Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation. Evolution occurs when evolutionary processes such as natural selection and genetic drift act on this variation, resulting in certain characteristics becoming more common or rare within a population, it is this process of evolution that has given rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms and molecules. The scientific theory of evolution by natural selection was proposed by Charles Darwin and Alfred Russel Wallace in the mid-19th century and was set out in detail in Darwin's book On the Origin of Species. Evolution by natural selection was first demonstrated by the observation that more offspring are produced than can survive.
This is followed by three observable facts about living organisms: 1) traits vary among individuals with respect to their morphology and behaviour, 2) different traits confer different rates of survival and reproduction and 3) traits can be passed from generation to generation. Thus, in successive generations members of a population are more to be replaced by the progenies of parents with favourable characteristics that have enabled them to survive and reproduce in their respective environments. In the early 20th century, other competing ideas of evolution such as mutationism and orthogenesis were refuted as the modern synthesis reconciled Darwinian evolution with classical genetics, which established adaptive evolution as being caused by natural selection acting on Mendelian genetic variation. All life on Earth shares a last universal common ancestor that lived 3.5–3.8 billion years ago. The fossil record includes a progression from early biogenic graphite, to microbial mat fossils, to fossilised multicellular organisms.
Existing patterns of biodiversity have been shaped by repeated formations of new species, changes within species and loss of species throughout the evolutionary history of life on Earth. Morphological and biochemical traits are more similar among species that share a more recent common ancestor, can be used to reconstruct phylogenetic trees. Evolutionary biologists have continued to study various aspects of evolution by forming and testing hypotheses as well as constructing theories based on evidence from the field or laboratory and on data generated by the methods of mathematical and theoretical biology, their discoveries have influenced not just the development of biology but numerous other scientific and industrial fields, including agriculture and computer science. The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers, such as Anaximander and Empedocles; such proposals survived into Roman times. The poet and philosopher Lucretius followed Empedocles in his masterwork De rerum natura.
In contrast to these materialistic views, Aristotelianism considered all natural things as actualisations of fixed natural possibilities, known as forms. This was part of a medieval teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and gave examples of how new types of living things could come to be. In the 17th century, the new method of modern science rejected the Aristotelian approach, it sought explanations of natural phenomena in terms of physical laws that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order. However, this new approach was slow to take root in the biological sciences, the last bastion of the concept of fixed natural types.
John Ray applied one of the more general terms for fixed natural types, "species," to plant and animal types, but he identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation. The biological classification introduced by Carl Linnaeus in 1735 explicitly recognised the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan. Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In 1751, Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species. Georges-Louis Leclerc, Comte de Buffon suggested that species could degenerate into different organisms, Erasmus Darwin proposed that all warm-blooded animals could have descended from a single microorganism; the first full-fledged evolutionary scheme was Jean-Baptiste Lamarck's "transmutation" theory of 1809, which envisaged spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, postulated that on a local level, these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents.
These ideas were cond
The Internet is the global system of interconnected computer networks that use the Internet protocol suite to link devices worldwide. It is a network of networks that consists of private, academic and government networks of local to global scope, linked by a broad array of electronic and optical networking technologies; the Internet carries a vast range of information resources and services, such as the inter-linked hypertext documents and applications of the World Wide Web, electronic mail and file sharing. Some publications no longer capitalize "internet"; the origins of the Internet date back to research commissioned by the federal government of the United States in the 1960s to build robust, fault-tolerant communication with computer networks. The primary precursor network, the ARPANET served as a backbone for interconnection of regional academic and military networks in the 1980s; the funding of the National Science Foundation Network as a new backbone in the 1980s, as well as private funding for other commercial extensions, led to worldwide participation in the development of new networking technologies, the merger of many networks.
