Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may be referred to as lowland equatorial evergreen rainforest. True rainforests are found between 10 degrees north and south of the equator. Within the World Wildlife Fund's biome classification, tropical rainforests are a type of tropical moist broadleaf forest that includes the more extensive seasonal tropical forests. Tropical rainforests can be characterized in two words: wet. Mean monthly temperatures exceed 18 °C during all months of the year. Average annual rainfall is no less than 1,680 mm and can exceed 10 m although it lies between 1,750 mm and 3,000 mm; this high level of precipitation results in poor soils due to leaching of soluble nutrients in the ground. Tropical rainforests exhibit high levels of biodiversity. Around 40% to 75% of all biotic species are indigenous to the rainforests. Rainforests are home to half of all the living plant species on the planet.
Two-thirds of all flowering plants can be found in rainforests. A single hectare of rainforest may contain 42,000 different species of insect, up to 807 trees of 313 species and 1,500 species of higher plants. Tropical rainforests have been called the "world's largest pharmacy", because over one quarter of natural medicines have been discovered within them, it is that there may be many millions of species of plants and microorganisms still undiscovered in tropical rainforests. Tropical rainforests are among the most threatened ecosystems globally due to large-scale fragmentation as a result of human activity. Habitat fragmentation caused by geological processes such as volcanism and climate change occurred in the past, have been identified as important drivers of speciation. However, fast human driven habitat destruction is suspected to be one of the major causes of species extinction. Tropical rain forests have been subjected to heavy logging and agricultural clearance throughout the 20th century, the area covered by rainforests around the world is shrinking.
Tropical rainforests have existed on earth for hundreds of millions of years. Most tropical rainforests today are on fragments of the Mesozoic era supercontinent of Gondwana; the separation of the landmass resulted in a great loss of amphibian diversity while at the same time the drier climate spurred the diversification of reptiles. The division left tropical rainforests located in five major regions of the world: tropical America, Southeast Asia and New Guinea, with smaller outliers in Australia. However, the specifics of the origin of rainforests remain uncertain due to an incomplete fossil record. Several biomes may appear similar-to, or merge via ecotones with, tropical rainforest: Moist seasonal tropical forest Moist seasonal tropical forests receive high overall rainfall with a warm summer wet season and a cooler winter dry season; some trees in these forests drop some or all of their leaves during the winter dry season, thus they are sometimes called "tropical mixed forest". They are found in parts of South America, in Central America and around the Caribbean, in coastal West Africa, parts of the Indian subcontinent, across much of Indochina.
Montane rainforests These are found in cooler-climate mountainous areas, becoming known as cloud forests at higher elevations. Depending on latitude, the lower limit of montane rainforests on large mountains is between 1500 and 2500 m while the upper limit is from 2400 to 3300 m. Flooded rainforests Tropical freshwater swamp forests, or "flooded forests", are found in Amazon basin and elsewhere. Rainforests are divided into different strata, or layers, with vegetation organized into a vertical pattern from the top of the soil to the canopy; each layer is a unique biotic community containing different plants and animals adapted for life in that particular strata. Only the emergent layer is unique to tropical rainforests, while the others are found in temperate rainforests; the forest floor, the bottom-most layer, receives only 2% of the sunlight. Only plants adapted to low light can grow in this region. Away from riverbanks and clearings, where dense undergrowth is found, the forest floor is clear of vegetation because of the low sunlight penetration.
