पृथ्वीचा इतिहास

पृथ्वीचा इतिहास या लेखात पृथ्वी ग्रहाच्या रचनेपासून ते आजपर्यंतच्या ४.६ अब्ज वर्षांच्या उत्पत्तीच्या कालावधीत घडलेल्या महत्त्वाच्या घटना आणि मूलभूत टप्प्यांची माहिती दिलेली आहे. पृथ्वीचे वय ब्रह्मांडाच्या वयाच्या साधारण एक तृतीयांश आहे. या कालावधीमधे भौगोलिक आणि जैविक स्तरावर अफाट बदल घडले आहेत.
पृथ्वीचा इतिहास दाखवण्यासाठी भूगर्भशास्त्रीय कालदर्शिका वापरतात. ज्यामध्ये पृथ्वीच्या इतिहासाचे प्रस्तरशास्त्रीय विश्लेषणानुसार वेगवेगळ्या कालखंडांमध्ये वर्गीकरण केले आहे, असे हे एक कोष्टक आहे.
धूळ आणि वायू यांपासून बनलेल्या, चक्राकार फिरणाऱ्या एका महाकाय ढगापासून पृथ्वीसकट संपूर्ण सूर्यमालेचा जन्म झाला. या ढगास सौर तेजोमेघ असे म्हणतात. साधारण १३.७ अब्ज वर्षांपूर्वी घडलेल्या महास्फोटामध्ये उत्पन्न झालेल्या हायड्रोजन, हेलियम, आणि इतर विस्फोटक ताऱ्यामधून उत्सर्जित झालेल्या जड मूलद्रव्यांपासून पासून सौर तेजोमेघ बनलेला होता. साधारण ४.६ अब्ज वर्षांपूर्वी हा तेजोमेघ आकुंचन पावू लागला. अशी कल्पना आहे की जवळपासच्या एका सुपरनोव्हाच्या आघात तरंगांमुळे आकुंचन प्रक्रियेला सुरुवात झाली असावी आणि त्याच बरोबर, तेजोमेघाला चक्राकार गती मिळाली असावी. तेजोमेघाच्या चक्राकार फिरण्याचा वेग जसा वाढत गेला तसा, गुरुत्वाकर्षण आणि जडत्वामुळे परिवलनाच्या अक्षालगत तो तेजोमेघ चपटा होऊन एका प्रोटोप्लॅनेटरी तबकडीमध्ये रूपांतरित झाला. बहुतांश द्रव्य केंद्रात स्थिरावले आणि त्याचे तापमान वाढू लागले. तबकडीतील द्रव्याच्या वाटणीत विषमता होत्या. तुलनात्मक दृष्ट्या कुठे द्रव्य विरळ होते तर दुसरीकडे घन होते. या विषमतांमुळे तबकडीच्या द्रव्यात अंतर्गत टकरी सुरू झाल्या. यामुळे व घन द्रव्याच्या स्वतःच्या अशा वेगळ्या परिवलन गतीमुळे विभागणीतील विषमता वाढत गेली आणि काही घनकिलोमीटर आकारमानाचे ग्रहपिंड बनू लागले.
द्रव्याचे आकुंचन, परिवलन गतीची वाढ आणि चिरडून टाकणारे गुरुत्वाकर्षण यामुळे तबकडीच्या केंद्रात प्रचंड प्रमाणावर गतिजन्य उष्णता निर्माण झाली. या साठणाऱ्या ऊर्जेचे उत्सर्जनाचे प्रमाण कमी होण्यासाठी कोणताही प्रयायी मार्ग उपलब्ध नसल्याने केंद्राचे तापमान प्रचंड वाढू लागले. अखेरीस, हायड्रोजनचे हेलियममधे रूपांतर होण्याची केन्द्रकीय संमीलन प्रक्रिया सुरू झाली. परिणामी, द्रव्याचे अधिक आकुंचन झाले आणि बनलेल्या टी टौरी ताऱ्याचे अंतर्गत प्रज्वलन होऊन सूर्याचा जन्म झाला.
दुसरीकडे, या सूर्याच्या गुरुत्वाकर्षण कक्षेच्या बाहेर राहिलेल्या घनतर विभागांभोवती द्रव्याचा संचय होऊ लागला व प्रोटोप्लॅनेटरी तबकडी कंकणाकृती पट्ट्यांमधे विभाजित झाली. प्रत्येक पट्ट्यातील संचयित द्रव्याच्या मोठ्या तुकड्यांमध्ये टकरी होऊन त्यांचे एकत्रीकरण घडू लागले, आणि ग्रहपिंड तयार झाले. केंद्रापासून साधारण १५ कोटी किलोमीटर अंतरावरील पट्टा म्हणजेचं पृथ्वी. साधारण ४.५४ अब्ज वर्षांपूर्वी (१%ची अनिश्चितता), पृथ्वीचा जन्म झाला आणि बहुतांश निर्माण प्रक्रिया १ कोटी ते २ कोटी वर्षांमधे पूर्ण झाली. नवजात सूर्याच्या सौर वाऱ्यांनी पसरलेल्या असंचयित वस्तुमानाची सफाई केली. संगणकीय प्रयोगांमध्ये असे दिसून आले आहे की एखाद्या प्रोटोप्लॅनेटरी तबकडीपासून सूर्यमालेसारखी ग्रह रचना तयार होऊ शकते.
