Animal

       Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Th eir body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also heterotrophs, meaning they must ingest other organisms or their products for sustenance.

Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.

Etymology

       The word "animal" comes from the Latin word animalis, meaning "having breath".In everyday colloquial usage, the word usually refers to non-human animals.[2] Sometimes, only closer relatives of humans such as mammals and other vertebrates are meant in colloquial use.[3] The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects and humans.

Characteristics

      Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and mostly multicellular,[5] which separates them from bacteria and most protists. They are heterotrophic,[6] generally digesting food in an internal chamber, which separates them from plants and algae.[7] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.[8] All animals are motile,[9] if only at certain life stages. In most animals, embryos pass through a blastula stage,[10] which is a characteristic exclusive to animals.

Structure

      With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals. Typically, there is also an internal digestive chamber, with one or two openings.[11] Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.

      All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.[13] This may be calcified to form structures like shells, bones, and spicules.[14] During development, it forms a relatively flexible framework[15] upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth.[11] Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.

Reproduction and development

    Nearly all animals undergo some form of sexual reproduction.[17] They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.[18] These fuse to form zygotes, which develop into new individuals.

Many animals are also capable of asexual reproduction.[20] This may take place through parthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation.

       A zygote initially develops into a hollow sphere, called a blastula,[22] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.[23] In most other groups, the blastula undergoes more complicated rearrangement.[24] It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm.[25] In most cases, a mesoderm also develops between them.[26] These germ layers then differentiate to form tissues and organs.

Food and energy sourcing

        All animals are heterotrophs, meaning that they feed directly or indirectly on other living things.[28] They are often further subdivided into groups such as carnivores, herbivores, omnivores, and parasites.

Predation is a biological interaction where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked).[30] Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey.[31] The other main category of consumption is detritivory, the consumption of dead organic matter.[32] It can at times be difficult to separate the two feeding behaviours, for example, where parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations.

        Most animals indirectly use the energy of sunlight by eating plants or plant-eating animals. Most plants use light to convert inorganic molecules in their environment into organic molecules, such as simple sugars, in photosynthesis. Starting with the molecules carbon dioxide (CO2) and water (H2O), photosynthesis converts the energy of sunlight into chemical energy stored as reduced carbon (e.g., glucose) and releases molecular oxygen. These sugars are then used as the building blocks for plant growth.[11] When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal.[34] They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion.[35] This process is known as glycolysis.

Animals living close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight.[37] Instead chemosynthetic archaea and bacteria form the base of the food chain.

Origin and fossil record

         Animals are generally considered to have evolved from a flagellated eukaryote.[40] Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges.[41] Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists.[42] The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.

The first fossils that might represent animals appear in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia.[44] These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.

         The next oldest possible animal fossils are found towards the end of the Precambrian, around 610 million years ago, and are known as the Ediacaran or Vendian biota.[45] These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all.

Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago.[47] It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible.

        Some paleontologists suggest that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.[48] Trace fossils such as tracks and burrows found in the Tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as earthworms.[49] During the beginning of the Tonian period around 1 billion years ago, there was a decrease in Stromatolite diversity, which may indicate the appearance of grazing animals, since stromatolite diversity increased when grazing animals went extinct at the End Permian and End Ordovician extinction events, and decreased shortly after the grazer populations recovered. However the discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist Gromia sphaerica casts doubt on their interpretation as evidence of early animal evolution.