This more open quality permits the easy movement of larger animals such as: ungulates like the okapi, Sumatran rhinoceros, apes like the western lowland gorilla, as well as many species of reptiles and insects. The forest floor contains decaying plant and animal matter, which disappears because the warm, humid conditions promote rapid decay. Many forms of fungi growing here help decay the plant waste; the understory layer lies between the forest floor. The understory is home to a number of birds, small mammals, insects and predators. Examples include leopard, poison dart frogs, ring-tailed coati, boa constrictor, many species of Coleoptera; the vegetation at this layer consists of shade-tolerant shrubs, small trees, large woody vines which climb into the trees to capture sunlight. Only about 5% of sunlight breaches the canopy to arrive at the understory causing true understory plants to grow to 3 m; as an
Republic of Formosa
The Republic of Formosa, was a short-lived republic that existed on the island of Taiwan in 1895 between the formal cession of Taiwan by the Qing Dynasty of China to the Empire of Japan by the Treaty of Shimonoseki and its being taken over by Japanese troops. The Republic was proclaimed on 23 May 1895 and extinguished on 21 October, when the Republican capital Tainan was taken over by the Japanese. Though sometimes claimed as the first Asian republic to have been proclaimed, it was predated by the Lanfang Republic in Borneo, established in 1777, as well as by the Republic of Ezo in Japan, established in 1869. In 1894, China and Japan went to war. In a few months the Japanese defeated China's Beiyang fleet, routed the Chinese armies in Manchuria, captured Port Arthur and Weihaiwei. Although nearly all the fighting took place in northern China, Japan had important territorial ambitions in southern China; as the war approached its end, the Japanese took steps to ensure that Taiwan would be ceded to Japan under the eventual peace treaty and that they were well placed militarily to occupy the island.
In March 1895 peace negotiations between Japan and China opened in the Japanese city of Shimonoseki. Although hostilities in northern China were suspended during these negotiations and the Pescadores were excluded from the scope of the armistice; this exclusion allowed the Japanese to mount a military operation against the Pescadores Islands in March 1895 without imperilling the negotiations. The Pescadores, lying midway between mainland China and Taiwan, were the key to a successful occupation of Taiwan. In a swift campaign in the last week of March the Japanese captured the islands, preventing further Chinese reinforcements from being sent across the Taiwan Strait to Taiwan; this brisk fait accompli influenced the peace negotiations, the ensuing Treaty of Shimonoseki, concluded on 17 April 1895, duly provided for the cession by China of Taiwan to Japan. On 10 May, Admiral Kabayama Sukenori was appointed the first Japanese governor-general of Taiwan; when the news of the treaty's contents reached Taiwan, a number of notables from central Taiwan led by Qiu Fengjia decided to resist the transfer of Taiwan to Japanese rule.
On 23 May, in Taipei, these men declared independence, proclaiming the establishment of a free and democratic Republic of Formosa. Tang Jingsong, the Qing governor-general of Taiwan, was prevailed upon to become the republic's first President, his old friend Liu Yongfu, the retired Black Flag Army commander who had become a national hero in China for his victories against the French in northern Vietnam a decade earlier, was invited to serve as Grand General of the Army. Chiu was appointed Grand Commander of Militia, with the power to raise local militia units throughout the island to resist the Japanese. On the Chinese mainland Zhang Zhidong, the powerful governor-general of Liangkiang, tacitly supported the Formosan resistance movement, the Republicans appointed Chen Jitong, a disgraced Chinese diplomat who understood European ways of thinking, as the Republic's foreign minister, his job would be to sell the Republic abroad. The declaration has not been preserved in its original Chinese version, although an English version of it was recorded by the American war correspondent James Wheeler Davidson, in Taipei when it was issued.
Davidson's version reads as follows: Official Declaration of Independence of the Republic of Formosa. The Japanese have affronted China by annexing our territory of Formosa, the supplications of us, the People of Formosa, at the portals of the Throne have been made in vain. We now learn. If we suffer this, the land of our hearths and homes will become the land of savages and barbarians, but if we do not suffer it, our condition of comparative weakness will not endure long. Frequent conferences have been held with the Foreign Powers, who all aver that the People of Formosa must establish their independence before the Powers will assist them. Now therefore we, the People of Formosa, are irrevocably resolved to die before we will serve the enemy, and we have in Council determined to convert the whole island of Formosa into a Republican state, that the administration of all our State affairs shall be organized and carried on by the deliberations and decisions of Officers publicly elected by us the People.