पृथ्वीच्या इतिहासातील प्रथम युगास आरकीअन युग म्हणतात. या युगाचा कालावधी ४.५ अब्ज वर्षांपूर्वी पासून २.५ अब्ज वर्षांपूर्वीपर्यंत गणला जातो. पृथ्वीवरील सर्वांत जुन्या खडकांची उत्पत्ती ४ अब्ज वर्षांपूर्वी घडल्याचे आढळते. पृथ्वीच्या रचनेपासून त्या खडकांच्या रचनेपर्यंतच्या कालखंडाला काही वेळा हेडिअन नामक वेगळे युग मानले जाते. हेडिअन युगातील तीव्र स्वरूपातील उल्का पातांच्या व अतिसक्रिय ज्वालामुखींच्या क्रियेमुळे कोणत्याही पदार्थांचे जतन होऊ शकले नसावे; म्हणून या युगाबद्दल अधिक माहिती मिळत नाही.
आरकीअन युगाच्या सुरुवातीला पृथ्वी बऱ्याच अंशी थंड झाली होती. परंतु ऑक्सिजन व ओझोन पटलाच्या अभावामुळे तत्कालीन वातावरण कोणत्याही प्रकारच्या जीवनास पोषक नव्हते.
पृथ्वी-पिंडाची वाढ अभिवृद्धी प्रक्रियेने झाली. ही वाढ होत असताना पृथ्वीच्या गाभ्याचे तापमान वाढले आणि पृथ्वी-पिंडातील siderophile धातू वितळले. द्रवरूपात आल्यानंतर हे धातू त्यांच्या अधिक घनतेमुळे पृथ्वीच्या केंद्राजवळ जमा होऊलागले. ह्या प्रक्रियेला लोहापत्ती असे नाव आहे. या प्रक्रियेमुळे पृथ्वी-पिंडाच्या निर्मितीनंतर १ कोटी वर्षांच्या आत पृथ्वीचे प्रावरण आणि गाभा या २ स्तरांची निर्मिती झाली तसेच पृथ्वीभोवती भूचुंबकीय क्षेत्र तयार झाले.
अभिवृद्धी प्रक्रिया सुरू असताना पृथ्वीभोवती वायुरूप सिलिकायुक्त मेघाचे आवरण असावे. कालांतराने सिलिकेचे घनीभवन होऊन पृथ्वीचा पृष्ठभाग बनला. उरलेल्या हलक्या मूलद्रव्यांपासून पृथ्वीचे वातावरण तयार झाले. हे वातावरण प्रामुख्याने हायड्रोजन आणि हेलियमचे बनलेले होते, पण सौर वारे आणि पृथ्वीची उष्णता यांमुळे हे वातावरण फार काळ टिकले नसावे. पृथ्वी-पिंडाचा आकार वाढून सध्याच्या आकाराच्या ४०% इतका झाल्यानंतर गुरुत्वाकर्षणामुळे वातावरण टिकून राहण्यास सुरुवात झाली.
पृथ्वीचे एक वैशिष्ट्य म्हणजे तिचा मोठा नैसर्गिक उपग्रह चंद्र. अपोलो मोहिमेमध्ये चंद्रावरून काही दगड पृथ्वीवर आणले गेले. या दगडांच्या अभ्यासावरून चंद्राचे वय ४५२.७ कोटी (± १ कोटी ) वर्षे येते. याचाच अर्थ चंद्र हा सूर्यमालेतील इतर ग्रहगोलांपेक्षा ३ ते ५.५ कोटी वर्षांनी लहान आहे. चंद्राचे अजून एक वैशिष्ट्य म्हणजे सूर्यमालेतील इतर ग्रहगोलांपेक्षा त्याची घनता कमी आहे, यावरून अन्य ग्रहगोलांप्रमाणे चंद्राचा गाभा ह जड धातूंचा बनलेला नसावा असा निष्कर्ष निघतो. पृथ्वीचा लोहयुक्त गाभा वगळल्यास चंद्राची घडण ही पृथ्वीचे बाह्यकवच आणि प्रावरण यांच्याशी खूप मिळतीजुळती आहे. यामुळे राक्षसी आघाताचा सिद्धान्त मांडला गेला. या सिद्धांतानुसार असे मानले जाते की पृथ्वी-पिंडावर दुसरा एखादा ग्रहपिंड येऊन आदळला असावा, व त्यातून (भूकवच आणि प्रावरण यांच्या) उडालेल्या ठिकऱ्यांमधून अभिवृद्धी प्रक्रियेने चंद्राची निर्मिती झाली असावी.
