Gondwana, was a supercontinent that existed from the Neoproterozoic until the Jurassic. It was formed by the accretion of several cratons. Gondwana became the largest piece of continental crust of the Paleozoic Era, covering an area of about 100,000,000 km2. During the Carboniferous Period, it merged with Laurussia to form a larger supercontinent called Pangaea. Gondwana broke up during the Mesozoic Era; the remnants of Gondwana make up about two thirds of today's continental area, including South America, Antarctica and the Indian Subcontinent. The formation of Gondwana began c. 800 to 650 Ma with the East African Orogeny, the collision of India and Madagascar with East Africa,and was completed c. 600 to 530 Ma with the overlapping Brasiliano and Kuunga orogenies, the collision of South America with Africa and the addition of Australia and Antarctica, respectively. The continent of Gondwana was named by Austrian scientist Eduard Suess, after the Gondwana region of central India, derived from Sanskrit for "forest of the Gonds".
The name had been used in a geological context, first by H. B. Medlicott in 1872, from which the Gondwana sedimentary sequences are described; the term "Gondwanaland" is preferred by some scientists in order to make a clear distinction between the region and the supercontinent. The assembly of Gondwana was a protracted process during the Neoproterozoic and Paleozoic, which however remains incompletely understood because of the lack of paleo-magnetic data. Several orogenies, collectively known as the Pan-African orogeny, led to the amalgamation of most of the continental fragments of a much older supercontinent, Rodinia. One of those orogenic belts, the Mozambique Belt, formed 800 to 650 Ma and was interpreted as the suture between East and West Gondwana. Three orogenies were recognized during the 1990s: the East African Orogeny and Kuunga orogeny, the collision between East Gondwana and East Africa in two steps, the Brasiliano orogeny, the successive collision between South American and African cratons.
The final stages of Gondwanan assembly overlapped with the opening of the Iapetus Ocean between Laurentia and western Gondwana. During this interval, the Cambrian explosion occurred. Laurentia was docked against the western shores of a united Gondwana for a short period near the Precambrian/Cambrian boundary, forming the short-lived and still disputed supercontinent Pannotia; the Mozambique Ocean separated the Congo–Tanzania–Bangweulu Block of central Africa from Neoproterozoic India. The Azania continent was an island in the Mozambique Ocean; the Australia/Mawson continent was still separated from India, eastern Africa, Kalahari by c. 600 Ma, when most of western Gondwana had been amalgamated. By c. 550 Ma, India had reached its Gondwanan position. Meanwhile, on the other side of the newly-forming Africa, Kalahari collided with Congo and Rio de la Plata which closed the Adamastor Ocean. C. 540–530 Ma, the closure of the Mozambique Ocean brought India next to Australia–East Antarctica, both North and South China were located in proximity to Australia.
As the rest of Gondwana formed, a complex series of orogenic events assembled the eastern parts of Gondwana c. 750 to 530 Ma. First the Arabian-Nubian Shield collided with eastern Africa in the East African Orogeny c.750 to 620 Ma. Australia and East Antarctica were merged with the remaining Gondwana c. 570 to 530 Ma in the Kuunga Orogeny. The Malagasy orogeny at about 550–515 Mya affected Madagascar, eastern East Africa and southern India. In it, Neoproterozoic India collided with the combined Azania and Congo–Tanzania–Bangweulu Block, suturing along the Mozambique Belt; the 18,000 km -long Terra Australis Orogen developed along Gondwana's western and eastern margins. Proto-Gondwanan Cambrian arc belts from this margin have been found in eastern Australia, New Zealand, Antarctica. Though these belts formed a continuous arc chain, the direction of subduction was different between the Australian-Tasmanian and New Zealand-Antarctica arc segments. A large number of terranes were accreted to Eurasia during Gondwana's existence but the Cambrian or Precambrian origin of many of these terranes remains uncertain.