But as in this new enterprise there is needed, as well for the resistance of Japanese aggression as for the organization of the new administration, a man to have chief control, in whom authority shall centre, by whom the peace of our homesteads shall be assured—therefore, in view of the respect and admiration in which we have long held the Governor and Commander-in-Chief, Tang Ching Sung, we have in Council determined to raise him to the position of President of the Republic. An official seal has been cut, on the second day of fifth moon, at the ssu hour, it will be publicly presented with all respect by the notables and people of the whole of Formosa. At early dawn on that day, all of us, notables and people and merchants, artizans and tradesmen, must assemble at the Tuan Fang Meeting House, that we may in grave and solemn manner inaugurate this undertaking Let there be neither delay nor mistake. A Declaration of the whole of Formosa. An announcement by the whole of Formosa. Since the island had been ceded to Japan in the Treaty of Shimonoseki, Western powers were not in a position to recognize the Republic of Formosa as a legitimate government.
Acting under the authority of the new Republic, Chinese troops would be able to resist the Japanese in Taiwan
The Negrito are several different ethnic groups who inhabit isolated parts of a region known today as Austronesia. Their current populations include the Andamanese peoples of the Andaman and Nicobar Islands, the Semang ethnic groups of Peninsular Malaysia, the Maniq people of Southern Thailand, the Aeta people, Ati people, 30 other official recognized ethnic groups in the Philippines; the word Negrito is the Spanish diminutive of negro, used to mean "little black person". This usage was coined by 16th-century Spanish missionaries operating in the Philippines, was borrowed by other European travellers and colonialists across Austronesia to label various peoples perceived as sharing small physical stature and dark skin. Contemporary usage of an alternative Spanish epithet, Negrillos tended to bundle these peoples with the pygmy peoples of Central Africa, based on perceived similarities in stature and complexion; the appropriateness of using the label "Negrito" to bundle peoples of different ethnicities based on similarities in stature and complexion has been challenged.
Many online dictionaries give the plural in English as either "Negritos" or "Negritoes", without preference. The plural in Spanish is "Negritos". Most Negrito groups lived as hunter-gatherers, while some used agriculture. Today most Negrito tribes live assimilated to the majority population of their homeland. Discrimination and poverty are problems; the Y-chromosome Haplogroup C-M130, as seen, for example, in the Semang of Malaysia, Haplogroup D-M174 among Andaman Islanders, are more prominent among Negritos than the general populations surrounding them. Haplogroup O-P31 is common among Austroasiatic-speaking Negrito peoples, such as the Maniq and the Semang. Aeta men are of great interest to genetic and historical researchers because at least 83% of them belong to haplogroup K2b, in the form of its rare primary clades K2b1* and P*. Most Aeta males carry K-P397, otherwise uncommon in the Philippines and is associated with the indigenous peoples of Melanesia and Micronesia. Basal P * is rare outside some other groups within Maritime Southeast Asia.
Genetic research has shown that the Negritos have existed as a separate group for a long time, comparable to the Australoid and Southwest Pacific groups. This has been interpreted to the effect that they are remnants of the original expansion from Africa some 70,000 years ago. Studies in osteology, cranial shape and dental morphology have connected the Semang to Australoid populations, while connecting the Andamanese to Africans in craniometry and to South Asians in dental morphology, Philippine Negritos to Southeast Asians. A possible conclusion of this is that the dispersal of mitochondrial haplogroup B4a1a is connected to the distinction between Philippine and other Negritos. However, another study suggests that the Onge are "more related to Southeast Asians than they are to present-day South Asians", that the Great Andamanese "appear to have received a degree of recent admixture from adjacent regional populations but share a significant degree of genetic ancestry with Malaysian negrito groups".
