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History of the Earth

The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation to the present day.[1][2] Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past. The age of Earth is approximately one-third of the age of the universe. An immense amount of biological and geological change has occurred in that time span.

Earth formed around 4.54 billion years ago by accretion from the solar nebula. Volcanic outgassing likely created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of extreme volcanism and frequent collisions with other bodies. One very large collision is thought to have been responsible for tilting the Earth at an angle and forming the Moon. Over time, such cosmic bombardments ceased, allowing the planet to cool and form a solid crust. Water that was brought here by comets and asteroids condensed into clouds and the oceans took shape. Earth was finally hospitable to life, and the earliest forms that arose enriched the atmosphere with oxygen. Life on Earth remained small and microscopic for at least one billion years. About 580 million years ago, complex multicellular life arose, and during the Cambrian period it experienced a rapid diversification into most major phyla. Around six million years ago, the primate lineage that would lead to chimpanzees (our closest living relatives) diverged from the lineage that would lead to modern humans.

Biological and geological change has been constantly occurring on our planet since the time of its formation. Organisms continuously evolve, taking on new forms or going extinct in response to an ever-changing planet. The process of plate tectonics has played a major role in the shaping of Earth's oceans and continents, as well as the life they harbor. The biosphere, in turn, has had a significant effect on the atmosphere and other abiotic conditions on the planet, such as the formation of the ozone layer, the proliferation of oxygen, and the creation of soil. Though humans are unable to perceive it due to their relatively brief life spans, this change is ongoing and will continue for the next few billion years.

Solar System formation

The standard model for the formation of the Solar System (including the Earth) is the solar nebula hypothesis.[4] In this model, the Solar system formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. It was composed of hydrogen and helium created shortly after the Big Bang 13.7 Ga (billion years ago) and heavier elements ejected by supernovae. About 4.5 Ga, the nebula began a contraction that may have been triggered by the shock wave of a nearby supernova.[5] A shock wave would have also made the nebula rotate. As the cloud began to accelerate, its angular momentum, gravity and inertia flattened it into a protoplanetary disk perpendicular to its axis of rotation. Small perturbations due to collisions and the angular momentum of other large debris created the means by which kilometer-sized protoplanets began to form, orbiting the nebular center.[6]

The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until nuclear fusion of hydrogen into helium began. After more contraction, a T Tauri star ignited and evolved into the Sun. Meanwhile, in the outer part of the nebula gravity caused matter to condense around density perturbations, dust particles and the rest of the protoplanetary disk began separating into rings. In a process known as runaway accretion, successively larger fragments of dust and debris clumped together to form planets.[6] Earth formed in this manner about 4.54 billion years ago (with an uncertainty of 1%)[7][8][9][10] and was largely completed within 10–20 million years.[11] The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies. The same process is expected to produce accretion disks around virtually all newly forming stars in the universe, some of which yield planets.[12]

The proto-Earth grew by accretion until its interior was hot enough to melt the heavy, siderophile metals. Having higher densities than the silicates, the metals sank. This iron catastrophe resulted in the separation of a primitive mantle and a (metallic) core only 10 million years after the Earth began to form, producing the layered structure of Earth and setting up the formation of Earth's magnetic field.[13] Earth's first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted in these elements compared to cosmic abundances.[14]

Hadean and Archean Eons

The first eon in Earth's history, the Hadean, begins with the Earth's formation and is followed by the Archean eon at 3.8 Ga.[2]:145 The oldest rocks found on Earth date to about 4.0 Ga, and the oldest detrital zircon crystals in rocks to about 4.4 Ga,[15][16][17] soon after the formation of the Earth's crust and the Earth itself. The giant impact hypothesis for the Moon's formation states that shortly after formation of an initial crust, the proto-Earth was impacted by a smaller protoplanet, which ejected part of the mantle and crust into space and created the Moon.[18][19][20]

From crater counts on other celestial bodies it is inferred that a period of intense meteorite impacts, called the Late Heavy Bombardment, began about 4.1 Ga, and concluded around 3.8 Ga, at the end of the Hadean.[21] In addition, volcanism was severe due to the large heat flow and geothermal gradient.[22] Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that the planet already had oceans or seas at that time.[15]