The linking of commercial networks and enterprises by the early 1990s marked the beginning of the transition to the modern Internet, generated a sustained exponential growth as generations of institutional and mobile computers were connected to the network. Although the Internet was used by academia since the 1980s, commercialization incorporated its services and technologies into every aspect of modern life. Most traditional communication media, including telephony, television, paper mail and newspapers are reshaped, redefined, or bypassed by the Internet, giving birth to new services such as email, Internet telephony, Internet television, online music, digital newspapers, video streaming websites. Newspaper and other print publishing are adapting to website technology, or are reshaped into blogging, web feeds and online news aggregators; the Internet has enabled and accelerated new forms of personal interactions through instant messaging, Internet forums, social networking. Online shopping has grown exponentially both for major retailers and small businesses and entrepreneurs, as it enables firms to extend their "brick and mortar" presence to serve a larger market or sell goods and services online.
Business-to-business and financial services on the Internet affect supply chains across entire industries. The Internet has no single centralized governance in either technological implementation or policies for access and usage; the overreaching definitions of the two principal name spaces in the Internet, the Internet Protocol address space and the Domain Name System, are directed by a maintainer organization, the Internet Corporation for Assigned Names and Numbers. The technical underpinning and standardization of the core protocols is an activity of the Internet Engineering Task Force, a non-profit organization of loosely affiliated international participants that anyone may associate with by contributing technical expertise. In November 2006, the Internet was included on USA Today's list of New Seven Wonders; when the term Internet is used to refer to the specific global system of interconnected Internet Protocol networks, the word is a proper noun that should be written with an initial capital letter.
In common use and the media, it is erroneously not capitalized, viz. the internet. Some guides specify that the word should be capitalized when used as a noun, but not capitalized when used as an adjective; the Internet is often referred to as the Net, as a short form of network. As early as 1849, the word internetted was used uncapitalized as an adjective, meaning interconnected or interwoven; the designers of early computer networks used internet both as a noun and as a verb in shorthand form of internetwork or internetworking, meaning interconnecting computer networks. The terms Internet and World Wide Web are used interchangeably in everyday speech. However, the World Wide Web or the Web is only one of a large number of Internet services; the Web is a collection of interconnected documents and other web resources, linked by hyperlinks and URLs. As another point of comparison, Hypertext Transfer Protocol, or HTTP, is the language used on the Web for information transfer, yet it is just one of many languages or protocols that can be used for communication on the Internet.
The term Interweb is a portmanteau of Internet and World Wide Web used sarcastically to parody a technically unsavvy user. Research into packet switching, one of the fundamental Internet technologies, started in the early 1960s in the work of Paul Baran and Donald Davies. Packet-switched networks such as the NPL network, ARPANET, the Merit Network, CYCLADES, Telenet were developed in the late 1960s and early 1970s; the ARPANET project led to the development of protocols for internetworking, by which multiple separate networks could be joined into a network of networks. ARPANET development began with two network nodes which were interconnected between the Network Measurement Center at the University of California, Los Angeles Henry Samueli School of Engineering and Applied Science directed by Leonard Kleinrock, the NLS system at SRI International by Douglas Engelbart in Menlo Park, California, on 29 October 1969; the third site was the Culler-Fried Interactive Mathematics Center at the University of California, Santa Barbara, followed by the University of
A hierarchy is an arrangement of items in which the items are represented as being "above", "below", or "at the same level as" one another. Hierarchy is an important concept in a wide variety of fields, such as philosophy, computer science, organizational theory, systems theory, the social sciences. A hierarchy can link entities either directly or indirectly, either vertically or diagonally; the only direct links in a hierarchy, insofar as they are hierarchical, are to one's immediate superior or to one of one's subordinates, although a system, hierarchical can incorporate alternative hierarchies. Hierarchical links can extend "vertically" upwards or downwards via multiple links in the same direction, following a path. All parts of the hierarchy which are not linked vertically to one another can be "horizontally" linked through a path by traveling up the hierarchy to find a common direct or indirect superior, down again; this is akin to colleagues. Organizational forms exist that are both complementary to hierarchy.