Bulbeck noted that the Andamanese's nuclear DNA clusters with that of other Andamanese Islanders, as they carry Haplogroup D-M174 and maternal mitochondrial Haplogroup M unique to their own. However, this is a subclade of the D haplogroup which has not been seen outside of the Andamans, a fact that underscores the insularity of these tribes. Analysis of mtDNA, inherited by maternal descent, confirms the above results. All Onge belong to M32 mtDNA, a subgroup of M, unique to Onge people, their parental Y-DNA is Haplogroup D, only found in Asia. A study of human blood group systems and proteins in the 1950s suggested that the Andamanese peoples were more related to Oceanic peoples than African pygmy peoples. Genetic studies on Philippine Negritos, based on polymorphic blood enzymes and antigens, showed that they were similar to their surrounding populations. Negrito peoples may descend from Australoid-Melanesian settlers of Austronesia. Despite being isolated, the different peoples do share genetic similarities with their neighboring populations.
They show relevant phenotypic variations which require explanation. In contrast, a recent genetic study found that unlike other early groups in Malesia, Andamanese Negritos lack Denisovan hominin admixture in their DNA. Denisovan ancestry is found among indigenous Melanesian and Aboriginal Australian populations between 4–6%; some studies have suggested that each group should be considered separately, as the genetic evidence refutes the notion of a specific shared ancestry between the "Negrito" groups of the Andaman Islands, the Malay Peninsula, the Philippines. Indeed, this sentiment is echoed in a more recent work from 2013 which concludes that "at the current level of genetic resolution... There is no evidence of a single ancestral population for the different groups traditionally defined as'negritos'." A number of features would seem to suggest a common origin for the Negrito and Negrillo, including short stature, dark skin, scant body hair, occasional steatopygia. The claim that the Andamanese more resemble African pygmies than other Austronesian populations in their cranial morphology in a study of 1973 added some weight to this theory, before genetic studies pointed to a closer relationship with their neighbours.
Multiple studies show that Negritos from Southeast Asia to New Guinea share a closer cranial affinity with Australo-Melane
The Holocene is the current geological epoch. It began 11,650 cal years before present, after the last glacial period, which concluded with the Holocene glacial retreat; the Holocene and the preceding Pleistocene together form the Quaternary period. The Holocene has been identified with the current warm period, known as MIS 1, it is considered by some to be an interglacial period within the Pleistocene Epoch. The Holocene has seen the growth and impacts of the human species worldwide, including all its written history, development of major civilizations, overall significant transition toward urban living in the present. Human impacts on modern-era Earth and its ecosystems may be considered of global significance for future evolution of living species, including synchronous lithospheric evidence, or more hydrospheric and atmospheric evidence of human impacts. In July 2018, the International Union of Geological Sciences split the Holocene epoch into three distinct subsections, Greenlandian and Meghalayan, as proposed by International Commission on Stratigraphy.
The boundary stratotype of Meghalayan is a speleothem in Mawmluh cave in India, the global auxiliary stratotype is an ice core from Mount Logan in Canada. The name Holocene comes from the Ancient Greek words ὅλος and καινός, meaning "entirely recent", it is accepted by the International Commission on Stratigraphy that the Holocene started 11,650 cal years BP. The Subcommission on Quaternary Stratigraphy quotes Gibbard and van Kolfschoten in Gradstein Ogg and Smith in stating the term'Recent' as an alternative to Holocene is invalid and should not be used and observe that the term Flandrian, derived from marine transgression sediments on the Flanders coast of Belgium has been used as a synonym for Holocene by authors who consider the last 10,000 years should have the same stage-status as previous interglacial events and thus be included in the Pleistocene; the International Commission on Stratigraphy, considers the Holocene an epoch following the Pleistocene and the last glacial period. Local names for the last glacial period include the Wisconsinan in North America, the Weichselian in Europe, the Devensian in Britain, the Llanquihue in Chile and the Otiran in New Zealand.