For example, some Palaeozoic terranes and microcontinents that now make up Central Asia called the "Kazakh" and "Mongolian terranes", were progressively amalgamated into the continent Kazakhstania in the Late Silurian. Whether these blocks originated on the shores of Gondwana is not known. In the Early Palaeozoic the Armorican terrane, which today form large parts of France, was part of either Peri-Gondwana or core Gondwana. Precambrian rocks from the Iberian Peninsula suggest it too formed part of core Gondwana before its detachment as an orocline in the Variscan orogeny close to the Carboniferous–Permian boundary. South-east Asia is made of Gondwanan and Cathaysian continental fragments that were assembled during the Mid-Palaeozoic and Cenozoic; this p
The flowering plants known as angiosperms, Angiospermae or Magnoliophyta, are the most diverse group of land plants, with 64 orders, 416 families 13,164 known genera and c. 369,000 known species. Like gymnosperms, angiosperms are seed-producing plants. However, they are distinguished from gymnosperms by characteristics including flowers, endosperm within the seeds, the production of fruits that contain the seeds. Etymologically, angiosperm means a plant; the term comes from the Greek words sperma. The ancestors of flowering plants diverged from gymnosperms in the Triassic Period, 245 to 202 million years ago, the first flowering plants are known from 160 mya, they diversified extensively during the Early Cretaceous, became widespread by 120 mya, replaced conifers as the dominant trees from 100 to 60 mya. Angiosperms differ from other seed plants in several ways, described in the table below; these distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans.
Angiosperm stems are made up of seven layers. The amount and complexity of tissue-formation in flowering plants exceeds that of gymnosperms; the vascular bundles of the stem are arranged such that the phloem form concentric rings. In the dicotyledons, the bundles in the young stem are arranged in an open ring, separating a central pith from an outer cortex. In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue known as cambium. By the formation of a layer of cambium between the bundles, a complete ring is formed, a regular periodical increase in thickness results from the development of xylem on the inside and phloem on the outside; the soft phloem becomes crushed, but the hard wood persists and forms the bulk of the stem and branches of the woody perennial. Owing to differences in the character of the elements produced at the beginning and end of the season, the wood is marked out in transverse section into concentric rings, one for each season of growth, called annual rings.
Among the monocotyledons, the bundles are more numerous in the young stem and are scattered through the ground tissue. They once formed the stem increases in diameter only in exceptional cases; the characteristic feature of angiosperms is the flower. Flowers show remarkable variation in form and elaboration, provide the most trustworthy external characteristics for establishing relationships among angiosperm species; the function of the flower is to ensure fertilization of the ovule and development of fruit containing seeds. The floral apparatus may arise terminally from the axil of a leaf; as in violets, a flower arises singly in the axil of an ordinary foliage-leaf. More the flower-bearing portion of the plant is distinguished from the foliage-bearing or vegetative portion, forms a more or less elaborate branch-system called an inflorescence. There are two kinds of reproductive cells produced by flowers. Microspores, which will divide to become pollen grains, are the "male" cells and are borne in the stamens.
The "female" cells called megaspores, which will divide to become the egg cell, are contained in the ovule and enclosed in the carpel. The flower may consist only of these parts, as in willow, where each flower comprises only a few stamens or two carpels. Other structures are present and serve to protect the sporophylls and to form an envelope attractive to pollinators; the individual members of these surrounding structures are known as petals. The outer series is green and leaf-like, functions to protect the rest of the flower the bud; the inner series is, in general, white or brightly colored, is more delicate in structure. It functions to attract bird pollinators. Attraction is effected by color and nectar, which may be secreted in some part of the flower; the characteristics that attract pollinators account for the popularity of flowers and flowering plants among humans. While the majority of flowers are perfect or hermaphrodite, flowering plants have developed numerous morphological and physiological mechanisms to reduce or prevent self-fertilization.
Heteromorphic flowers have short carpels and long stamens, or vice versa, so animal pollinators cannot transfer pollen to the pistil. Homomorphic flowers may employ a biochemical mechanism called self-incompatibility to discriminate between self and non-self pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers; the botanical term "Angiosperm", from the Ancient Greek αγγείον, angeíon and σπέρμα, was coined in the form Angiospermae by Paul Hermann in 1690, as the name of one of his primary divisions of the plant kingdom. This included flowering plants possessing seeds enclosed in capsules, distinguished from his Gymnospermae, or flowering plants with achenial or schizo-carpic fruits, the whole fruit or each of its pieces being here regarded as a seed and naked; the term and its antonym were maintained by Carl Linnaeus with the same sense, but with restricted application, in the names of the orders of his class Didynamia. Its use with any
The Cambrian explosion or Cambrian radiation was an event 541 million years ago in the Cambrian period when most major animal phyla appeared in the fossil record. It lasted for about 20–25 million years and resulted in the divergence of most modern metazoan phyla; the event was accompanied by major diversification of other organisms. Before the Cambrian explosion, most organisms were simple, composed of individual cells organized into colonies; as the rate of diversification subsequently accelerated, the variety of life began to resemble that of today. All present animal phyla appeared during this period; the rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the 1840s, in his 1859 book On the Origin of Species, Charles Darwin discussed the inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through natural selection. The long-running puzzlement about the appearance of the Cambrian fauna abruptly, without precursor, centers on three key points: whether there was a mass diversification of complex organisms over a short period of time during the early Cambrian.