By the beginning of the Archean, the Earth had cooled significantly. Most present life forms could not have survived in the Archean atmosphere, which lacked oxygen and an ozone layer. Nevertheless it is believed that primordial life began to evolve by the early Archean, with candidate fossils dated to around 3.5 Ga.[23] Some scientists even speculate that life could have begun during the early Hadean, as far back as 4.4 Ga, surviving the possible Late Heavy Bombardment period in hydrothermal vents below the Earth's surface.[24]

Formation of the Moon
The Earth's relatively large natural satellite, the Moon, is larger relative to its planet than any other satellite in the solar system.[nb 1] During the Apollo program, rocks from the Moon's surface were brought to Earth. Radiometric dating of these rocks has shown the Moon to be 4.53 ± .01 billion years old,[27] at least 30 million years after the solar system was formed.[28] New evidence suggests the Moon formed even later, 4.48 ± 0.02 Ga, or 70–110 million years after the start of the Solar System.[29]

Theories for the formation of the Moon must explain its late formation as well as the following facts. First, the Moon has a low density (3.3 times that of water, compared to 5.5 for the earth[30]) and a small metallic core. Second, there is virtually no water or other volatiles on the moon. Third, the Earth and Moon have the same oxygen isotopic signature (relative abundance of the oxygen isotopes). Of the theories that have been proposed to account for these phenomena, only one is widely accepted: The giant impact hypothesis proposes that the Moon originated after a body the size of Mars struck the proto-Earth a glancing blow.[1]:256[31][32]

The collision between the impactor, sometimes named Theia,[28] and the Earth released about 100 million times more energy than the impact that caused the extinction of the dinosaurs. This was enough to vaporize some of the Earth's outer layers and melt both bodies.[31][1]:256 A portion of the mantle material was ejected into orbit around the Earth. The giant impact hypothesis predicts that the Moon was depleted of metallic material,[33] explaining its abnormal composition.[34] The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.[35]

First continents
Mantle convection, the process that drives plate tectonics today, is a result of heat flow from the Earth's interior to the Earth's surface.[36]:2 It involves the creation of rigid tectonic plates at mid-oceanic ridges. These plates are destroyed by subduction into the mantle at subduction zones. During the early Archean (about 3.0 Ga) the mantle was much hotter than today, probably around 1600 °C,[37]:82 so convection in the mantle was faster. While a process similar to present day plate tectonics did occur, this would have gone faster too. It is likely that during the Hadean and Archean, subduction zones were more common, and therefore tectonic plates were smaller.[1]:258

The initial crust, formed when the Earth's surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. However, it is thought that it was basaltic in composition, like today's oceanic crust, because little crustal differentiation had yet taken place.[1]:258 The first larger pieces of continental crust, which is a product of differentiation of lighter elements during partial melting in the lower crust, appeared at the end of the Hadean, about 4.0 Ga. What is left of these first small continents are called cratons. These pieces of late Hadean and early Archean crust form the cores around which today's continents grew.[38]

The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites from about 4.0 Ga. They show traces of metamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing rivers and seas existed then.[39] Cratons consist primarily of two alternating types of terranes. The first are so called greenstone belts, consisting of low grade metamorphosed sedimentary rocks. These "greenstones" are similar to the sediments today found in oceanic trenches, above subduction zones. For this reason, greenstones are sometimes seen as evidence for subduction during the Archean. The second type is a complex of felsic magmatic rocks. These rocks are mostly tonalite, trondhjemite or granodiorite, types of rock similar in composition to granite (hence such terranes are called TTG-terranes). TTG-complexes are seen as the relicts of the first continental crust, formed by partial melting in basalt.[40]:Chapter 5

Oceans and atmosphere
Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted in these elements compared to cosmic abundances.[14] After the impact, the molten Earth released volatile gases; and later more gases were released by volcanoes, completing a second atmosphere rich in greenhouse gases but poor in oxygen. [1]:256 Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga.[42]:83–84,116–117

In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of volatiles from the Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as impact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would therefore have started to form even as the Earth formed.[43] The new atmosphere probably contained water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.[44]

