Animal
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. 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.
For other uses, see Animal (disambiguation).
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 is known as zoology, and the study of animal behaviors is known as ethology.
Most living animal species belong to the infrakingdom Bilateria, a highly proliferative clade whose members have a bilaterally symmetric body plan. The vast majority belong to two large superphyla: the protostomes, which includes organisms such as the arthropods, molluscs, flatworms, annelids and nematodes; and the deuterostomes, which include the echinoderms, hemichordates and chordates, the latter of which contains the vertebrates. The simple Xenacoelomorpha have an uncertain position within Bilateria.
Animals first appear in the fossil record in the late Cryogenian period, and diversified in the subsequent Ediacaran. Earlier evidence of animals is still controversial; the sponge-like organism Otavia has been dated all the way back to the Tonian period, but its identity as an animal is heavily contested.[4] Nearly all modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 539 million years ago (Mya), and most classes during the Ordovician radiation 485.4 Mya. 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 the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between taxa.
Humans make use of many other animal species for food (including meat, eggs and dairies), 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 the earliest times, and are frequently featured in mythology, religion, arts, literature, heraldry, politics and sports.
Etymology
The word animal comes from the Latin noun animal of the same meaning, which is itself derived from Latin animalis 'having breath or soul'.[5] The biological definition includes all members of the kingdom Animalia.[6] In colloquial usage, the term animal is often used to refer only to nonhuman animals.[7][8][9][10] 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'.[11][12]
Dickinsonia costata from the Ediacaran biota (c. 635–542 mya) is one of the earliest animal species known.[93]
![Dickinsonia costata from the Ediacaran biota (c. 635–542 mya) is one of the earliest animal species known.[93]](http://upload.wikimedia.org/wikipedia/commons/thumb/f/fb/DickinsoniaCostata.jpg/320px-DickinsoniaCostata.jpg)
![Auroralumina attenboroughii, an Ediacaran predator (c. 560 mya)[100]](http://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Auroralumina_attenboroughii_reconstruction.jpg/209px-Auroralumina_attenboroughii_reconstruction.jpg)
Anomalocaris canadensis is one of the many animal species that emerged in the Cambrian explosion, starting some 539 mya, and found in the fossil beds of the Burgess shale.
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 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.[88][89]
The first body fossils of animals appear in the Ediacaran, represented by forms such as Charnia and Spriggina. It had long been doubted whether these fossils truly represented animals,[90][91][92] but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature.[93] 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.[94]
Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 539 million years ago, in beds such as the Burgess shale.[95] 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 the fossil record, rather than showing that all these animals appeared simultaneously.[96][97][98][99] 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.[100]
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.[101] 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.[102]
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.[103] However, similar tracks are produced by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution.[104][105] Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals.[106] 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.[107][108]