Heterarchy is one such form. Hierarchies have their own special vocabulary; these terms are easiest to understand. In an organizational context, the following terms are used related to hierarchies: Object: one entity System: the entire set of objects that are being arranged hierarchically Dimension: another word for "system" from on-line analytical processing Member: an at any in a Terms about Positioning Rank: the relative value, complexity, importance, level etc. of an object Level or Tier: a set of objects with the same rank OR importance Ordering: the arrangement of the Hierarchy: the arrangement of a particular set of members into. Multiple hierarchies are possible per, in which selected levels of the dimension are omitted to flatten the structure Terms about Placement Hierarch, the apex of the hierarchy, consisting of one single orphan in the top level of a dimension; the root of an inverted-tree structure Member, a in any level of a hierarchy in a dimension to which members are attached Orphan, a member in any level of a dimension without a parent member.
The apex of a disconnected branch. Orphans can be grafted back into the hierarchy by creating a relationship with a parent in the superior level Leaf, a member in any level of a dimension without subordinates in the hierarchy Neighbour: a member adjacent to another member in the same. Always a peer. Superior: a higher level or an object ranked at a higher level Subordinate: a lower level or an object ranked at a lower level Collection: all of the objects at one level Peer: an object with the same rank Interaction: the relationship between an object and its direct superior or subordinate a direct interaction occurs when one object is on a level one higher or one lower than the other Distance: the minimum number of connections between two objects, i.e. one less than the number of objects that need to be "crossed" to trace a path from one object to another Span: a qualitative description of the width of a level when diagrammed, i.e. the number of subordinates an object has Terms about Nature Attribute: a heritable characteristic of in a level Attribute-value: the specific value of a heritable characteristic In a mathematical context, the general terminology used is different.
Most hierarchies use a more specific vocabulary pertaining to their subject, but the idea behind them is the same. For example, with data structures, objects are known as nodes, superiors are called parents and subordinates are called children. In a business setting, a superior is a supervisor/boss and a peer is a colleague. Degree of branching refers to the number of direct subordinates or children an object has a node has. Hierarchies can be categorized based on the "maximum degree", the highest degree present in the system as a whole. Categorization in this way yields two broad classes: branching. In a linear hierarchy, the maximum degree is 1. In other words, all of the objects can be visualized in a line-up, each object has one direct subordinate and one direct superior. Note that this is referring to the objects and not the levels. An example of a linear hierarchy is the hierarchy of life. In a branching hierarchy, one or more objects has a degree of 2 or more. For many people, the word "hierarchy" automatically evokes an image of a branching hierarchy.
Branching hierarchies are present within numerous systems, including organizations and classification schemes. The broad category of branching hierarchies can be furt
An author is the creator or originator of any written work such as a book or play, is thus a writer. More broadly defined, an author is "the person who originated or gave existence to anything" and whose authorship determines responsibility for what was created; the first owner of a copyright is the person who created the work i.e. the author. If more than one person created the work a case of joint authorship can be made provided some criteria are met. In the copyright laws of various jurisdictions, there is a necessity for little flexibility regarding what constitutes authorship; the United States Copyright Office, for example, defines copyright as "a form of protection provided by the laws of the United States to authors of "original works of authorship". Holding the title of "author" over any "literary, musical, certain other intellectual works" gives rights to this person, the owner of the copyright the exclusive right to engage in or authorize any production or distribution of their work.
Any person or entity wishing to use intellectual property held under copyright must receive permission from the copyright holder to use this work, will be asked to pay for the use of copyrighted material. After a fixed amount of time, the copyright expires on intellectual work and it enters the public domain, where it can be used without limit. Copyright laws in many jurisdictions – following the lead of the United States, in which the entertainment and publishing industries have strong lobbying power – have been amended since their inception, to extend the length of this fixed period where the work is controlled by the copyright holder. However, copyright is the legal reassurance that one owns his/her work. Technically, someone owns their work from the time. An interesting aspect of authorship emerges with copyright in that, in many jurisdictions, it can be passed down to another upon one's death; the person who inherits the copyright enjoys the same legal benefits. Questions arise as to the application of copyright law.