थीआ नावाने ओळखला जाणारा हा ग्रहपिंड आकाराने सध्याच्या मंगळ ग्रहापेक्षा थोडा लहान असावा. याची निर्मिती पृथ्वी आणि सूर्य यांच्या चौथ्या/पाचव्या लॅग्रांजिअन बिंदूजवळील (दोहोंपासून सुमारे १५ कोटी किमी अंतरावरील) द्रव्याच्या अभिवृद्धी प्रक्रियेने झाली असावी. त्याची कक्षा सुरुवातीला स्थिर असावी, पण अभिवृद्धी प्रक्रियेने त्याचे वस्तुमान वाढत जाऊन ती अस्थिर झाली असावी. हळूहळू थीआची कक्षा अधिकाधिक मोठी झाली आणि त्याचे पर्यवसान थीआ आणि पृथ्वीची टक्कर होण्यात झाले.
वेगवेगळ्या प्रतिकृतींमधून असे दिसते की ही टक्कर कमी कोनात आणि कमी वेगात (८-२० किमी प्रतिसेकंद) झाली. दोन्ही ग्रहपिंडांच्या कवच आणि प्रावरणाच्या ठिकऱ्या उडून त्या पृथ्वीभोवती फिरू लागल्या. या टकरीतून प्रचंड ऊर्जा निर्माण झाली आणि काही काळासाठी पृथ्वीचे भूकवच आणि प्रावरण वितळले. यामुळे थीआच्या गाभ्यातील द्रव्य पृथ्वीच्या वितळलेल्या प्रावरणात बुडाले आणि पृथ्वीच्या गाभ्याशी एकरूप झाले. उडालेल्या ठिकऱ्या काही आठवड्यात एकत्र येऊन चंद्राची निर्मिती झाली. या ठिकऱ्यांच्या स्वतःच्या गुरुत्वाकर्षणामुळे चंद्राला त्याचा गोल आकार प्राप्त झाला.
या टकरीमुळे पृथ्वीवर काही दूरगामी परिणाम झाले. पृथ्वी वितळलेल्या अवस्थेत जाऊन तिच्या प्रावरणात मोठ्या प्रमाणात हालचाली सुरूझाल्या. पृथ्वीवरील वातावरण पूर्णतः नष्ट झाले. या टकरीमुळेच पृथ्वीचा अक्ष २३.५ अंशांनी कलला ज्यामुळे कालांतराने ऋतूंची सुरुवात झाली.
पृथ्वीच्या बाल्यात तिजभोवती वातावरण नसल्याने ती थंड पडण्याचा वेग अधिक असणार. १५ कोटी वर्षांच्या सुमारास पृथ्वीभोवती एक बॅसाॅल्टमय कडक आवरण निर्माण झाले असणार. The felsic continental crust of today did not yet exist. Within the Earth, further differentiation could only begin when the mantle had at least partly solidified again. Nevertheless, during the early Archaean (about 3.0 Ga) the mantle was still much hotter than today, probably around 1600°C. This means its fraction that was partially molten was still much larger than today.
Steam escaped from the crust, and more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity.
The large amount of water on Earth can never have been produced by volcanism and degassing alone. It is assumed the water was derived from impacting comets that contained ice. Though most comets are today in orbits further away from the Sun than Neptune, computer simulations show they were originally far more common in the inner parts of the solar system. However, most of the water on Earth was probably derived from small impacting protoplanets, objects comparable with today's small icy moons of the outer planets. Impacts of these objects can have enriched the terrestrial planets (Mercury, Venus, the Earth and Mars) with water, carbon dioxide, methane, ammonia, nitrogen and other volatiles. If all water in the Earth's oceans was derived from comets alone, a million impacting comets are required to explain the oceans. Computer simulations show this is not an unreasonable number.
As the planet cooled, clouds formed. Rain gave rise to the oceans. Recent evidence suggests the oceans may have begun forming by 4.2 Ga. At the start of the Archaean eon, the Earth was already covered with oceans. The new atmosphere probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface.
Mantle convection, the process that drives plate tectonics today, is a result of heat flow from the core 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. The inner Earth was warmer during the Hadean and Archaean eons, so convection in the mantle must have been faster. When a process similar to present day plate tectonics did occur, this will have gone faster too. Most geologists think that in the Hadean and Archaean subduction zones were more common, and therefore tectonic plates were smaller.
