How The Life Begin

Main article:  Supercontinent cycle A reconstruction of Pannotia (550 Ma). Reconstructions of tectonic plate movement in the past 250 million years (the Cenozoic and Mesozoic eras) can be made reliably using fitting of continental margins, ocean floor magnetic anomalies and paleomagnetic poles. 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. About 1000 to 830 Ma, most continental mass was united in the supercontinent Rodinia.Rodinia may have been preceded by Early-Middle Proterozoic continents called Nun

Further information:  Eukaryote § Origin of eukaryotes Chloroplasts in the cells of a moss 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. [ citation needed ] 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 o

Main article:  Snowball Eart h The  natural evolution of the Sun  made it progressively more  luminous  during the Archean and Proterozoic eons; the Sun's luminosity increases 6% every billion years. 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 suggests it cooled dramatically during the early Proterozoic.  Glacial deposits  found in South Africa date back to 2.2 Ga, at which time, based on  paleomagnetic  evidence, they must have been located near the equator. 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 greenho

Main article:  Great Oxygenation Event See also:  Ozone layer Lithified   stromatolites  on the shores of  Lake Thetis ,  Western Australia . Archean stromatolites are the first direct fossil traces of life on Earth. A  banded iron formation  from the 3.15 Ga Moories Group,  Barberton Greenstone Belt ,  South Africa . Red layers represent the times when oxygen was available; gray layers were formed in anoxic circumstances. 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

The Proterozoic eon lasted from 2.5 Ga to 542 Ma (million years) ago.In this time span,  cratons  grew into continents with modern sizes. 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 . After the last Snowball Earth about 600 Ma, the evolution of life on Earth accelerated. About 580 Ma, the  Ediacaran biota formed the prelude for the  Cambrian Explosion .

Further information:  Graham Cairns-Smith § Clay hypothesis 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.Although this idea has not become the scientific consensus, it still has active supporters. Cross-section through a  liposome 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. A similar hypothesis presents self-replicating iron-rich clays as the progen

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.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.

The replicator in virtually all known life is  deoxyribonucleic acid . DNA is far more complex than the original replicator and its replication systems are highly elaborate. Main article:  iron–sulfur world theory 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. 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 kilometers (4.3 mi) of rock. Hence, self-sustaining synthesis of proteins could have occurred near hydrot

Main article:  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.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.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.Ribozymes remain as the main components of  ribosomes ,

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 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 .Although atmospheric composition was probably different from that used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules.Computer simulations show that  extraterrestrial organi

See also:  Origin of the world's oceans Graph showing range of estimated  partial pressure of atmospheric oxygen through geologic time  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.  After the impact which created the moon, 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.   Finally, the third atmosphere, rich in oxygen, emerged when bacteria  began to produce oxygen  about 2.8 Ga. In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of  volatiles  from t

Geologic map of North America, color-coded by age. The reds and pinks indicate rock from the  Archean . Mantle convection , the process that drives plate tectonics, is a result of heat flow from the Earth's interior to the Earth's surface. 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 1,600 °C (2,910 °F),so convection in the mantle was faster. 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 Bombardm

Main articles:  Moon ,  Origin of the Moon , and  Giant impact hypothesis Artist's impression of the enormous collision that probably formed the Moon Earth's only  natural satellite , the Moon, is larger relative to its planet than any other satellite in the solar system.During the  Apollo program , rocks from the Moon's surface were brought to Earth.  Radiometric dating  of these rocks shows that the Moon is 4.53 ± 0.01 billion years old,formed at least 30 million years after the solar system.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. 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) 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

Main articles:  Hadean  and  Archean Artist's conception of  Hadean Eon Earth, when it was much hotter and inhospitable to all forms of life. 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 . :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,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. 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. In addition, volcanism was severe due to t

See also:  Planetary differentiation The standard model for the formation of the  Solar System  (including the  Earth ) is the  solar nebula hypothesis . 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 13.8  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  from a nearby  supernova .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. The center of the nebula, not hav

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 following four timelines show the geologic time scale. 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. Millions of Years

Eons In  geochronology , time is generally measured in mya ( megayears  or million years), each unit representing the period of approximately 1,000,000 years in the past. The history of Earth is divided into four great  eons , starting 4,540 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 . EonTime (mya)Description Hadean 4,540–4,000The Earth is formed out of debris around the solar  protoplanetary disk . There is no life. Temperatures are extremely hot, with frequent volcanic activity and hellish environments. The atmosphere is nebular. Possible early oceans or bodies of liquid water. The moon is formed around this time, probably due to a  protoplanet's collision into Earth . Archean 4,000–2,500 Prokaryote  life, the first form of life, emerges at the very beginning of this eon