How does it, for example, apply to the complex issue of fan fiction? If the media agency responsible for the authorized production allows material from fans, what is the limit before legal constraints from actors and other considerations, come into play? Additionally, how does copyright apply to fan-generated stories for books? What powers do the original authors, as well as the publishers, have in regulating or stopping the fan fiction? This particular sort of case illustrates how complex intellectual property law can be, since such fiction may involved trademark law, likeness rights, fair use rights held by the public, many other interacting complications. Authors may portion out different rights they hold to different parties, at different times, for different purposes or uses, such as the right to adapt a plot into a film, but only with different character names, because the characters have been optioned by another company for a television series or a video game. An author may not have rights when working under contract that they would otherwise have, such as when creating a work for hire, or when writing material using intellectual property owned by others.
In literary theory, critics find complications in the term author beyond what constitutes authorship in a legal setting. In the wake of postmodern literature, critics such as Roland Barthes and Michel Foucault have examined the role and relevance of authorship to the meaning or interpretation of a text. Barthes challenges the idea, he writes, in his essay "Death of the Author", that "it is language which speaks, not the author". The words and language of a text itself determine and expose meaning for Barthes, not someone possessing legal responsibility for the process of its production; every line of written text is a mere reflection of references from any of a multitude of traditions, or, as Barthes puts it, "the text is a tissue of quotations drawn from the innumerable centres of culture". With this, the perspective of the author is removed from the text, the limits imposed by the idea of one authorial voice, one ultimate and universal meaning, are destroyed; the explanation and meaning of a work does not have to be sought in the one who produced it, "as if it were always in the end, through the more or less transparent allegory of the fiction, the voice of a single person, the author'confiding' in us".
The psyche, fanaticism of an author can be disregarded when interpreting a text, because the words are rich enough themselves with all of the traditions of language. To expose meanings in a written work without appealing to the celebrity of an author, their tastes, vices, is, to Barthes, to allow language to speak, rather than author. Michel Foucault argues in his essay "What is an author?" that all authors are writers, but not all writers are authors. He states that "a private letter may have a signatory—it does not have an author". For a reader to assign the title of author upon any written work is to attribute certain standards upon the text which, for Foucault, are working in conjunction with the idea of "the author function". Foucault's author function is the idea that an author exists only as a fun
Cladistics is an approach to biological classification in which organisms are categorized in groups based on the most recent common ancestor. Hypothesized relationships are based on shared derived characteristics that can be traced to the most recent common ancestor and are not present in more distant groups and ancestors. A key feature of a clade is that all its descendants are part of the clade. All descendants stay in their overarching ancestral clade. For example, if within a strict cladistic framework the terms animals, bilateria/worms, fishes/vertebrata, or monkeys/anthropoidea were used, these terms would include humans. Many of these terms are used paraphyletically, outside of cladistics, e.g. as a'grade'. Radiation results in the generation of new subclades by bifurcation; the techniques and nomenclature of cladistics have been applied to other disciplines. Cladistics is now the most used method to classify organisms; the original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics.
Cladistics in the original sense refers to a particular set of methods used in phylogenetic analysis, although it is now sometimes used to refer to the whole field. What is now called the cladistic method appeared as early as 1901 with a work by Peter Chalmers Mitchell for birds and subsequently by Robert John Tillyard in 1921, W. Zimmermann in 1943; the term "clade" was introduced in 1958 by Julian Huxley after having been coined by Lucien Cuénot in 1940, "cladogenesis" in 1958, "cladistic" by Cain and Harrison in 1960, "cladist" by Mayr in 1965, "cladistics" in 1966. Hennig referred to his own approach as "phylogenetic systematics". From the time of his original formulation until the end of the 1970s, cladistics competed as an analytical and philosophical approach to systematics with phenetics and so-called evolutionary taxonomy. Phenetics was championed at this time by the numerical taxonomists Peter Sneath and Robert Sokal, evolutionary taxonomy by Ernst Mayr. Conceived, if only in essence, by Willi Hennig in a book published in 1950, cladistics did not flourish until its translation into English in 1966.
