Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals Heterotroph, breathe oxygen, are Motility, can reproduce sexually, and go through an ontogenetic stage in which their body consists of a hollow sphere of cells, the blastula, during Embryogenesis. Over 1.5 million Extant taxon animal species have been described—of which around 1 million are Insecta—but it has been estimated there are over 7 million animal species in total. Animals range in length from to . They have Ecology with each other and their environments, forming intricate . The scientific study of animals is known as zoology.
Most living animal species are in Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes, containing animals such as , , , and , and the deuterostomes, containing the and the , the latter including the . Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal Phylum became clearly established in the fossil record as Marine life during the Cambrian explosion, which began around 539 million years ago. 6,331 groups of common to all living animals have been identified; these may have arisen from a single common ancestor that lived Cryogenian.
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 Phylum by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous for 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 relationships between Taxon.
make use of many animal species, such as for food (including meat, milk, and ), for materials (such as leather and wool), as , and as including for transport. have been Hunting dog, as have Falconry, while many terrestrial and were hunted for sports. Nonhuman animals have appeared in art from the earliest times and are featured in mythology and religion.
With few exceptions—in particular, the sponges and —animal bodies are differentiated into tissues. These include , which enable locomotion, and , which transmit signals and coordinate the body. Typically, there is also an internal Digestion chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).
Repeated instances of inbreeding during sexual reproduction generally leads to inbreeding depression within 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 ; or parthenogenesis, where fertile eggs are produced without mating, such as in .
Most animals rely on the biomass and energy produced by plants through photosynthesis. Herbivores eat plant material directly, while carnivores, and other animals on higher typically acquire it indirectly by eating other animals. Animals oxidize , , , and other biomolecules, which allows the animal to grow and to sustain biological processes such as locomotion. Animals living close to hydrothermal vents and on the dark Seabed consume organic matter of archaea and bacteria produced in these locations through chemosynthesis (by oxidizing inorganic compounds, such as hydrogen sulfide).
Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as , probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician. such as the Sarcopterygii Tiktaalik started to move on to land in the late Devonian, about 375 million years ago. Animals occupy virtually all of earth's and microhabitats, including salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of animals, plants, fungi and rocks. Animals are however not particularly Thermophile; very few of them can survive at constant temperatures above . Only very few species of animals (mostly nematodes) inhabit the most extreme cold deserts of continental Antarctica.
Arthropods | 1,257,000 | 1,000,000 (insects) Stork notes that 1m insects have been named, making much larger predicted estimates. | >40,000 (Malac- ostraca) | 94,000 | Yes | >45,000 | |
Molluscs | 85,000 107,000 | 35,000 | 60,000 | 5,000 12,000 | Yes | >5,600 | |
Chordates | >70,000 | 23,000 | 13,000 | 18,000 9,000 | Yes | 40 (catfish) | |
Platyhelminthes | 29,500 | Yes | Yes | 1,300 | Yes 3,000–6,500 | >40,000 4,000–25,000 | |
Nematodes | 25,000 | Yes (soil) | 4,000 | 2,000 | 11,000 | 14,000 | |
17,000 | Yes (soil) | Yes | 1,750 | Yes | 400 | ||
Cnidaria | 16,000 | Yes | Yes (few) | Yes | >1,350 (Myxozoa) | ||
Sponges | 10,800 | Yes | 200–300 | Yes | Yes | ||
Echinoderms | 7,500 | 7,500 | Yes | ||||
Bryozoa | 6,000 | Yes | 60–80 | Yes | |||
2,000 | >400 | 2,000 | Yes | ||||
Animals are found as long ago as the Ediacaran biota, towards the end of the Precambrian, and possibly somewhat earlier. It had long been doubted whether these life-forms included 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 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 , , , , , and , along with numerous now-extinct forms such as the Anomalocaris. The apparent suddenness of the event may however be an artefact of the fossil record, rather than showing that all these animals appeared simultaneously.
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.