Interpretation is difficult due to a limited supply of evidence, based on an incomplete fossil record and chemical signatures remaining in Cambrian rocks. The first discovered Cambrian fossils were trilobites, described by Edward Lhuyd, the curator of Oxford Museum, in 1698. Although their evolutionary importance was not known, on the basis of their old age, William Buckland realised that a dramatic step-change in the fossil record had occurred around the base of what we now call the Cambrian. Nineteenth-century geologists such as Adam Sedgwick and Roderick Murchison used the fossils for dating rock strata for establishing the Cambrian and Silurian periods. By 1859, leading geologists including Roderick Murchison, were convinced that what was called the lowest Silurian stratum showed the origin of life on Earth, though others, including Charles Lyell, differed. In On the Origin of Species, Charles Darwin considered this sudden appearance of a solitary group of trilobites, with no apparent antecedents, absence of other fossils, to be "undoubtedly of the gravest nature" among the difficulties in his theory of natural selection.
He reasoned that earlier seas had swarmed with living creatures, but that their fossils had not been found due to the imperfections of the fossil record. In the sixth edition of his book, he stressed his problem further as: To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer. American paleontologist Charles Walcott, who studied the Burgess Shale fauna, proposed that an interval of time, the "Lipalian", was not represented in the fossil record or did not preserve fossils, that the ancestors of the Cambrian animals evolved during this time. Earlier fossil evidence has since been found; the earliest claim is that the history of life on earth goes back 3,850 million years: Rocks of that age at Warrawoona, were claimed to contain fossil stromatolites, stubby pillars formed by colonies of microorganisms. Fossils of more complex eukaryotic cells, from which all animals and fungi are built, have been found in rocks from 1,400 million years ago, in China and Montana.
Rocks dating from 580 to 543 million years ago contain fossils of the Ediacara biota, organisms so large that they are multicelled, but unlike any modern organism. In 1948, Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian, but as as the 1970s, no sign was seen of how the'relatively' modern-looking organisms of the Middle and Late Cambrian arose; the intense modern interest in this "Cambrian explosion" was sparked by the work of Harry B. Whittington and colleagues, who, in the 1970s, reanalysed many fossils from the Burgess Shale and concluded that several were as complex as, but different from, any living animals; the most common organism, was an arthropod, but not a member of any known arthropod class. Organisms such as the five-eyed Opabinia and spiny slug-like Wiwaxia were so different from anything else known that Whittington's team assumed they must represent different phyla unrelated to anything known today. Stephen Jay Gould's popular 1989 account of this work, Wonderful Life, brought the matter into the public eye and raised questions about what the explosion represented.
While differing in details, both Whittington and Gould proposed that all modern animal phyla had appeared simultaneously in a rather short span of geological period. This view led to the modernization of Darwin's tree of life and the theory of punctuated equilibrium, which Eldredge and Gould developed in the early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change. Other analyses, some more recent and some dating back to the 1970s, argue that complex animals similar to modern types evolved well before the start of the Cambrian. Radiometric dates for much of the Cambrian, obtained by analysis of radioactive elements contained within rocks, have only become available, for only a few regions. Relative dating is assumed sufficient for studying processes of evolution, but this, has been difficult, because of the problems involved in matching up rocks of the same age across different continents. Therefore, dates or descriptions of sequences of events should be regarded with some caution until better data become
Pannotia known as Vendian supercontinent, Greater Gondwana, the Pan-African supercontinent, was a short-lived Neoproterozoic supercontinent that formed at the end of the Precambrian during the Pan-African orogeny and broke apart 560 Ma with the opening of the Iapetus Ocean. Pannotia formed when Laurentia was located adjacent to the two major South American cratons, Amazonia and Río de la Plata; the opening of the Iapetus Ocean separated Laurentia from Baltica, Río de la Plata. Piper 1976 was the first to propose a Proterozoic supercontinent preceding Pangaea, today known as Rodinia. At that time he referred to it as "the Proterozoic super-continent", but much he named this "symmetrical crescent-shaped analogue of Pangaea"'Palaeopangaea' and still insists there is neither a need nor any evidences for Rodinia or its daughter supercontinent Pannotia or a series of other proposed supercontinents since Archaean times; the existence of a Late Proterozoic supercontinent, much different from Pangaea, was first proposed by McWilliams 1981 based on paleomagnetic data and the break-up of this supercontinent around 625–550 Ma was documented by Bond, Nickeson & Kominz 1984.