Planetesimals at a distance of 1 astronomical unit (AU), the distance of the Earth from the Sun, probably did not contribute any water to the Earth because the solar nebula was too hot for ice to form and the hydration of rocks by water vapor would have taken too long.[43][45] The water must have been supplied by meteorites from the outer asteroid belt and some large planetary embryos from beyond 2.5 AU.[43][46] Comets may also have contributed. Though most comets are today in orbits farther away from the Sun than Neptune, computer simulations show they were originally far more common in the inner parts of the solar system.[39]:130-132

As the planet cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4.4 Ga.[15] By the start of the Archean eon they already covered the Earth. This early formation has been difficult to explain because of a problem known as the faint young Sun paradox. Stars are known to get brighter as they age, and at the time of its formation the Sun would have been emitting only 70% of its current power. Many models predict that the Earth would have been covered in ice.[47][43] A likely solution is that there was enough carbon dioxide and methane to produce a greenhouse effect. The carbon dioxide would have been produced by volcanos and the methane by early microbes. Another greenhouse gas, ammonium would have been ejected by volcanos but quickly destroyed by ultraviolet radiation.[42]:83

Origin of life
One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are a lot of models, but little consensus, on how life emerged from non-living chemicals; chemical systems that have been created in the laboratory still fall well short of the minimum complexity for a living organism.[48][49]

The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids, that are the building blocks of life. An experiment in 1953 by Stanley Miller and Harold Urey showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of lightning.[50] Although the atmospheric composition was likely different from the composition used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules.[51] Recent computer simulations have even shown that extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of the Earth.[52]

The next stage of complexity could have been reached from at least three possible starting points: self-replication, an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances.[53]

Replication first: RNA world
Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance and self-replication.

The discovery that a kind of RNA molecule called a ribozyme can catalyze both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA.[54] They could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that the offspring in each generation were quite likely to have different genomes from those that their parents started with.[55] RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have.[56] Ribozymes remain as the main components of ribosomes, the "protein factories" of modern cells.[57]

Although short, self-replicating RNA molecules have been artificially produced in laboratories,[58] doubts have been raised about whether natural non-biological synthesis of RNA is possible.[59][60][61] The earliest ribozymes may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA.[62][63] Other pre-RNA replicators have been posited, including crystals[64]:150 and even quantum systems.[65]

In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and ocean-bottom pressures near hydrothermal vents. In this hypothesis, lipid membranes would be the last major cell components to appear and until they did the proto-cells would be confined to the pores.[66]

Metabolism first: Iron-sulfur world
Another long-standing hypothesis is that the first life was composed of protein molecules. Amino acids, the building blocks of proteins, are easily synthesized in plausible prebiotic conditions, as are small peptides (polymers of amino acids) that make good catalysts.[67]:295–297 A series of experiments starting in 1997 showed that amino acids and peptides could form in the presence of carbon monoxide and hydrogen sulfide with iron sulfide and nickel sulfide as catalysts. Most of the steps in their assembly required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometres (4.3 mi) of rock. Hence self-sustaining synthesis of proteins could have occurred near hydrothermal vents.[68]

A difficulty with the metabolism-first scenario is finding a way for organisms to evolve. Without the ability to replicate as individuals, aggregates of molecules would have "compositional genomes" (counts of molecular species in the aggregate) as the target of natural selection. However, a recent model shows that such a system is unable to evolve in response to natural selection.[69]

Membranes first: Lipid world
It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step.[70] Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside.[71]

The clay theory
Some clays, notably montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern, are subject to an analog of natural selection (as the clay "species" that grows fastest in a particular environment rapidly becomes dominant), and can catalyze the formation of RNA molecules.[72] Although this idea has not become the scientific consensus, it still has active supporters.[73]:150–158[64]

Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles", and that the bubbles could encapsulate RNA attached to the clay. Bubbles can then grow by absorbing additional lipids and dividing. The formation of the earliest cells may have been aided by similar processes.[74]

A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids.[75]

Last common ancestor
It is believed that of this multiplicity of protocells, only one line survived. Current phylogenetic evidence suggests that the last universal common ancestor (LUCA) lived during the early Archean eon, perhaps 3.5 Ga or earlier.[76][77] This LUCA cell is the ancestor of all life on Earth today. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts. Like all modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to catalyze reactions. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes by lateral gene transfer.[76]


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