These genes are found in the Placozoa and the higher animals, the Bilateria. 6,331 groups of common to all living animals have been identified; these may have arisen from a single common ancestor that lived Cryogenian in the Precambrian. 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of 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.
The phylogenetic tree (of major lineages only) indicates approximately how many millions of years ago () the lineages split.
The Ctenophora (comb jellies) and Cnidaria (which includes jellyfish, , and corals) are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus. Animals in both phyla have distinct tissues, but these are not organised into organs. They are diploblastic, having only two main germ layers, ectoderm and endoderm. The tiny are similar, but they do not have a permanent digestive chamber.
Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with and a mouth. Many bilaterians have a combination of circular that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis. They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), while some Helminths have extremely simplified body structures.
Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, the protostomes and the deuterostomes. The basalmost bilaterians are the Xenacoelomorpha.
The main deuterostome phyla are the Echinodermata and the Chordata. Echinoderms are exclusively marine and include starfish, , and . The chordates are dominated by the vertebrates (animals with Vertebral column), which consist of , , , , and mammals. The deuterostomes also include the Hemichordata (acorn worms).
The Lophotrochozoa includes the , , , , bryozoa and Entoprocta. The molluscs, the second-largest animal phylum by number of described species, includes , , and , while the annelids are the segmented worms, such as , , and . These two groups have long been considered close relatives because they share trochophore larvae.
In 1758, Carl Linnaeus created the first hierarchical classification in his Systema Naturae. In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the chordate, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Jean-Baptiste de Lamarck, who called the Vermes une espèce de chaos (a chaotic mess) and split the group into three new phyla: worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique, Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely , annelids, crustaceans, arachnids, insects, worms, Radiata, polyps, and .
In his 1817 Le Règne Animal, Georges Cuvier used comparative anatomy to group the animals into four embranchements ("branches" with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and zoophytes (echinoderms, cnidaria and other forms).
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. The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia.
Invertebrates including , , and bivalve or gastropod molluscs are hunted or farmed for food. , cattle, sheep, , and other animals are raised as livestock for meat across the world.
Animal fibres such as wool are used to make textiles, while animal have been used as lashings and bindings, and leather is widely used to make shoes and other items. Animals have been hunted and farmed for their fur to make items such as coats and hats. Dyestuffs including carmine (cochineal), shellac, and kermes have been made from the bodies of insects. Working animals including cattle and horses have been used for work and transport from the first days of agriculture.
Animals such as the fruit fly Drosophila melanogaster serve a major role in science as model organism. Animals have been used to create since their discovery in the 18th century. Some medicines such as the cancer drug Yondelis are based on or other molecules of animal origin.
People have used to help chase down and retrieve animals, and birds of prey to catch birds and mammals, while tethered have been used to catch fish. Poison dart frogs have been used to poison the tips of blowdart.
A wide variety of animals are kept as pets, from invertebrates such as tarantulas and octopuses, insects including , reptiles such as and , and birds including Domestic canary, , and all finding a place. However, the most kept pet species are mammals, namely , , and . There is a tension between the role of animals as companions to humans, and their existence as animal rights of their own.
A wide variety of terrestrial and aquatic animals are hunted for sport.
Animals including insects and mammals feature in mythology and religion. In both Japan and Europe, a butterfly was seen as the personification of a person's soul,Hutchins, M., Arthur V. Evans, Rosser W. Garrison and Neil Schlager (Eds) (2003) Grzimek's Animal Life Encyclopedia, 2nd edition. Volume 3, Insects. Gale, 2003. while the scarab beetle was sacred in ancient Egypt. Among the mammals, cattle, deer, Horse worship, lions, bats, bear worship, and wolves are the subjects of myths and worship. The signs of the Western and are based on animals.
Protostomes and deuterostomes
Ecdysozoa
Spiralia
History of classification
In human culture
Practical uses
Symbolic uses
See also
Notes
External links
target="_blank" rel="nofollow"> ARKive – multimedia database of endangered/protected species
|
|