The Holocene can be subdivided into five time intervals, or chronozones, based on climatic fluctuations: Preboreal, Atlantic and Subatlantic. Note: "ka" means "kilo-annum" Before Present, i.e. 1,000 years before 1950 The Blytt–Sernander classification of climatic periods defined by plant remains in peat mosses, is being explored. Geologists working in different regions are studying sea levels, peat bogs and ice core samples by a variety of methods, with a view toward further verifying and refining the Blytt–Sernander sequence, they find a general correspondence across Eurasia and North America, though the method was once thought to be of no interest. The scheme was defined for Northern Europe, but the climate changes were claimed to occur more widely; the periods of the scheme include a few of the final pre-Holocene oscillations of the last glacial period and classify climates of more recent prehistory. Paleontologists have not defined any faunal stages for the Holocene. If subdivision is necessary, periods of human technological development, such as the Mesolithic and Bronze Age, are used.
However, the time periods referenced by these terms vary with the emergence of those technologies in different parts of the world. Climatically, the Holocene may be divided evenly into the Neoglacial periods. According to some scholars, a third division, the Anthropocene, has now begun; the International Commission on Stratigraphy Subcommission on Quaternary Stratigraphy’s working group on the'Anthropocene' note this term is used to denote the present time interval in which many geologically significant conditions and processes have been profoundly altered by human activities. The'Anthropocene' is not a formally defined geological unit. Continental motions due to plate tectonics are less than a kilometre over a span of only 10,000 years. However, ice melt caused world sea levels to rise about 35 m in the early part of the Holocene. In addition, many areas above about 40 degrees north latitude had been depressed by the weight of the Pleistocene glaciers and rose as much as 180 m due to post-glacial rebound over the late Pleistocene and Holocene, are still rising today.
The sea level rise and temporary land depression allowed temporary marine incursions into areas that are now far from the sea. Holocene marine fossils are known, from Vermont and Michigan. Other than higher-latitude temporary marine incursions associated with glacial depression, Holocene fossils are found in lakebed and cave deposits. Holocene marine deposits along low-latitude coastlines are rare because the rise in sea levels during the period exceeds any tectonic uplift of non-glacial origin. Post-glacial rebound in the Scandinavia region resulted in the formation of the Baltic Sea; the region continues to rise, still causing weak earthquakes across Northern Europe. The equivalent event in North America was the rebound of Hudson Bay, as it shrank from its larger, immediate post-glacial Tyrrell Sea phase, to near its present boundaries. Climate has been stable over the Holocene. Ice core
Homo is the genus which emerged in the otherwise extinct Australopithecus genus that encompasses the extant species Homo sapiens, plus several extinct species classified as either ancestral to or related to modern humans, most notably Homo erectus and Homo neanderthalensis. The genus is taken to emerge with the appearance of Homo habilis, just over two million years ago. Genus Homo, together with the genus Paranthropus is sister to A. africanus in the genus Australopithecus, which itself had split from the lineage of Pan, the chimpanzees. Homo erectus appeared about two million years ago and, in several early migrations, it spread throughout Africa and Eurasia, it was the first human species to live in a hunter-gatherer society and to control fire. An adaptive and successful species, Homo erectus persisted for more than a million years, diverged into new species by around 500,000 years ago. Homo sapiens emerges close to 300,000 to 200,000 years ago, most in Africa, Homo neanderthalensis emerged at around the same time in Europe and Western Asia.
H. sapiens dispersed from Africa in several waves, from as early as 250,000 years ago, by 130,000 years ago, the so-called Southern Dispersal beginning about 70,000 years ago leading to the lasting colonisation of Eurasia and Oceania by 50,000 years ago. Both in Africa and Eurasia, H. sapiens interbred with archaic humans. Separate archaic human species are thought to have survived until around 40,000 years ago, with possible late survival of hybrid species as late as 12,000 years ago. See Homininae for an overview of taxonomy; the Latin noun homō means "human being" or "man" in the generic sense of "human being, mankind". The binomial name Homo sapiens was coined by Carl Linnaeus. Names for other species of the genus were introduced beginning in the second half of the 19th century. Today, the genus Homo has not been properly defined. Since the early human fossil record began to emerge from the earth, the boundaries and definitions of the genus Homo have been poorly defined and in flux; because there was no reason to think it would have any additional members, Carl Linnaeus did not bother to define Homo when he first created it for humans in the 18th century.
The discovery of Neanderthal brought the first addition. The genus Homo was given its taxonomic name to suggest that its member species can be classified as human. And, over the decades of the 20th century, fossil finds of pre-human and early human species from late Miocene and early Pliocene times produced a rich mix for debating classifications. There is continuing debate on delineating Homo from Australopithecus—or, delineating Homo from Pan, as one body of scientists argue that the two species of chimpanzee should be classed with genus Homo rather than Pan. So, classifying the fossils of Homo coincides with evidence of: 1) competent human bipedalism in Homo habilis inherited from the earlier Australopithecus of more than four million years ago, as demonstrated by the Laetoli footprints. From the late-19th to mid-20th centuries, a number of new taxonomic names including new generic names were proposed for early human fossils. Many such names are now dubbed as "synonyms" with Homo, including Pithecanthropus,Protanthropus,Sinanthropus,Cyphanthropus,Africanthropus,Telanthropus,Atlanthropus, Tchadanthropus.
Classifying the genus Homo into species and subspecies is subject to incomplete information and remains poorly done. This has led to using common names in scientific papers to avoid trinomial names or the ambiguity of classifying groups as incertae sedis —for example, H. neanderthalensis vs. H. sapiens neanderthalensis, or H. georgicus vs. H. erectus georgicus. Some extinct species in the genus Homo are only discovered and do not as yet have consensus binomial names. Since the beginning of the Holocene, it is that Homo sapiens has been the only extant species of Homo. John Edward Gray was an early advocate of classifying taxa by designating families. Wood and Richmond proposed that Hominini be designated as a tribe that comprised all species of early humans and pre-humans ancestral to humans back to after the chimpanzee-human last common ancestor. Designations alternative to Hominina existed, or were offered: Australopithecinae and Preanthropinae. See Hominini and Chimpanzee–human last common ancestor for the separation of Australopithecina and Panina.
Several species, including Australopithecus garhi, Australopithecus sediba, Australopithecus africanus, Australopithecus afarensis, have been proposed as the direct ancestor of the Homo lineage. These species have morphological features that align them with Homo
Fault blocks are large blocks of rock, sometimes hundreds of kilometres in extent, created by tectonic and localized stresses in the Earth's crust. Large areas of bedrock are broken up into blocks by faults. Blocks are characterized by uniform lithology; the largest of these fault blocks are called crustal blocks. Large crustal blocks broken off from tectonic plates are called terranes; those terranes which are the full thickness of the lithosphere are called microplates. Continent-sized blocks are called variously microcontinents, continental ribbons, H-blocks, extensional allochthons and outer highs; because most stresses relate to the tectonic activity of moving plates, most motion between blocks is horizontal, parallel to the Earth's crust by strike-slip faults. However vertical movement of blocks produces much more dramatic results. Landforms are sometimes formed. Adjacent raised down-dropped blocks can form high escarpments; the movement of these blocks is accompanied by tilting, due to compaction or stretching of the crust at that point.
Fault-block mountains result from rifting, another indicator of tensional tectonic forces. These can form extensive rift valley systems, such as the East African Rift zone. Death Valley in California is a smaller example. There are two types of block mountains. Lifted type block mountains have two steep sides exposing both sides scarps, leading to the horst and graben terrain seen in various parts of Europe including the Upper Rhine valley, a graben between two horsts - the Vosges mountains and the Black Forest, the Rila - Rhodope Massif in Bulgaria, Southeast Europe, including the well defined horsts of Belasitsa, Rila mountain and Pirin mountain - a horst forming a massive anticline situated between the complex graben valleys of Struma and that of Mesta. Tilted type block mountains have one sloping side and one steep side with an exposed scarp, are common in the Basin and Range region of the western United States. Example of graben is the basin of the Narmada River in India, between the Vindhya and Satpura horsts.