The initial crust that 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. It is however assumed that this crust must have been basaltic in composition like today's oceanic crust, because little crustal differentiation had yet taken place. 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 start of the Archaean, about 4.0 Ga. What is left of these first small continents are called cratons. These pieces of Archaean 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 and 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 at the time.
Cratons consist mostly 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 Archaean. The second type are complexes 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. The alternation between greenstone belts and TTG-complexes is interpreted as a tectonic situation in which small proto-continents were separated by a thorough network of subduction zones.
The details of the origin of life are unknown, but the broad principles have been established. There are two schools of thought about the origin of life. One suggests that organic components arrived on Earth from space (see “Panspermia”), while the other argues that they originated on Earth. Nevertheless, both schools propose similar mechanisms by which life initially arose.
पृथ्वीवरील सजीवसृष्टीची सुरुवात नक्की कधी झाली याबद्दल मतांतरे आहेत. Perhaps it arose around 4 Ga. In the energetic chemistry of early Earth, a molecule gained the ability to make copies of itself–a replicator. (More accurately, it promoted the chemical reactions which produced a copy of itself.) The replication was not always accurate: some copies were slightly different from their parent.
If the change destroyed the copying ability of the molecule, the molecule did not produce any copies, and the line “died out”. On the other hand, a few rare changes might make the molecule replicate faster or better: those “strains” would become more numerous and “successful”. This created evolution. As choice raw materials (“food”) became depleted, strains which could exploit different materials, or perhaps halt the progress of other strains and steal their resources, became more numerous.
The nature of the first replicator is unknown because its function was long since superseded by life’s current replicator, DNA. Several models have been proposed explaining how a replicator might have developed. Different replicators have been posited, including organic chemicals such as modern proteins, nucleic acids, phospholipids, crystals, or even quantum systems. There is currently no way to determine whether any of these models closely fits the origin of life on Earth.
One of the older theories, and one which has been worked out in some detail, will serve as an example of how this might occur. The high energy from volcanoes, lightning, and ultraviolet radiation could help drive chemical reactions producing more complex molecules from simple compounds such as methane and ammonia. Among these were many of the relatively simple organic compounds that are the building blocks of life. As the amount of this “organic soup” increased, different molecules reacted with one another. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material.
The presence of certain molecules could speed up a chemical reaction. All this continued for a very long time, with reactions occurring more or less at random, until by chance it produced a replicator molecule. In any case, at some point, the function of the replicator was superseded by DNA; all known life (except some viruses and prions) use DNA as their replicator, in an almost identical manner (see Genetic code).
Modern life has its replicating material packaged inside a cellular membrane. It is easier to understand the origin of the cell membrane than the origin of the replicator, because a cell membrane is made of phospholipid molecules which often form a bilayer spontaneously when placed in water. Under certain conditions, many such spheres can be formed (see “The bubble theory”).
The prevailing theory is that the membrane formed after the replicator, which perhaps by then was RNA (the RNA world hypothesis), along with its replicating apparatus and maybe other biomolecules. Initial protocells may have simply burst when they grew too large; the scattered contents may then have recolonized other “bubbles”. Proteins that stabilized the membrane, or that later assisted in an orderly division, would have promoted the proliferation of those cell lines.
RNA is a likely candidate for an early replicator, because it can both store genetic information and catalyze reactions. At some point DNA took over the genetic storage role from RNA, and proteins known as enzymes took over the catalysis role, leaving RNA to transfer information and modulate the process. There is increasing belief that these early cells evolved in association with underwater volcanic vents known as black smokers or even hot, deep rocks.
It is believed that of this multiplicity of protocells, only one survived. Current evidence suggests that the last universal common ancestor lived during the early Archean eon, perhaps roughly 3.5 Ga or earlier. 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 in lateral gene transfer.
The Proterozoic is the eon of Earth's history that lasted from 2.500 billion to 542 million years ago. In this time span, the cratons grew into continents with modern sizes. For the first time it is clear that plate tectonics took place in a more or less modern sense. Another important development was the change to an oxygen rich atmosphere. Life developed from prokaryotes into eukaryotes and multicellular forms. The Proterozoic saw a couple of very severe ice ages called snowball Earths. After the end of the last Snowball Earth about 600 million years ago, the evolution of life on Earth accelerated. About 580 million years ago, the Ediacara biota formed the prelude for the Cambrian Explosion.
It is likely that the initial cells were all heterotrophs, using surrounding organic molecules (including those from other cells) as raw material and an energy source. As the food supply diminished, a new strategy evolved in some cells. Instead of relying on the diminishing amounts of free-existing organic molecules, these cells adopted sunlight as an energy source. Estimates vary, but by about 3 Ga, something similar to modern photosynthesis had probably developed. This made the sun’s energy available not only to autotrophs but also to the heterotrophs that consumed them. Photosynthesis used the plentiful carbon dioxide and water as raw materials and, with the energy of sunlight, produced energy-rich organic molecules (carbohydrates).
Moreover, oxygen was produced as a waste product of photosynthesis. At first it became bound up with limestone, iron, and other minerals. There is substantial proof of this in iron-oxide rich layers in geological strata that correspond with this time period. The reaction of the minerals with oxygen would have turned the oceans green. When most of the exposed readily-reacting minerals were oxidized, oxygen finally began to accumulate in the atmosphere. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast period of time transformed Earth’s atmosphere to its current state. Among the oldest examples of oxygen-producing lifeforms are fossil stromatolites. This was Earth’s third atmosphere.
Some of the oxygen was stimulated by incoming 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 ultimately the land: without the ozone layer, ultraviolet radiation bombarding the surface would have caused unsustainable levels of mutation in exposed cells.
Photosynthesis had another, major, and world-changing impact. Oxygen was toxic; probably much life on Earth 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 enhance their metabolism and derive more energy from the same food.
An oxygen-rich atmosphere had two important advantages for life. Organisms not using oxygen for their metabolism, such as anaerobe bacteria, base their metabolism on fermentation. The abundance of oxygen makes respiration possible, a much more effective energy source for life. The second advantage of an oxygen-rich atmosphere is that oxygen forms ozone in the higher atmosphere, causing the origin of the Earth's ozone layer. The ozone layer protects the Earth's surface from ultraviolet radiation, which is harmful for life. Without the ozone layer, the development of more complex life later on would probably have been impossible.
The natural evolution of the Sun made it gradually more luminous during the Archaean 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 seems to suggest it cooled dramatically during the early Proterozoic. Glacial deposits found on all cratons show that about 2.3 Ga, the Earth underwent its first big ice age (the Makganyene ice age). Some scientists suggest this and following Proterozoic ice ages were so severe that the planet was totally frozen over from the poles to the equator, a hypothesis called Snowball Earth. Not all geologists agree with this scenario and older, Archaean ice ages have been postulated, but the ice age 2.3 Ga is the first such event for which the evidence is universally accepted.
The ice age around 2.3 Ga could have been directly caused by the increased oxygen concentration in the atmosphere, which caused the decrease of methane (CH4) in the atmosphere. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO2, a less effective greenhouse gas. When free oxygen became available in the atmosphere, the concentration of methane could have decreased dramatically, enough to counter the effect of the increasing heat flow from the Sun.
Modern taxonomy classifies life into three domains. The time of the origin of these domains is speculative. 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 coming to light.
Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today’s Rickettsia entered a larger prokaryotic cell. Perhaps the large cell attempted to ingest the smaller one but failed (maybe due to the evolution of prey defenses). Or, perhaps the smaller cell tried to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it was able to metabolize the larger cell’s waste products and derive more energy. Some of this surplus 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 of the genes of the smaller cells, and the two kinds became dependent on each other: the larger cell could not survive without the energy produced by the smaller ones, and these in turn could not survive without the raw materials provided by the larger cell. 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 larger heterotrophic cells and becoming chloroplasts. Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes more than 1 billion years ago. There were probably several such inclusion events, as the figure at right suggests. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, it has been suggested that cells gave rise to peroxisomes, spirochetes gave rise to cilia and flagella, and that perhaps a DNA virus gave rise to the cell nucleus, though none of these theories are generally accepted.
Archaeans, bacteria, and eukaryotes continued to diversify and to become more sophisticated 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. Around 1.1 Ga, the supercontinent Rodinia was assembling. The plant, animal, and fungi lines had all split, though they still existed as solitary cells. Some of these lived in colonies, and gradually some 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. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 1 billion years ago the first multicellular plants emerged, probably green algae. Possibly by around 900 million years ago true multicellularity had also evolved in animals.
At first it probably somewhat resembled that of today’s sponges, where all cells were totipotent and a disrupted organism could reassemble itself. As the division of labor became more complete in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die.
When the theory of plate tectonics was developed around 1960, geologists began to reconstruct the movements and positions of the continents in the past. This appeared relatively easy until about 250 million years back, when all continents were united in what is called the "supercontinent" Pangaea. Before that time, reconstructions cannot rely on apparent similarities in coastlines or ages of oceanic crust, but instead solely rely on geologic observations and, more importantly, on paleomagnetic data.
Throughout the history of the Earth, there have been times when the continental mass came together to form a supercontinent, followed by the break-up of the supercontinent and new continents moving apart again. This repetition of tectonic events is called a Wilson cycle. The further back in time, the scarcer and harder to interpret the data get. It is at least clear that, about 1000 to 830 million years ago, most continental mass was united in the supercontinent Rodinia. It is very probable Rodinia was not the first supercontinent and a number of early Proterozoic supercontinents have been proposed. This means plate tectonic processes similar to today's must have been active during the Proterozoic.
After the break-up of Rodinia about 800 million years ago, it is possible the continents joined again around 550 million years ago. The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia. The evidence for it is a phase of continental collision known as the Pan-African orogeny, which joined the continental masses of current-day Africa, South-America, Antarctica and Australia. It is very likely however, that the assemblage of continental masses was not complete, since a continent called Laurentia (roughly equal to current-day North America) had already started breaking off around 610 million years ago. It is at least certain that by the end of the Proterozoic eon, most of the continental mass lay united in a position around the south pole.
Proterozoic शेवटी समुद्र पृष्ठभाग शकते पूर्णपणे जप्त केली आहेत की त्यांना गंभीर किमान दोन बर्फाचा Earths पाहिले. या Cryogenian [| कालावधीत] [कालावधी (जिऑलॉजी)] सुमारे 710 आणि 640 दशलक्ष वर्षांपूर्वी झाले. या गंभीर glaciations लवकर Proterozoic बर्फाचा पृथ्वी पेक्षा स्पष्ट करण्यासाठी कमी सोपे आहेत. सर्वात paleoclimatologists थंड भाग supercontinent Rodinia निर्मिती काहीतरी होते वाटते. Rodinia विषुववृत्त सुमारे केंद्रीत होते कारण, दर [[रासायनिक] weathering] कार्बन डायऑक्साइड (सह <उप> 2 </ उप>) वातावरण काढून घेण्यात आले वाढ झाली. कारण एकत्र <उप> 2 </ उप> एक महत्त्वाचा हरितगृह वायू हवामान जागतिक स्तरावर थंड आहे. बर्फाचा Earths दरम्यान समान मार्ग नाही, मध्ये युरोपिअन बहुतांश पृष्ठभाग glaciations शेवटी अग्रगण्य, पुन्हा weathering रासायनिक रक्कम कमी जे ध्रुवप्रदेशाकडील कायम गाठलेली जमीन मध्ये होते. एक पर्यायी गृहीते ते पुरेसे कार्बन डायऑक्साइड परिणामी हरितगृह परिणाम आणि जागतिक तापमान असण्याचा ज्वालामुखीचा outgassing सुटका आहे. Whereas the Ediacaran life forms appear yet primitive and not easy to put in any modern group, at the end of the Cambrian most modern phyla were already present. Because of 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 fossilisation 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. The boundary between the Cambrian and Ordovician (the following period, 488-444 million years ago) is characterized by a large mass-extinction, in which some of the new groups disappeared altogether. Some of these Cambrian groups appear complex but totally different compared to modern life, examples are Anomalocaris and Haikouichthys.
During the Cambrian, the first vertebrate animals, among them the first fishes, had appeared. A creature that could have been the ancestor of the fishes or was probably closely related to it, was Pikaia. 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 Ordovician. The colonisation of new niches resulted in gigantic body sizes. In this way, fishes with increasing sizes evolved in the course of the early Paleozoic, such as the gigantic placoderm Dunkleosteus, which could become 7 meters long.
At the end of the Proterozoic, the supercontinent Pannotia had broken apart in the smaller continents Laurentia, Baltica, Siberia and Gondwana. During periods when continents move apart, more oceanic crust is formed by volcanic activity. Because young volcanic crust is relatively hot and less dense than old oceanic crust, the ocean floors will 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 generally warmer than today, but the end of the Ordovician saw a short ice age, in 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 number of mass extinctions took place, in which many brachiopods, trilobites, Bryozoa and corals disappeared. These marine species could probably not cope with the decreasing temperature of the sea water. After the extinctions new species evolved, more diverse and better adapted. They would fill the niches left by the extinct species.
The continents Laurentia and Baltica collided between 450 and 400 million years ago, during the Caledonian Orogeny, to form Laurussia. Traces of the mountain belt which resulted from this collision can be found in Scandinavia, Scotland and the northern Appalachians. In the Devonian period (416-359 million years ago) Gondwana and Siberia began to move towards Laurussia. The collision of Siberia with Laurussia caused the Uralian Orogeny, the collision of Gondwana with Laurussia is called the Variscan or Hercynian Orogeny in Europe or the Alleghenian Orogeny in North America. The latter phase took place during the Carboniferous period (359-299 million years ago) and resulted in the formation of the last supercontinent, Pangaea.
Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of 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. Prokaryotes had probably colonized the land as early as 2.6 Ga even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600 Ma and then broke apart a short 50 million years later. Fish, the earliest vertebrates, evolved in the oceans around 530 Ma. A major extinction event occurred near the end of the Cambrian period, which ended 488 Ma.
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 oldest fossils of land fungi and plants date to 480–460 Ma, though molecular evidence suggests the fungi may have colonized the land as early as 1000 Ma and the plants 700 Ma. Initially remaining close to the water’s edge, mutations and variations resulted in further colonization of this new environment. The timing of the first animals to leave the oceans is not precisely known: the oldest clear evidence is of arthropods on land around 450 Ma, perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also some unconfirmed evidence that arthropods may have appeared on land as early as 530 Ma.
At the end of the Ordovician period, 440 Ma, additional extinction events occurred, perhaps due to a concurrent ice age. Around 380 to 375 Ma, the first tetrapods evolved from fish. It is thought that perhaps fins evolved to become limbs which allowed the first tetrapods to lift their heads out of the water to breathe air. This would let them survive in oxygen-poor water or pursue small prey in shallow water. They may have later ventured on land for brief periods. 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. About 365 Ma, another period of extinction occurred, perhaps as a result of global cooling. Plants evolved seeds, which dramatically accelerated their spread on land, around this time (by approximately 360 Ma).
Some 20 million years later (340 Ma), the amniotic egg evolved, which could be laid on land, giving a survival advantage to tetrapod embryos. This resulted in the divergence of amniotes from amphibians. Another 30 million years (310 Ma) saw the divergence of the synapsids (including mammals) from the sauropsids (including birds and reptiles). Other groups of organisms continued to evolve and lines diverged—in fish, insects, bacteria, and so on—but less is known of the details. 300 million years ago, the most recent hypothesized supercontinent formed, called Pangaea.
The most severe extinction event to date took place 250 Ma, at the boundary of the Permian and Triassic periods; 95% of life on Earth died out, possibly due to the Siberian Traps volcanic event. The discovery of the Wilkes Land crater in Antarctica may suggest a connection with the Permian-Triassic extinction, but the age of that crater is not known. But life persevered, and around 230 Ma, dinosaurs split off from their reptilian ancestors. An extinction event between the Triassic and Jurassic periods 200 Ma spared many of the dinosaurs, and they soon became dominant among the vertebrates. Though some of the mammalian lines began to separate during this period, existing mammals were probably all small animals resembling shrews.
By 180 Ma, Pangaea broke up into Laurasia and Gondwana. The boundary between avian and non-avian dinosaurs is not clear, but Archaeopteryx, traditionally considered one of the first birds, lived around 150 Ma. The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period, some 20 million years later (132 Ma).
Competition with birds drove many pterosaurs to extinction and the dinosaurs were probably already in decline when, 65 Ma, a १०-किलोमीटर (६.२ मैल) meteorite probably struck Earth just off the Yucatán Peninsula where the Chicxulub crater is today. This ejected vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. Most large animals, including the non-avian dinosaurs, became extinct, marking the end of the Cretaceous period and Mesozoic era. Thereafter, in the Paleocene epoch, mammals rapidly diversified, grew larger, and became the dominant vertebrates. Perhaps a couple of million years later (around 63 Ma), the last common ancestor of primates lived. By the late Eocene epoch, 34 Ma, some terrestrial mammals had returned to the oceans to become animals such as Basilosaurus which later gave rise to dolphins and baleen whales.
A small African ape living around six million years ago was the last animal whose descendants would include both modern humans and their closest relatives, the bonobos, and chimpanzees. Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still debated, apes in one branch developed the ability to walk upright. Brain size increased rapidly, and by 2 Ma, the very first animals classified in the genus Homo had appeared. Of course, the line between different species or even genera is rather arbitrary as organisms continuously change over generations. Around the same time, the other branch split into the ancestors of the common chimpanzee and the ancestors of the bonobo as evolution continued simultaneously in all life forms.
The ability to control fire probably began in Homo erectus (or Homo ergaster), probably at least 790,000 years ago but perhaps as early as 1.5 Ma. In addition it has sometimes suggested that the use and discovery of controlled fire may even predate Homo erectus. Fire was possibly used by the early Lower Paleolithic (Oldowan) hominid Homo habilis and/or by robust australopithecines such as Paranthropus.
It is more difficult to establish the origin of language; it is unclear whether Homo erectus could speak or if that capability had not begun until Homo sapiens. As brain size increased, babies were born sooner, before their heads grew too large to pass through the pelvis. As a result, they exhibited more plasticity, and thus possessed an increased capacity to learn and required a longer period of dependence. Social skills became more complex, language became more advanced, and tools became more elaborate. This contributed to further cooperation and brain development. Anatomically modern humans — Homo sapiens — are believed to have originated somewhere around 200,000 years ago or earlier in Africa; the oldest fossils date back to around 160,000 years ago.
The first humans to show evidence of spirituality are the Neanderthals (usually classified as a separate species with no surviving descendants); they buried their dead, often apparently with food or tools. However, evidence of more sophisticated beliefs, such as the early Cro-Magnon cave paintings (probably with magical or religious significance) did not appear until some 32,000 years ago. Cro-Magnons also left behind stone figurines such as Venus of Willendorf, probably also signifying religious belief. By 11,000 years ago, Homo sapiens had reached the southern tip of South America, the last of the uninhabited continents (except for Antarctica, which remained undiscovered until 1820 AD). Tool use and language continued to improve; interpersonal relationships became more complex.
मानवाच्या इतिहासातील ९०%पेक्षा जास्त काळ त्याने भटक्या जमातींच्या स्वरूपात व्यतीत केला आहे. hunter-gatherers. जसजशी भाषांची उत्क्रांती होत गेली, तसतशी माणसाची भूतकाळातील घटना लक्षात ठेवण्याची आणि ती इतरांना सांगण्याची शक्तीही वाढली. यामुळे संचितमाहिती शतगुणित होण्यासाठीचे एक साधनही मिळाले. यामुळे माणसाला आपल्याला सुचलेल्या कल्पनांचे एकमेकांशी आदानप्रदान करणे तसेच आपल्यानंतरच्या पिढीला सांगणे सोपे झाले.
Cultural evolution quickly outpaced biological evolution, and history proper began. Somewhere between 8500 and 7000 BC, humans in the Fertile Crescent in Middle East began the systematic husbandry of plants and animals: agriculture. This spread to neighboring regions, and also developed independently elsewhere, until most Homo sapiens lived sedentary lives in permanent settlements as farmers.
Not all societies abandoned nomadism, especially those in isolated areas of the globe poor in domesticable plant species, such as Australia. However, among those civilizations that did adopt agriculture, the relative security and increased productivity provided by farming allowed the population to expand.
Agriculture had a major impact; humans began to affect the environment as never before. Surplus food allowed a priestly or governing class to arise, followed by increasing division of labor. This led to Earth’s first civilization at Sumer in the Middle East, between 4000 and 3000 BC. Additional civilizations quickly arose in ancient Egypt, at the Indus River valley and in China.
Starting around 3000 BC, Hinduism, one of the oldest religions still practiced today, began to take form. Others soon followed. The invention of writing enabled complex societies to arise: record-keeping and libraries served as a storehouse of knowledge and increased the cultural transmission of information. Humans no longer had to spend all their time working for survival—curiosity and education drove the pursuit of knowledge and wisdom.
Various disciplines, including science (in a primitive form), arose. New civilizations sprang up, traded with one another, and engaged in war for territory and resources: empires began to form. By around 500 BC, there were empires in the Middle East, Iran, India, China, and Greece, approximately on equal footing; at times one empire expanded, only to decline or be driven back later.
In the fourteenth century, the Renaissance began in Italy with advances in religion, art, and science. Starting around 1500, European civilization began to undergo changes leading to the scientific and industrial revolutions: that continent began to exert political and cultural dominance over human societies around the planet. From 1914 to 1918 and 1939 to 1945, nations around the world were embroiled in world wars.
Established following World War I, the League of Nations was a first step in establishing international institutions to resolve disputes peacefully; after its failure to prevent World War II and the subsequent end of the conflict it was replaced by the United Nations. In 1992, several European nations joined together in the European Union. As transportation and communication improved, the economies and political affairs of nations around the world have become increasingly intertwined. This globalization has often produced both discord and collaboration.
१९४० च्या दशकापासून आजमितीस मोठ्या गतीने बदल घडत आहेत. तंत्रज्ञानातील मोठ्या वाटचालींमध्ये आण्विक शस्त्रे, संगणक, अतिसूक्ष्मतंत्रज्ञान, आणि जनुक तंत्रज्ञान यांचा समावेश आहे. Economic globalization spurred by advances in communication and transportation technology has influenced everyday life in many parts of the world. Cultural and institutional forms such as democracy, capitalism, and environmentalism have increased influence. Major concerns and problems such as disease, war, poverty, violent radicalism, and more recently, human-caused climate change have risen as the world population increases.
१९५७मध्ये सोवियेत संघाने पहिला कृत्रिम उपग्रह अंतराळात प्रक्षेपित केला आणि नंतर युरी गागारिन अंतराळात जाणारा पहिला मानव झाला. अमेरिकेचा नील आर्मस्ट्राँग चंद्रावर पाउल ठेवलेला पहिला मानव होता. त्यानंतर मानवविरहित अंतराळयाने सूर्यमालेच्या दूर भागांतून गेली आहेत आणि व्हॉयेजर मालिकेतील याने सूर्यमालेच्या पलीकडेही गेली आहेत. The Soviet Union and the United States were the primary early leaders in space exploration in the 20th Century. Five space agencies, representing over fifteen countries, have worked together to build the International Space Station. Aboard it, there has been a continuous human presence in space since 2000.