Today, cladistics is the most popular method for constructing phylogenies from morphological data. In the 1990s, the development of effective polymerase chain reaction techniques allowed the application of cladistic methods to biochemical and molecular genetic traits of organisms, vastly expanding the amount of data available for phylogenetics. At the same time, cladistics became popular in evolutionary biology, because computers made it possible to process large quantities of data about organisms and their characteristics; the cladistic method interprets each character state transformation implied by the distribution of shared character states among taxa as a potential piece of evidence for grouping. The outcome of a cladistic analysis is a cladogram – a tree-shaped diagram, interpreted to represent the best hypothesis of phylogenetic relationships. Although traditionally such cladograms were generated on the basis of morphological characters and calculated by hand, genetic sequencing data and computational phylogenetics are now used in phylogenetic analyses, the parsimony criterion has been abandoned by many phylogeneticists in favor of more "sophisticated" but less parsimonious evolutionary models of character state transformation.
Cladists contend. Every cladogram is based on a particular dataset analyzed with a particular method. Datasets are tables consisting of molecular, ethological and/or other characters and a list of operational taxonomic units, which may be genes, populations, species, or larger taxa that are presumed to be monophyletic and therefore to form, all together, one large clade. Different datasets and different methods, not to mention violations of the mentioned assumptions result in different cladograms. Only scientific investigation can show, more to be correct; until for example, cladograms like the following have been accepted as accurate representations of the ancestral relations among turtles, lizards and birds: If this phylogenetic hypothesis is correct the last common ancestor of turtles and birds, at the branch near the ▼ lived earlier than the last common ancestor of lizards and birds, near the ♦. Most molecular evidence, produces cladograms more like this: If this is accurate the last common ancestor of turtles and birds lived than the last common ancestor of lizards and birds.
Since the cladograms provide competing accounts of real events, at most one of them is correct. The cladogram to the right represents the current universally accepted hypothesis that all primates, including strepsirrhines like the lemurs and lorises, had a common ancestor all of whose descendants were primates, so form a clade. Within the primates, all anthropoids are hypothesized to have had a common ancestor all of whose descendants were anthropoids, so they form the clade called Anthropoidea; the "prosimians", on the other hand, form a paraphyletic taxon. The name Prosimii is not used in phylogenetic nomenclature, whic
Tree of life (biology)
The tree of life or universal tree of life is a metaphor and research tool used to explore the evolution of life and describe the relationships between organisms, both living and extinct, as described in a famous passage in Charles Darwin's On the Origin of Species. The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile speaks the truth. Tree diagrams originated in the medieval era to represent genealogical relationships. Phylogenetic tree diagrams in the evolutionary sense date back to at least the early 19th century; the term phylogeny for the evolutionary relationships of species through time was coined by Ernst Haeckel, who went further than Darwin in proposing phylogenic histories of life. In contemporary usage, tree of life refers to the compilation of comprehensive phylogenetic databases rooted at the last universal common ancestor of life on Earth; the Open Tree of Life, first published 2015, is a project to compile such a database for free public access.
Although the mutability of species may have appeared in paintings and trees have been used as a metaphor for other purposes earlier than 1800, the combination of the concept of branching evolution and the tree image did not appear before 1800. The earliest tree of life was published by the French botanist Augustin Augier in 1801, it shows the relationships between members of the plant kingdom. Jean-Baptiste Lamarck produced the first branching tree of animals in his Philosophie Zoologique, it was an upside-down tree ending with mammals. However, Lamarck did not believe in common descent of all life. Instead, he believed; the American geologist Edward Hitchcock published in 1840 the first tree of life based on paleontology in his Elementary Geology. On the vertical axis are paleontological periods. Hitchcock made a separate tree for animals; the plant and the animal tree are not connected at the bottom of the chart. Furthermore, each tree starts with multiple origins. Hitchcock's tree was more realistic than Darwin's 1859 theoretical tree because Hitchcock used real names in his trees.
