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Earth formed around 4. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. While Earth was in its earliest stage Early Earth , a giant impact collision with a planet-sized body named Theia is thought to have formed the Moon. Over time, the Earth cooled, causing the formation of a solid crust , and allowing liquid water on the surface. The Hadean eon represents the time before a reliable fossil record of life; it began with the formation of the planet and ended 4.
The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution. The succeeding eon is the Phanerozoic, divided into three eras: Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale. The earliest undisputed evidence of life on Earth dates at least from 3. There are microbial mat fossils such as stromatolites found in 3.
History of Earth
Photosynthetic organisms appeared between 3. Life remained mostly small and microscopic until about million years ago , when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about million years ago. This sudden diversification of life forms produced most of the major phyla known today, and divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era. It is estimated that 99 percent of all species that ever lived on Earth, over five billion,  have gone extinct. The Earth's crust has constantly changed since its formation, as has life has since its first appearance.
Species continue to evolve , taking on new forms, splitting into daughter species, or going extinct in the face of ever-changing physical environments. The process of plate tectonics continues to shape the Earth's continents and oceans and the life they harbor. Human activity is now a dominant force affecting global change, harming the biosphere , the Earth's surface, hydrosphere , and atmosphere with the loss of wild lands, over-exploitation of the oceans, production of greenhouse gases , degradation of the ozone layer , and general degradation of soil, air, and water quality.
In geochronology , time is generally measured in mya megayears or million years , each unit representing the period of approximately 1,, years in the past. The history of Earth is divided into four great eons , starting 4, mya with the formation of the planet. Each eon saw the most significant changes in Earth's composition, climate and life. Each eon is subsequently divided into eras , which in turn are divided into periods , which are further divided into epochs.
The history of the Earth can be organized chronologically according to the geologic time scale , which is split into intervals based on stratigraphic analysis. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. Therefore, the second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, and the most recent period is expanded in the fourth timeline.
The standard model for the formation of the Solar System including the Earth is the solar nebula hypothesis. It was composed of hydrogen and helium created shortly after the Big Bang 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.
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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 and 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. The same process is expected to produce accretion disks around virtually all newly forming stars in the universe, some of which yield planets. The proto-Earth grew by accretion until its interior was hot enough to melt the heavy, siderophile metals. Having higher densities than the silicates, these metals sank. Jacobs  was the first to suggest that the inner core —a solid center distinct from the liquid outer core —is freezing and growing out of the liquid outer core due to the gradual cooling of Earth's interior about degrees Celsius per billion years .
The first eon in Earth's history, the Hadean , begins with the Earth's formation and is followed by the Archean eon at 3. 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. 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.
By the beginning of the Archean, the Earth had cooled significantly. Present life forms could not have survived at Earth's surface, because the Archean atmosphere lacked oxygen hence had no ozone layer to block ultraviolet light. Nevertheless, it is believed that primordial life began to evolve by the early Archean, with candidate fossils dated to around 3.
Earth's only natural satellite , the Moon, is larger relative to its planet than any other satellite in the solar system. Radiometric dating of these rocks shows that the Moon is 4. 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. 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 proposed to account for these phenomena, one is widely accepted: The giant impact hypothesis proposes that the Moon originated after a body the size of Mars sometimes named Theia  struck the proto-Earth a glancing blow.
The collision released about million times more energy than the more recent Chicxulub impact that is believed to have caused the extinction of the dinosaurs. It was enough to vaporize some of the Earth's outer layers and melt both bodies. The giant impact hypothesis predicts that the Moon was depleted of metallic material,  explaining its abnormal composition.
Under the influence of its own gravity, the ejected material became a more spherical body: Mantle convection , the process that drives plate tectonics, is a result of heat flow from the Earth's interior to the Earth's surface. These plates are destroyed by subduction into the mantle at subduction zones.
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During the early Archean about 3. Although 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. 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. 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.