The reconstruction of Bond et al. is identical to that of Dalziel 1997 and others. Another term for the supercontinent, thought to have existed at the end of Neoproterozoic time is "Greater Gondwanaland", suggested by Stern 1994; this term recognizes that the supercontinent of Gondwana, which formed at the end of the Neoproterozoic, was once part of the much larger end-Neoproterozoic supercontinent. Pannotia was named by Powell 1995, based on the term "Pannotios" proposed by Stump 1987 for "the cycle of tectonic activity common to the Gondwana continents that resulted in the formation of the supercontinent." Young 1995 proposed renaming the older Proterozoic supercontinent "Kanatia", the St. Lawrence Iroquoian word from which the name'Canada' is derived, while keeping the name Rodinia for the latter Neoproterozoic supercontinent. Powell, objected to this renaming and instead proposed Stump's term for the latter supercontinent. Reconstructions of Rodinia varies but most include five elements: Laurentia or the Canadian Shield is located at the centre.
Less certain position of continental blocks includes: the West African Craton was an extension of the Amazonian Craton. The formation of Pannotia began during the Pan-African orogeny when the Congo continent got caught between the northern and southern halves of the previous supercontinent Rodinia some 750 Ma; the peak in this mountain building event was around 640–610 Ma, but these continental collisions may have continued into the Early Cambrian some 530 Ma. The formation of Pannotia was the result of Rodinia turning itself inside out; when Pannotia had formed, Africa was located at the centre surrounded by the rest of Gondwana: South America, Madagascar, India and Australia. Laurentia, which'escaped' out of Rodinia and Siberia kept the relative positions they had in Rodinia; the Cathaysian and Cimmerian terranes were located along the northern margins of east Gondwana. The Avalonian-Cadomian terranes were located along the active northern margins of western Gondwana; this orogeny extended north into the Uralian margin of Baltica.
Pannotia formed by subduction of exterior oceans over a geoid low, whereas Pangaea formed by subduction of interior oceans over a geoid high caused by superplumes and slab avalanche events. The oceanic crust subducted by Pannotia formed within the Mirovoi superocean that surrounded Rodinia before its 830–750 Ma break-up and were accreted during the Late Proterozoic orogenies that resulted from the assembly of Pannotia. One of the major of these orogenies was the collision between East and West Gondwana or the East African Orogeny; the Trans-Saharan Belt in West Africa is the result of the collision between the East Saharan Shield and the West African Craton when 1200–710 Ma-old volcanic and arc-related rocks were accreted to the margin of this craton. 600–500 Ma two Brazilian interior orogenies got deformed and metamorphosed between a series of colliding cratons: Amazonia, West Africa-São Luís, São Francisco-Congo-Kasai. The material, accreted included 950–850 Ma mafic meta-igneous complexes and younger arc-related rocks.
The break-up of Pannotia was accompanied by sea level rise, dramatic changes in climate and ocean water chemistry, rapid metazoan diversification. Bond, Nickeson & Kominz 1984 found Neoproterozoic passive margin sequences worldwide – the first indication of a Late Neoproterozoic supercontinent but the traces of its demise; the Iapetus Ocean started to open while Pannotia was being assembled, 200 Ma after the break-up of Rodinia. This opening of the Iapetus and other Cambrian seas coincided with the first steps in the evolution of soft-bodied metazoans, made a myriad of habitats available for them.
An arthropod is an invertebrate animal having an exoskeleton, a segmented body, paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids and crustaceans; the term Arthropoda as proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin mineralised with calcium carbonate; the arthropod body plan consists of each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton; some species have wings. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments, they have over a million described species, making up more than 80 per cent of all described living animal species, some of which, unlike most other animals, are successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.
Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, through which their haemolymph – analogue of blood – circulates. Like their exteriors, the internal organs of arthropods are built of repeated segments, their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, their brains are formed by fusion of the ganglia of these segments and encircle the esophagus; the respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey.
Arthropods have a wide range of chemical and mechanical sensors based on modifications of the many setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; the evolutionary ancestry of arthropods dates back to the Cambrian period. The group is regarded as monophyletic, many analyses support the placement of arthropods with cycloneuralians in a superphylum Ecdysozoa. Overall, the basal relationships of Metazoa are not yet well resolved; the relationships between various arthropod groups are still debated. Aquatic species use either external fertilization. All arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and undergo a total metamorphosis to produce the adult form; the level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions. Arthropods contribute to the human food supply both directly as food, more indirectly as pollinators of crops.
Some species are known to spread severe disease to humans and crops. The word arthropod comes from the Greek ἄρθρον árthron, "joint", πούς pous, i.e. "foot" or "leg", which together mean "jointed leg". Arthropods are invertebrates with jointed limbs; the exoskeleton or cuticles consists of a polymer of glucosamine. The cuticle of many crustaceans, beetle mites, millipedes is biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments occur in some opiliones. Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 per cent of all known living animal species; the number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.
They are important members of marine, freshwater and air ecosystems, are one of only two major animal groups that have adapted to life in dry environments. One arthropod sub-group, insects, is the most species-rich member of all ecological guilds in land and freshwater environments; the lightest insects weigh less than 25 micrograms. Some living crustaceans are much larger; the embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways; the three-
Tetrapods are four-limbed animals constituting the superclass Tetrapoda. It includes existing and extinct amphibians and mammals. Tetrapods evolved from a group of animals known as the Tetrapodomorpha which, in turn, evolved from ancient Sarcopterygii around 390 million years ago in the middle Devonian period; the first tetrapods appeared by the late Devonian, 367.5 million years ago. The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known; the first tetrapods were aquatic. Modern amphibians, which evolved from earlier groups, are semiaquatic. However, most tetrapod species today are amniotes, most of those are terrestrial tetrapods whose branch evolved from earlier tetrapods about 340 million years ago; the key innovation in amniotes over amphibians is laying of eggs on land or having further evolved to retain the fertilized egg within the mother. Amniote tetrapods drove most amphibian tetrapods to extinction.
One group of amniotes diverged into the reptiles, which includes lepidosaurs, crocodilians and extinct relatives. Amniotes include the tetrapods that further evolved for flight—such as birds from among the dinosaurs, bats from among the mammals; some tetrapods, such as the snakes, have lost some or all of their limbs through further speciation and evolution. Others, such as amphibians, returned to or aquatic lives, the first during the Carboniferous period. Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic ancestors; these include the structure of the jaw and teeth for feeding on land, limb girdles and extremities for land locomotion, lungs for respiration in air, a heart for circulation, eyes and ears for seeing and hearing in air. Tetrapods can be defined in cladistics as the nearest common ancestor of all living amphibians and all living amniotes, along with all of the descendants of that ancestor; this is a node-based definition. The group so defined is crown tetrapods.
The term tetrapodomorph is used for the stem-based definition: any animal, more related to living amphibians, reptiles and mammals than to living dipnoi. The group so defined is known as the tetrapod total group. Stegocephalia is a larger group equivalent to some broader uses of the word tetrapod, used by scientists who prefer to reserve tetrapod for the crown group; such scientists use the term "stem-tetrapod" to refer to those tetrapod-like vertebrates that are not members of the crown group, including the tetrapodomorph fishes. The two subclades of crown tetrapods are Reptiliomorpha. Batrachomorphs are all animals sharing a more recent common ancestry with living amphibians than with living amniotes. Reptiliomorphs are all animals sharing a more recent common ancestry with living amniotes than with living amphibians. Tetrapoda includes four living classes: amphibians, reptiles and birds. Overall, the biodiversity of lissamphibians, as well as of tetrapods has grown exponentially over time. However, that diversification process was interrupted at least a few times by major biological crises, such as the Permian–Triassic extinction event, which at least affected amniotes.
