How did the earth formed.pptx

7/31/2019 How did the earth formed.pptx

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The history of the Earth describes themost important events and

fundamental stages in the developmentof the planet Earth from its formationto the present day. Nearly all branchesof natural science have contributed to

the understanding of the main events ofthe Earth's past. The age of Earth isapproximately one-third of the age of

the universe. An immense amount

of biological and geological change hasoccurred in that time span.


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It all started with a tremendous bang. Somewhere in our galaxy a star

exploded, throwing out masses of gas and dust. This supernova, as theseexplosions are called, happened about 5bn years ago. The wreckage fromthe explosion then crashed into a nearby cloud of gas, bringing together

the ingredients for our solar system to form.

Because the explosion was so energetic it made the dust mixture very hotand things began to cook. Little bits of dust began to cluster, makingbigger and bigger lumps, and the mixture began to pull together under

its own gravity


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The standard model for the formation of the SolarSystem (including the Earth) is the solar nebulahypothesis. In this model, the Solar system formed from

a large, rotating cloud of interstellar dust and gascalled the solar nebula. It was composedof hydrogen and helium created shortly after the BigBang 13.7 Ga (billion years ago) andheavier elements ejected by supernovae. About 4.5 Ga,

the nebula began a contraction that may have beentriggered by the shock wave of a nearby supernova.Ashock wave would have also made the nebula rotate. Asthe cloud began to accelerate, its angularmomentum, gravity and inertia flattened it into

a protoplanetary disk perpendicular to its axis ofrotation. Small perturbations due to collisions and theangular momentum of other large debris created themeans by which kilometer-sized protoplanets began toform, orbiting the nebular center


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Mantle convection, the process that drives platetectonics today, is a result of heat flow from the

Earth's interior to the Earth's surface. It involvesthe 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 so convection in themantle was faster. While a process similar to

present day plate tectonics did occur, this wouldhave gone faster too. It is likely that during the

Hadean and Archean, subduction zones weremore common, and therefore tectonic plates

were smaller.


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One of the reasons for interest in the early atmosphere and ocean is thatthey 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 laboratorystill 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 reactionsthat produced many of the simpler organic compounds,

including nucleobases and amino acids, that are the building blocks oflife. An experiment in 1953 by Stanley Miller and Harol

Urey showed that such molecules could form in an atmosphere ofwater, methane, ammonia and hydrogen with the aid of sparks to

mimic the effect of lightning.[50] Although the atmospheric compositionwas likely different from the composition used by Miller and Urey,later experiments with more realistic compositions also managed tosynthesize organic molecules.[51] Recent computer simulations have

even shown that extraterrestrial organic molecules could have formedin the protoplanetary disk before the formation of the Earth.


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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 solarwind 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 about2.8 Ga.[42]:8384,116117

In early models for the formation of the atmosphere and ocean, the second

atmosphere was formed by out gassing 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 degassingin which incoming bodies vaporize on

impact. The ocean and atmosphere would therefore have started to form even asthe Earth formed.[43] The new atmosphere probably contained water vapor,

carbon dioxide, nitrogen, and smaller amounts of other gases


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Even the simplest members of the three modern domains of lifeuse DNA to record their "recipes" and a complex arrayof RNA and protein molecules to "read" these instructions anduse them for growth, maintenance and self-replication.

The discovery that a kind of RNA molecule calleda ribozyme can catalyze both its own replication and theconstruction of proteins led to the hypothesis that earlierlife-forms were based entirely on RNA.[54] They could haveformed an RNA world in which there were individuals but nospecies, as mutations and horizontal gene transfers wouldhave meant that the offspring in each generation were quitelikely to have different genomes from those that theirparents started with.[55] RNA would later have been replacedby DNA, which is more stable and therefore can build longer

genomes, expanding the range of capabilities a singleorganism can have.[56] Ribozymes remain as the maincomponents ofribosomes, the "protein factories" of moderncells.[57]