The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites from about 4. They show traces of metamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing that rivers and seas existed then. 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. 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 of these elements compared to cosmic abundances.
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. Though most comets are today in orbits farther away from the Sun than Neptune , computer simulations show that they were originally far more common in the inner parts of the solar system. As the Earth cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4. This early formation has been difficult to explain because of a problem known as the faint young Sun paradox.
The carbon dioxide would have been produced by volcanoes and the methane by early microbes. Another greenhouse gas, ammonia, would have been ejected by volcanos but quickly destroyed by ultraviolet radiation. 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 many models, but little consensus, on how life emerged from non-living chemicals; chemical systems created in the laboratory fall well short of the minimum complexity for a living organism. 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 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. Additional complexity could have been reached from at least three possible starting points: 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. Although short, self-replicating RNA molecules have been artificially produced in laboratories,  doubts have been raised about whether natural non-biological synthesis of RNA is possible.
In this hypothesis, the proto-cells would be confined in the pores of the metal substrate until the later development of lipid membranes. 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. Hence, self-sustaining synthesis of proteins could have occurred near hydrothermal vents. 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. 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. 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. Some clays , notably montmorillonite , have properties that make them plausible accelerators for the emergence of an RNA world: Research in reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles", and that the bubbles could encapsulate RNA attached to the clay.
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Bubbles can then grow by absorbing additional lipids and dividing. The formation of the earliest cells may have been aided by similar processes. A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides , lipids and amino acids. It is believed that of this multiplicity of protocells, only one line survived.
Current phylogenetic evidence suggests that the last universal ancestor LUA lived during the early Archean eon, perhaps 3. 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 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.
The Proterozoic eon lasted from 2. The change to an oxygen-rich atmosphere was a crucial development. Life developed from prokaryotes into eukaryotes and multicellular forms.
The Proterozoic saw a couple of severe ice ages called snowball Earths. The earliest cells absorbed energy and food from the surrounding environment. They used fermentation , the breakdown of more complex compounds into less complex compounds with less energy, and used the energy so liberated to grow and reproduce. Fermentation can only occur in an anaerobic oxygen-free environment. The evolution of photosynthesis made it possible for cells to derive energy from the Sun. Most of the life that covers the surface of the Earth depends directly or indirectly on photosynthesis.
The most common form, oxygenic photosynthesis, turns carbon dioxide, water, and sunlight into food. It captures the energy of sunlight in energy-rich molecules such as ATP, which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product. Such extremophile organisms are restricted to otherwise inhospitable environments such as hot springs and hydrothermal vents. The simpler anoxygenic form arose about 3. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2.
At first, the released oxygen was bound up with limestone , iron , and other minerals. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast time transformed Earth's atmosphere to its current state. This was Earth's third atmosphere. Some oxygen was stimulated by solar ultraviolet radiation to form ozone , which collected in a layer near the upper part of the atmosphere.
The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonize the surface of the ocean and eventually the land: Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died out as its levels rose in what is known as the oxygen catastrophe.
Resistant forms survived and thrived, and some developed the ability to use oxygen to increase their metabolism and obtain more energy from the same food. However, the Earth did not get warmer.
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Instead, the geological record suggests it cooled dramatically during the early Proterozoic. Glacial deposits found in South Africa date back to 2. Thus, this glaciation, known as the Huronian glaciation , may have been global. Some scientists suggest this was so severe that the Earth was frozen over from the poles to the equator, a hypothesis called Snowball Earth. The Huronian ice age might have been caused by the increased oxygen concentration in the atmosphere, which caused the decrease of methane CH 4 in the atmosphere. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO 2 , a less effective greenhouse gas.
However, the term Snowball Earth is more commonly used to describe later extreme ice ages during the Cryogenian period. Carbon dioxide combines with rain to weather rocks to form carbonic acid, which is then washed out to sea, thus extracting the greenhouse gas from the atmosphere. When the continents are near the poles, the advance of ice covers the rocks, slowing the reduction in carbon dioxide, but in the Cryogienian the weathering of Rodinia was able to continue unchecked until the ice advanced to the tropics.
The process may have finally been reversed by the emission of carbon dioxide from volcanoes or the destabilization of methane gas hydrates. According to the alternative Slushball Earth theory, even at the height of the ice ages there was still open water at the Equator. Modern taxonomy classifies life into three domains. The time of their origin is uncertain. The Bacteria domain probably first split off from the other forms of life sometimes called Neomura , but this supposition is controversial.
Soon after this, by 2 Ga,  the Neomura split into the Archaea and the Eukarya. Eukaryotic cells Eukarya are larger and more complex than prokaryotic cells Bacteria and Archaea , and the origin of that complexity is only now becoming known. Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today's Rickettsia ,  which had evolved to metabolize oxygen , entered a larger prokaryotic cell, which lacked that capability. Perhaps the large cell attempted to digest the smaller one but failed possibly due to the evolution of prey defenses.
The smaller cell may have tried to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it metabolized the larger cell's waste products and derived more energy. Part of this excess energy was returned to the host. The smaller cell replicated inside the larger one. Soon, a stable symbiosis developed between the large cell and the smaller cells inside it. Over time, the host cell acquired some genes from the smaller cells, and the two kinds became dependent on each other: The whole cell is now considered a single organism , and the smaller cells are classified as organelles called mitochondria.
A similar event occurred with photosynthetic cyanobacteria  entering large heterotrophic cells and becoming chloroplasts. There were probably several such inclusion events. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, there are theories that cells led to peroxisomes , spirochetes led to cilia and flagella , and that perhaps a DNA virus led to the cell nucleus,   though none of them are widely accepted.
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Archaeans, bacteria, and eukaryotes continued to diversify and to become more complex and better adapted to their environments. Each domain repeatedly split into multiple lineages, although little is known about the history of the archaea and bacteria. Some of these lived in colonies, and gradually a division of labor began to take place; for instance, cells on the periphery might have started to assume different roles from those in the interior. At first, it probably resembled today's sponges , which have totipotent cells that allow a disrupted organism to reassemble itself.
No ocean crust dates back further than that, so earlier reconstructions are more difficult. Paleomagnetic poles are supplemented by geologic evidence such as orogenic belts , which mark the edges of ancient plates, and past distributions of flora and fauna. The further back in time, the scarcer and harder to interpret the data get and the more uncertain the reconstructions. Throughout the history of the Earth, there have been times when continents collided and formed a supercontinent, which later broke up into new continents.
The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia. The existence of Pannotia depends on the timing of the rifting between Gondwana which included most of the landmass now in the Southern Hemisphere, as well as the Arabian Peninsula and the Indian subcontinent and Laurentia roughly equivalent to current-day North America.
The end of the Proterozoic saw at least two Snowball Earths, so severe that the surface of the oceans may have been completely frozen. This happened about Because CO 2 is an important greenhouse gas, climates cooled globally. An alternative hypothesis is that enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures. The Cryogenian period was followed by the Ediacaran period, which was characterized by a rapid development of new multicellular lifeforms. The new forms of life, called Ediacara biota, were larger and more diverse than ever.
Though the taxonomy of most Ediacaran life forms is unclear, some were ancestors of groups of modern life. None of the Ediacaran fossils had hard body parts like skeletons.
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These first appear after the boundary between the Proterozoic and Phanerozoic eons or Ediacaran and Cambrian periods. The Phanerozoic is the current eon on Earth, which started approximately million years ago. It consists of three eras: The Paleozoic , Mesozoic , and Cenozoic ,  and is the time when multi-cellular life greatly diversified into almost all the organisms known today. Life colonized the land, first plants, then animals. Two major extinctions occurred. The continents formed at the break-up of Pannotia and Rodinia at the end of the Proterozoic slowly moved together again, forming the supercontinent Pangaea in the late Paleozoic.
These three periods are further split into seven sub-divisions, with the Paleogene composed of The Paleocene , Eocene , and Oligocene , the Neogene divided into the Miocene , Pliocene , and the Quaternary composed of the Pleistocene , and Holocene. At the end of the Proterozoic, the supercontinent Pannotia had broken apart into the smaller continents Laurentia, Baltica , Siberia and Gondwana.
Because young volcanic crust is relatively hotter and less dense than old oceanic crust, the ocean floors rise during such periods. This causes the sea level to rise. Therefore, in the first half of the Paleozoic, large areas of the continents were below sea level. Early Paleozoic climates were warmer than today, but the end of the Ordovician saw a short ice age during which glaciers covered the south pole, where the huge continent Gondwana was situated.
Traces of glaciation from this period are only found on former Gondwana.
During the Late Ordovician ice age, a few mass extinctions took place, in which many brachiopods , trilobites, Bryozoa and corals disappeared. These marine species could probably not contend with the decreasing temperature of the sea water. The biological fomenting in the Cambrian Explosion was unpreceded before and since that time. The development of hard body parts such as shells, skeletons or exoskeletons in animals like molluscs , echinoderms , crinoids and arthropods a well-known group of arthropods from the lower Paleozoic are the trilobites made the preservation and fossilization of such life forms easier than those of their Proterozoic ancestors.
For this reason, much more is known about life in and after the Cambrian than about that of older periods. Some of these Cambrian groups appear complex but are seemingly quite different from modern life; examples are Anomalocaris and Haikouichthys. More recently, however, these seem to have found a place in modern classification.
During the Cambrian, the first vertebrate animals, among them the first fishes , had appeared. It had a primitive notochord , a structure that could have developed into a vertebral column later. The first fishes with jaws Gnathostomata appeared during the next geological period, the Ordovician. The colonisation of new niches resulted in massive body sizes. The diversity of life forms did not increase greatly because of a series of mass extinctions that define widespread biostratigraphic units called biomeres.
Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of the Sun's ultraviolet radiation , meaning unicellular organisms that reached land were less likely to die, and prokaryotes began to multiply and become better adapted to survival out of the water. Prokaryote lineages  had probably colonized the land as early as 2. For a long time, the land remained barren of multicellular organisms. Several hundred million years ago, plants probably resembling algae and fungi started growing at the edges of the water, and then out of it.
The timing of the first animals to leave the oceans is not precisely known: There is also unconfirmed evidence that arthropods may have appeared on land as early as Ma. This would let them live in oxygen-poor water, or pursue small prey in shallow water. Eventually, some of them became so well adapted to terrestrial life that they spent their adult lives on land, although they hatched in the water and returned to lay their eggs.
This was the origin of the amphibians. This resulted in the divergence of amniotes from amphibians. Other groups of organisms continued to evolve, and lines diverged—in fish, insects, bacteria, and so on—but less is known of the details. Though some mammalian lines began to separate during this period, existing mammals were probably small animals resembling shrews.
The first of five great mass extinctions was the Ordovician-Silurian extinction. Its possible cause was the intense glaciation of Gondwana, which eventually led to a snowball earth. The second mass extinction was the Late Devonian extinction , probably caused by the evolution of trees, which could have led to the depletion of greenhouse gases like CO2 or the eutrophication of water. The third mass extinction was the Permian-Triassic, or the Great Dying , event was possibly caused by some combination of the Siberian Traps volcanic event, an asteroid impact, methane hydrate gasification, sea level fluctuations, and a major anoxic event.
Either the proposed Wilkes Land crater  in Antarctica or Bedout structure off the northwest coast of Australia may indicate an impact connection with the Permian-Triassic extinction. But it remains uncertain whether either these or other proposed Permian-Triassic boundary craters are either real impact craters or even contemporaneous with the Permian-Triassic extinction event. The fourth mass extinction was the Triassic-Jurassic extinction event in which almost all synapsids and archosaurs became extinct, probably due to new competition from dinosaurs.