The overall composition of biodiversity was driven by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown, so has the number of niches that tetrapods have occupied; the first tetrapods were aquatic and fed on fish. Today, the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets; the following table shows summary estimates for each tetrapod class from the IUCN Red List of Threatened Species, 2014.3, for the number of extant species that have been described in the literature, as well as the number of threatened species. The classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on gross anatomical and physiological traits. Snakes and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have a full complement of limbs. Similar considerations apply to aquatic mammals.
Newer taxonomy is based on cladistics instead, giving a variable number of major "branches" of the tetrapod family tree. As is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological classification sometimes fa
Water is a transparent, tasteless and nearly colorless chemical substance, the main constituent of Earth's streams and oceans, the fluids of most living organisms. It is vital for all known forms of life though it provides no calories or organic nutrients, its chemical formula is H2O, meaning that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. Water is the name of the liquid state of H2O at standard ambient pressure, it forms precipitation in the form of rain and aerosols in the form of fog. Clouds are formed from suspended droplets of its solid state; when finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is water vapor. Water moves continually through the water cycle of evaporation, condensation and runoff reaching the sea. Water covers 71% of the Earth's surface in seas and oceans. Small portions of water occur as groundwater, in the glaciers and the ice caps of Antarctica and Greenland, in the air as vapor and precipitation.
Water plays an important role in the world economy. 70% of the freshwater used by humans goes to agriculture. Fishing in salt and fresh water bodies is a major source of food for many parts of the world. Much of long-distance trade of commodities and manufactured products is transported by boats through seas, rivers and canals. Large quantities of water and steam are used for cooling and heating, in industry and homes. Water is an excellent solvent for a wide variety of chemical substances. Water is central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, sport fishing, diving; the word water comes from Old English wæter, from Proto-Germanic *watar, from Proto-Indo-European *wod-or, suffixed form of root *wed-. Cognate, through the Indo-European root, with Greek ύδωρ, Russian вода́, Irish uisce, Albanian ujë; the identification of water as a substance Water is a polar inorganic compound, at room temperature a tasteless and odorless liquid, nearly colorless with a hint of blue.
This simplest hydrogen chalcogenide is by far the most studied chemical compound and is described as the "universal solvent" for its ability to dissolve many substances. This allows it to be the "solvent of life", it is the only common substance to exist as a solid and gas in normal terrestrial conditions. Water is a liquid at the pressures that are most adequate for life. At a standard pressure of 1 atm, water is a liquid between 0 and 100 °C. Increasing the pressure lowers the melting point, about −5 °C at 600 atm and −22 °C at 2100 atm; this effect is relevant, for example, to ice skating, to the buried lakes of Antarctica, to the movement of glaciers. Increasing the pressure has a more dramatic effect on the boiling point, about 374 °C at 220 atm; this effect is important in, among other things, deep-sea hydrothermal vents and geysers, pressure cooking, steam engine design. At the top of Mount Everest, where the atmospheric pressure is about 0.34 atm, water boils at 68 °C. At low pressures, water cannot exist in the liquid state and passes directly from solid to gas by sublimation—a phenomenon exploited in the freeze drying of food.
At high pressures, the liquid and gas states are no longer distinguishable, a state called supercritical steam. Water differs from most liquids in that it becomes less dense as it freezes; the maximum density of water in its liquid form is 1,000 kg/m3. The density of ice is 917 kg/m3. Thus, water expands 9% in volume as it freezes, which accounts for the fact that ice floats on liquid water; the details of the exact chemical nature of liquid water are not well understood. Pure water is described as tasteless and odorless, although humans have specific sensors that can feel the presence of water in their mouths, frogs are known to be able to smell it. However, water from ordinary sources has many dissolved substances, that may give it varying tastes and odors. Humans and other animals have developed senses that enable them to evaluate the potability of water by avoiding water, too salty or putrid; the apparent color of natural bodies of water is determined more by dissolved and suspended solids, or by reflection of the sky, than by water itself.
Light in the visible electromagnetic spectrum can traverse a couple meters of pure water without significant absorption, so that it looks transparent and colorless. Thus aquatic plants and other photosynthetic organisms can live in water up to hundreds of meters deep, because sunlight can reach them. Water vapour is invisible as a gas. Through a thickness of 10 meters or more, the intrinsic color of water is visibly turquoise, as its absorption spectrum has