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Another long-standing hypothesis is that the first life wascomposed of protein molecules. Amino acids, the buildingblocks of proteins, are easily synthesized in plausible prebiotic conditions, as are small peptides (polymers of aminoacids) that make good catalysts.[67]:295297 A series of

experiments starting in 1997 showed that amino acids andpeptides could form in the presence of carbonmonoxide and hydrogen sulfide with iron sulfide and nickelsulfide as catalysts. Most of the steps in their assembly requiredtemperatures of about 100 C (212 F) and moderate pressures,

although one stage required 250 C (482 F) and a pressureequivalent to that found under 7 kilometres (4.3 mi) of rock.Hence self-sustaining synthesis of proteins could have occurrednear hydrothermal vents.[68]


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It has been suggested that double-walled "bubbles"of lipids like those that form the external membranesof cells may have been an essential firststep.[70] Experiments that simulated the conditions ofthe 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 subjectto natural selection for longevity and reproduction.

Nucleic acids such as RNA might then have formedmore easily within the liposomes than they wouldhave outside.[71]


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Some clays, notably montmorillonite, have properties that makethem plausible accelerators for the emergence of an RNA world: theygrow by self-replication of their crystalline pattern, are subject to ananalog of natural selection (as the clay "species" that grows fastest ina particular environment rapidly becomes dominant), andcan catalyze the formation of RNA molecules.[72] Although this ideahas not become the scientific consensus, it still has activesupporters.[73]:150158[64]

Research in 2003 reported that montmorillonite could alsoaccelerate the conversion of fatty acids into "bubbles", and that thebubbles could encapsulate RNA attached to the clay. Bubbles can

then grow by absorbing additional lipids and dividing. The formationof the earliest cells may have been aided by similar processes.[74]

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


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It is believed that of this multiplicity of proto cells,only one line survived. Current phylo genetic evidencesuggests that the last universal commonancestor (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 probablya prokaryote, possessing a cell membrane and probablyribosome's 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, andenzymes to catalyze reactions. Some scientists believethat instead of a single organism being the lastuniversal common ancestor, there were populations oforganisms exchanging genes by lateral gene

transfer.[76]


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The Proterozoic eon lasted from 2.5 Ga to 542 Ma (million yearsago).[2]:130 In this time span, cratons grew into continents withmodern sizes. The change to an oxygen-rich atmosphere was acrucial development. Life developedfrom prokaryotes into eukaryotes and multicellular forms. TheProterozoic saw a couple of severe ice ages called snowball Earths.After the last Snowball Earth about 600 Ma, the evolution of life

on Earth accelerated. About 580 Ma, the Ediacara biota formedthe prelude for the Cambrian Explosion.


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The earliest cells absorbed energy and food from the environment around them.

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

manufacture their own food.[78]:377

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 uses the energy of sunlight to

power an electric circuit that makes 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.[79] Some organisms,

including purple bacteria and green sulfur bacteria, use an anoxygenic form of

photosynthesis that use alternatives to hydrogen stripped from water as electron

donors; examples are hydrogen sulfide, sulfur and iron. Such organisms are mainly

restricted to extreme environments such as hot springs and hydrothermal

vents.[78]:379382[80]

The simpler anoxygenic form arose about 3.8 Ga, not long after the appearance of

life. The timing of oxygenic photosynthesis is more controversial; it had certainly

appeared by about 2.4 Ga, but some researchers put it back as far as

3.2 Ga.[79] The latter "probably increased global productivity by at least two or

three orders of magnitude."[81][82] Among the oldest remnants of oxygen-producing

lifeforms are fossil stromatolites [81][82][41]
http://en.wikipedia.org/wiki/History_of_the_Earthhttp://en.wikipedia.org/wiki/History_of_the_Earth