Orogeny – The formation of mountain ranges Plummer, David McGeary, Diane Carlson. Physical Geology 8th ed. McGraw-Hill, Boston, 1999. Monroe, James S. and Reed Wicander. The Changing Earth: Exploring Geology and Evolution. 2nd educational Belmont: Wadsworth Publishing Company, 1997. ISBN 0-314-09577-2 Fault-Block Mountains — Universe Today
Vertebrates comprise all species of animals within the subphylum Vertebrata. Vertebrates represent the overwhelming majority of the phylum Chordata, with about 69,276 species described. Vertebrates include the jawless fishes and jawed vertebrates, which include the cartilaginous fishes and the bony fishes; the bony fishes in turn, cladistically speaking include the tetrapods, which include amphibians, reptiles and mammals. Extant vertebrates range in size from the frog species Paedophryne amauensis, at as little as 7.7 mm, to the blue whale, at up to 33 m. Vertebrates make up less than five percent of all described animal species; the vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution, though their closest living relatives, the lampreys, do. Hagfish do, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology. Molecular analysis since 1992 has suggested that hagfish are most related to lampreys, so are vertebrates in a monophyletic sense.
Others consider them a sister group of vertebrates in the common taxon of craniata. The word vertebrate derives from the Latin word vertebratus. Vertebrate is derived from the word vertebra, which refers to any of the bones or segments of the spinal column. All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal, with a hollow tube of nervous tissue above it and the gastrointestinal tract below. In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the anus opens to the exterior before the end of the body; the remaining part of the body continuing after the anus forms a tail with vertebrae and spinal cord, but no gut. The defining characteristic of a vertebrate is the vertebral column, in which the notochord found in all chordates has been replaced by a segmented series of stiffer elements separated by mobile joints. However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the sturgeon and coelacanth.
Jawed vertebrates are typified by paired appendages, but this trait is not required in order for an animal to be a vertebrate. All basal vertebrates breathe with gills; the gills are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior. Each gill is supported by a cartilagenous or bony gill arch; the bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches than this, as some of their chordate relatives have more than 50 pairs of gills. In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches; these are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish being irrevocably lost early in the evolution of tetrapods.
While the more derived vertebrates lack gills, the gill arches form during fetal development, form the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella and, in mammals, the malleus and incus. The central nervous system of vertebrates is based on a hollow nerve cord running along the length of the animal. Of particular importance and unique to vertebrates is the presence of neural crest cells; these are progenitors of stem cells, critical to coordinating the functions of cellular components. Neural crest cells migrate through the body from the nerve cord during development, initiate the formation of neural ganglia and structures such as the jaws and skull; the vertebrates are the only chordate group to exhibit cephalisation, the concentration of brain functions in the head. A slight swelling of the anterior end of the nerve cord is found in the lancelet, a chordate, though it lacks the eyes and other complex sense organs comparable to those of vertebrates.
Other chordates do not show any trends towards cephalisation. A peripheral nervous system branches out from the nerve cord to innervate the various systems; the front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon and rhombencephalon, further differentiated in the various vertebrate groups. Two laterally placed eyes form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss; the forebrain is well developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vesicles of the forebrain are paired, giving rise to hemispheres like the cerebral hemispheres in mammals; the resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of vesicles, is unique to vertebrates. All invertebrates with well-developed brains, such as insects and squids, have a ventral rather than dorsal system of ganglions, with a split brain stem running on each side of the mouth or gut.
Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw