at very early dates, and thus take priority over those of other, later authors. In zoology there is one exception, which is a monograph on Swedish spiders, Svenska Spindlar , published by Carl Clerck in 1757, so the names established there take priority over

3069-483: 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

3162-643: The Xenophyophorea that can reach 20 cm. Multicellularity has evolved independently at least 25 times in eukaryotes , and also in some prokaryotes , like cyanobacteria , myxobacteria , actinomycetes , Magnetoglobus multicellularis or Methanosarcina . However, complex multicellular organisms evolved only in six eukaryotic groups: animals , symbiomycotan fungi , brown algae , red algae , green algae , and land plants . It evolved repeatedly for Chloroplastida (green algae and land plants), once for animals, once for brown algae, three times in

3255-665: 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

3348-472: 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

3441-429: The fungi ( chytrids , ascomycetes , and basidiomycetes ) and perhaps several times for slime molds and red algae. The first evidence of multicellular organization, which is when unicellular organisms coordinate behaviors and may be an evolutionary precursor to true multicellularity, is from cyanobacteria -like organisms that lived 3.0–3.5 billion years ago. To reproduce, true multicellular organisms must solve

3534-584: The ligature æ ) is one of the major works of the Swedish botanist, zoologist and physician Carl Linnaeus (1707–1778) and introduced the Linnaean taxonomy . Although the system, now known as binomial nomenclature , was partially developed by the Bauhin brothers, Gaspard and Johann , Linnaeus was the first to use it consistently throughout his book. The first edition was published in 1735. The full title of

3627-493: 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

3720-577: The placenta and the brain body separation. Two viral components have been identified. The first is syncytin , which came from a virus. The second identified in 2002 is called EFF-1 , which helps form the skin of Caenorhabditis elegans , part of a whole family of FF proteins. Felix Rey, of the Pasteur Institute in Paris, has constructed the 3D structure of the EFF-1 protein and shown it does

3813-500: 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

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3906-716: The "ish" qualifiers (e.g. albus "white" instead of albescens "whitish") and revising the characterization of Asiaticus from fuscus (tawny) to luridus (pale yellow). It also incorporates behavioral and cultural traits that the Linnean Society recognizes as having cemented colonial stereotypes and provided the foundations for scientific racism . The orders and classes of plants, according to his Systema Sexuale , were never intended to represent natural groups (as opposed to his ordines naturales in his Philosophia Botanica ) but only for use in identification. They were used in that sense well into

3999-685: The 10th edition (1758), which was the most important one, was Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis , which appeared in English in 1806 with the title: "A General System of Nature, Through the Three Grand Kingdoms of Animals, Vegetables, and Minerals, Systematically Divided Into their Several Classes, Orders, Genera, Species, and Varieties, with their Habitations, Manners, Economy, Structure and Peculiarities". The tenth edition of this book (1758)

4092-475: The 1753 publication of Species Plantarum . The system eventually developed into modern Linnaean taxonomy , a hierarchically organized biological classification . After Linnaeus' health declined in the early 1770s, publication of editions of Systema Naturae went in two directions. Another Swedish scientist, Johan Andreas Murray issued the Regnum Vegetabile section separately in 1774 as

4185-734: The 19th century. The Linnaean classes for plants, in the Sexual System, were: Linnaeus's taxonomy of minerals has long since fallen out of use. In the 10th edition, 1758, of the Systema Naturæ , the Linnaean classes were: Gmelin's thirteenth ( decima tertia ) edition of Systema Naturae (1788–1793) should be carefully distinguished from the more limited Systema Vegetabilium first prepared and published by Johan Andreas Murray in 1774 (but labelled as "thirteenth edition"). The dates of publication for Gmelin's edition were

4278-456: The Animal, Vegetable and Mineral Kingdoms, survives until today in the popular mind, notably in the form of parlour games: "Is it animal, vegetable or mineral ?" The classification was based on five levels: kingdom , class , order , genus , and species . While species and genus was seen as God-given (or "natural"), the three higher levels were seen by Linnaeus as constructs. The concept behind

4371-712: 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

4464-711: 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

4557-531: The Cryogenian period in Earth's history could have been the catalyst for the evolution of complex multicellular life. Brocks suggests that the time between the Sturtian Glacian and the more recent Marinoan Glacian allowed for planktonic algae to dominate the seas making way for rapid diversity of life for both plant and animal lineages. Complex life quickly emerged and diversified in what is known as

4650-503: The Linnean names. His exceptional importance to science was less in the value of his taxonomy, more his deployment of skilful young students abroad to collect specimens. At the close of the 18th century, his system had effectively become the standard for biological classification. Only in the animal kingdom is the higher taxonomy of Linnaeus still more or less recognizable and some of these names are still in use, but usually not quite for

4743-475: The appearance of metazoans are deregulated in cancer cells, including genes that control cell differentiation , adhesion and cell-to-cell communication . There is a discussion about the possibility of existence of cancer in other multicellular organisms or even in protozoa . For example, plant galls have been characterized as tumors , but some authors argue that plants do not develop cancer. In some multicellular groups, which are called Weismannists ,

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4836-425: The challenge of cancer , which occurs when cells fail to regulate their growth within the normal program of development. Changes in tissue morphology can be observed during this process. Cancer in animals ( metazoans ) has often been described as a loss of multicellularity and an atavistic reversion towards a unicellular-like state. Many genes responsible for the establishment of multicellularity that originated around

4929-704: The clump, the clump now reproduces by peeling off smaller clumps. Multicellularity allows an organism to exceed the size limits normally imposed by diffusion : single cells with increased size have a decreased surface-to-volume ratio and have difficulty absorbing sufficient nutrients and transporting them throughout the cell. Multicellular organisms thus have the competitive advantages of an increase in size without its limitations. They can have longer lifespans as they can continue living when individual cells die. Multicellularity also permits increasing complexity by allowing differentiation of cell types within one organism. Whether all of these can be seen as advantages however

5022-449: The composite lichen , although dependent on each other for survival, have to separately reproduce and then re-form to create one individual organism once more. This theory states that a single unicellular organism, with multiple nuclei , could have developed internal membrane partitions around each of its nuclei. Many protists such as the ciliates or slime molds can have several nuclei, lending support to this hypothesis . However,

5115-461: 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

5208-400: 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

5301-459: The expression of genes associated with reproduction and survival likely changed. In the unicellular state, genes associated with reproduction and survival are expressed in a way that enhances the fitness of individual cells, but after the transition to multicellularity, the pattern of expression of these genes must have substantially changed so that individual cells become more specialized in their function relative to reproduction and survival. As

5394-505: The first edition (1735) to 2,400 pages in the 12th edition (1766–1768). Also, as the work progressed, he made changes: in the first edition, whales were classified as fishes , following the work of Linnaeus' friend and "father of ichthyology " Peter Artedi ; in the 10th edition, published in 1758, whales were moved into the mammal class. In this same edition, he introduced two-part names (see binomen ) for animal species, something that he had done for plant species (see binary name ) in

5487-700: The first edition of Systema Naturae in the year 1735, during his stay in the Netherlands . As was customary for the scientific literature of its day, the book was published in Latin . In it, he outlined his ideas for the hierarchical classification of the natural world, dividing it into the animal kingdom ( regnum animale ), the plant kingdom ( regnum vegetabile ), and the " mineral kingdom " ( regnum lapideum ). Linnaeus's Systema Naturae lists only about 10,000 species of organisms, of which about 6,000 are plants and 4,236 are animals. According to

5580-570: 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

5673-446: The first multicellular organisms occurred from symbiosis (cooperation) of different species of single-cell organisms, each with different roles. Over time these organisms would become so dependent on each other that they would not be able to survive independently, eventually leading to the incorporation of their genomes into one multicellular organism. Each respective organism would become a separate lineage of differentiated cells within

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5766-445: 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,

5859-514: 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

5952-736: The fossil record. One exception may be the demosponge , which may have left a chemical signature in ancient rocks. The earliest fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon ( Gabonionta ). The Doushantuo Formation has yielded 600 million year old microfossils with evidence of multicellular traits. Until recently, phylogenetic reconstruction has been through anatomical (particularly embryological ) similarities. This

6045-496: The four known continents and temperaments . The tenth edition expanded on these varieties with behavioral and cultural traits that the Linnean Society acknowledges as having cemented colonial stereotypes and provided the foundations for scientific racism . As a result of the popularity of the work, and the number of new specimens sent to him from around the world, Linnaeus kept publishing new and ever-expanding editions of his work. It grew from eleven very large pages in

6138-734: 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

6231-436: The historian of botany William T. Stearn , "Even in 1753 he believed that the number of species of plants in the whole world would hardly reach 10,000; in his whole career he named about 7,700 species of flowering plants." Linnaeus developed his classification of the plant kingdom in an attempt to describe and understand the natural world as a reflection of the logic of God 's creation. His sexual system , where species with

6324-1082: 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

6417-438: 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

6510-453: The latter of which consists of up to 500–50,000 cells (depending on the species), only a fraction of which reproduce. For example, in one species 25–35 cells reproduce, 8 asexually and around 15–25 sexually. However, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular". Some authors suggest that

6603-745: 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

6696-509: The multicellular organism emerged, gene expression patterns became compartmentalized between cells that specialized in reproduction ( germline cells) and those that specialized in survival ( somatic cells ). As the transition progressed, cells that specialized tended to lose their own individuality and would no longer be able to both survive and reproduce outside the context of the group. Systema Naturae Systema Naturae (originally in Latin written Systema Naturæ with

6789-453: The newly created species. This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between clown fish and Riterri sea anemones . In these cases, it is extremely doubtful whether either species would survive very long if the other became extinct. However, the problem with this theory is that it is still not known how each organism's DNA could be incorporated into one single genome to constitute them as

6882-614: 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

6975-527: The origin of multicellularity, at least in Metazoa, occurred due to a transition from temporal to spatial cell differentiation , rather than through a gradual evolution of cell differentiation, as affirmed in Haeckel 's gastraea theory . About 800 million years ago, a minor genetic change in a single molecule called guanylate kinase protein-interaction domain (GK-PID) may have allowed organisms to go from

7068-524: 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;

7161-412: The other way round. To be deemed valid, this theory needs a demonstrable example and mechanism of generation of a multicellular organism from a pre-existing syncytium. The colonial theory of Haeckel , 1874, proposes that the symbiosis of many organisms of the same species (unlike the symbiotic theory , which suggests the symbiosis of different species) led to a multicellular organism. At least some - it

7254-506: The oxygen available in the atmosphere of early Earth could have been the limiting factor for the emergence of multicellular life. This hypothesis is based on the correlation between the emergence of multicellular life and the increase of oxygen levels during this time. This would have taken place after the Great Oxidation Event but before the most recent rise in oxygen. Mills concludes that the amount of oxygen present during

7347-516: 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

7440-645: The presence of this predator, C. reinhardtii does indeed evolve simple multicellular features. It is impossible to know what happened when single cells evolved into multicellular organisms hundreds of millions of years ago. However, we can identify mutations that can turn single-celled organisms into multicellular ones. This would demonstrate the possibility of such an event. Unicellular species can relatively easily acquire mutations that make them attach to each other—the first step towards multicellularity. Multiple normally unicellular species have been evolved to exhibit such early steps: C. reinhartii normally starts as

7533-773: The problem of regenerating a whole organism from germ cells (i.e., sperm and egg cells), an issue that is studied in evolutionary developmental biology . Animals have evolved a considerable diversity of cell types in a multicellular body (100–150 different cell types), compared with 10–20 in plants and fungi. Loss of multicellularity occurred in some groups. Fungi are predominantly multicellular, though early diverging lineages are largely unicellular (e.g., Microsporidia ) and there have been numerous reversions to unicellularity across fungi (e.g., Saccharomycotina , Cryptococcus , and other yeasts ). It may also have occurred in some red algae (e.g., Porphyridium ), but they may be primitively unicellular. Loss of multicellularity

7626-399: The result of many identical individuals joining together to form a colony . However, it can often be hard to separate colonial protists from true multicellular organisms, because the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular". There are also macroscopic organisms that are multinucleate though technically unicellular, such as

7719-480: The same groups as used by Linnaeus. He divided the Animal Kingdom into six classes; in the tenth edition (1758), these were: Linnaeus was one of the first scientists to classify humans as primates (originally Anthropomorpha for "manlike"), eliciting some controversy for placing people among animals and thus not ruling over nature . He distinguished humans ( Homo sapiens ) from Homo troglodytes ,

7812-624: The same number of stamens were treated in the same group, was convenient but in his view artificial. Linnaeus believed in God's creation and that there were no deeper relationships to be expressed. He is frequently quoted as saying: "God created, Linnaeus organized" (Latin: Deus creavit, Linnaeus disposuit ). The classification of animals was more natural. For instance, humans were for the first time placed together with other primates , as Anthropomorpha . They were also divided into four varieties , as distinguished by skin color and corresponding with

7905-544: The set ranks being applied to all groups was to make a system that was easy to remember and navigate, a task which most say he succeeded in. Linnaeus's work had a huge impact on science; it was indispensable as a foundation for biological nomenclature , now regulated by the Nomenclature Codes . Two of his works, the first edition of the Species Plantarum (1753) for plants and the 10th edition of

7998-425: The simple presence of multiple nuclei is not enough to support the theory. Multiple nuclei of ciliates are dissimilar and have clear differentiated functions. The macro nucleus serves the organism's needs, whereas the micro nucleus is used for sexual reproduction with exchange of genetic material. Slime molds syncitia form from individual amoeboid cells, like syncitial tissues of some multicellular organisms, not

8091-746: 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

8184-403: 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

8277-471: 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 ;

8370-410: 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

8463-521: 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

8556-461: 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

8649-417: The work of linking one cell to another, in viral infections. The fact that all known cell fusion molecules are viral in origin suggests that they have been vitally important to the inter-cellular communication systems that enabled multicellularity. Without the ability of cellular fusion, colonies could have formed, but anything even as complex as a sponge would not have been possible. This theory suggests that

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