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The natural evolution of the Sun made it progressivelymore luminous during the Archean and Proterozoic eons; the Sun'sluminosity increases 6% every billion years.[39]:165 As a result, the Earth

began to receive more heat from the Sun in the Proterozoic eon. However,the Earth did not get warmer. Instead, the geological record seems tosuggest it cooled dramatically during the early Proterozoic. Glacialdeposits found in South Africa date back to 2.2 Ga, at whichtime paleomagnetic evidence puts them near the equator. Thus, thisglaciation, known as the Makganyene glaciation, may have been global.Some scientists suggest this and following Proterozoic ice ages were sosevere that the planet was totally frozen over from the poles to the equator,a hypothesis called Snowball Earth.[85]

The ice age around 2.3 Ga could have been directly caused bythe increased oxygen concentration in the atmosphere, which caused thedecrease of methane (CH4) in the atmosphere. Methane is astrong greenhouse gas, but with oxygen it reacts to form CO2, a less effectivegreenhouse gas.[39]:172 When free oxygen became available in theatmosphere, the concentration of methane could have decreaseddramatically, enough to counter the effect of the increasing heat flow fromthe Sun.[86]


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Modern taxonomy classifies life into three domains. The time of the origin of these domains is uncertain.Th B d b bl f l ff f h h f f l f ( ll d N ) b h


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y f f f g fThe Bacteria domain probably first split off from the other forms of life (sometimes called Neomura), but thissupposition is controversial. Soon after this, by 2 Ga,[87] the Neomura split into the Archaea and the Eukarya.Eukaryotic cells (Eukarya) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and theorigin of that complexity is only now becoming known.

Around this time, the first proto-mitochondrion was formed. A bacterial cell related to

todaysRickettsia,[88] which had evolved tometabolize oxygen, entered a larger prokaryotic cell, which lackedthat capability. Perhaps the large cell attempted to digest the smaller one but failed (possibly due to theevolution of prey defenses). The smaller cell may have tried to parasitize the larger one. In any case, the smallercell survived inside the larger cell. Usingoxygen, it metabolized the larger cells waste products and derivedmore energy. Part of this excess energy was returned to the host. The smaller cell replicated inside the largerone. Soon, a stable symbiosis developed between the large cell and the smaller cells inside it. Over time, the hostcell acquired some of the genes of the smaller cells, and the two kinds became dependent on each other: thelarger cell could not survive without the energy produced by the smaller ones, and these in turn could notsurvive without the raw materials provided by the larger cell. The whole cell is now considered asingle organism, and the smaller cells are classified as organelles called mitochondria.[89]

A similar event occurred with photosynthetic cyano bacteria[90] entering large heterotrophic cells andbecoming chloroplasts.[83]:6061[91]:536539 Probably as a result of these changes, a line of cells capable ofphotosynthesis split off from the other eukaryotes more than 1 billion years ago. There were probably several

such inclusion events. Besides the well-established endosymbiotic theory of the cellular origin of mitochondriaand chloroplasts, there are theories that cells led to peroxisomes, spirochetes led to cilia and flagella, and thatperhaps a DNA virus led to the cell nucleus,[92],[93] though none ofthem is widely accepted.[94]


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When the theory of plate tectonics was developed around 1960, geologistsbegan to reconstruct the movements and positions of the continents in thepast. This appeared relatively easy until about 250 million years ago, whenall continents were united in the supercontinent Pangaea. Before that time,reconstructions cannot rely on apparent similarities in coastlines or agesof oceanic crust, but only on geologic observationsand paleomagnetic data.[39]:95

Throughout the history of the Earth, there have been times when thecontinental mass came together to form a supercontinent, followed by thebreak-up of the supercontinent and new continents moving apart again.This repetition of tectonic events is called a Wilson cycle. The further backin time, the scarcer and harder to interpret the data get. It is at least clearthat, about 1000 to 830 Ma, most continental mass was united in thesupercontinent Rodinia.[99] Rodinia was not the first supercontinent; it

formed at ~1.0 Ga by accretion and collision of fragments produced bybreakup of the older supercontinent, called Nuna or Columbia, which wasassembled by global-scale 2.01.8 Ga collision events.[100][101] This meansplate tectonic processes similar to today's were likely active during theProterozoic.


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