It is true that Hitchcock's trees were branching trees. However, they were not evolutionary trees, because Hitchcock believed that a deity was the agent of change; that was an important difference with Darwin. The first edition of Robert Chambers' Vestiges of the Natural History of Creation, published anonymously in 1844 in England, contained a tree-like diagram in the chapter "Hypothesis of the development of the vegetable and animal kingdoms", it shows a model of embryological development where fish and birds represent branches from a path leading to mammals. In the text this branching tree idea is tentatively applied to the history of life on earth: "there may be branching", but the branching diagram is not displayed again for this purpose. However, the image of a branching tree could have inspired others to use it explicitly as a representation of the history of life on earth. In 1858, a year before Darwin's Origin, the paleontologist Heinrich Georg Bronn published a hypothetical tree labeled with letters.
Although not a creationist, Bronn did not propose a mechanism of change. Charles Darwin used the concept of a tree of life in the context of his theory of evolution. In On the Origin of Species Chapter IV he presented an abstract diagram of a theoretical tree of life for species of an unnamed large genus. On the horizontal base line hypothetical species within this genus are labelled A – L and are spaced irregularly to indicate how distinct they are from each other, are above broken lines at various angles suggesting that they have diverged from one or more common ancestors. On the vertical axis divisions labelled I – XIV each represent a thousand generations. From A, diverging lines show branching descent producing new varieties, some of which become extinct, so that after ten thousand generations descendants of A have become distinct new varieties or sub-species a10, f10, m10; the descendants of I have diversified to become the new varieties w10 and z10. The process is extrapolated for a further four thousand generations so that the descendants of A and I become fourteen new species labelled a14 to z14.
While F has continued for fourteen thousand generations unchanged, species B,C,D,E,G,H,K and L have gone extinct. In Darwin's own words: "Thus the small differences distinguishing varieties of the same species, will tend to increase till they come to equal the greater differences between species of the same genus, or of distinct genera.". This is a branching pattern with no names given to species, unlike the more linear tree Ernst Haeckel made years which includes the names of species and shows a more linear development from "lower" to "higher" species. In his summary to the section, Darwin put his concept in terms of the metaphor of the tree of life: The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile speaks the truth; the green and budding twigs may represent existing species. At each period of growth all the growing twigs have tried to branch out on all sides, to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life.
The limbs divided into great branches, these into lesser and lesser branches, w
Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times. Earth's axis of rotation is tilted with respect to its orbital plane; the gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, slows its rotation. Earth is the largest of the four terrestrial planets. Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water by oceans; the remaining 29% is land consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere.
The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, a convecting mantle that drives plate tectonics. Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms; some geological evidence indicates. Since the combination of Earth's distance from the Sun, physical properties, geological history have allowed life to evolve and thrive. In the history of the Earth, biodiversity has gone through long periods of expansion punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely. Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival.
Humans have developed diverse cultures. The modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most spelled eorðe, it has cognates in every Germanic language, their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was being used to translate the many senses of Latin terra and Greek γῆ: the ground, its soil, dry land, the human world, the surface of the world, the globe itself; as with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus, Norse mythology included Jörð, a giantess given as the mother of Thor. Earth was written in lowercase, from early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, the earth became the Earth when referenced along with other heavenly bodies. More the name is sometimes given as Earth, by analogy with the names of the other planets.
House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name but writes it in lowercase when preceded by the, it always appears in lowercase in colloquial expressions such as "what on earth are you doing?" The oldest material found in the Solar System is dated to 4.5672±0.0006 billion years ago. By 4.54±0.04 Bya the primordial Earth had formed. The bodies in the Solar System evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, dust. According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10–20 million years to form. A subject of research is the formation of some 4.53 Bya. A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, hit Earth.
In this view, the mass of Theia was 10 percent of Earth, it hit Earth with a glancing blow and some of its mass merged with Earth. Between 4.1 and 3.8 Bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth. Earth's atmosphere and oceans were formed by volcanic outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 3.5 Bya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind. A crust formed; the two models that explain land mass propose either a steady growth to the present-day forms or, more a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics