For more detail of the biological history of Earth, seeHistory of life.
Earth's history with time-spans of theeons to scale.Ma means "million years ago".
The naturalhistory of Earth concerns the development ofplanetEarth from its formation to the present day.[1][2] Nearly all branches ofnatural science have contributed to understanding of the main events of Earth's past, characterized by constantgeological change and biologicalevolution.
Thegeological time scale (GTS), as defined by international convention,[3] depicts the large spans of time from the beginning of Earth to the present, and its divisions chronicle some definitive events of Earth history. Earth formed around 4.54 billion years ago, approximately one-third theage of the universe, byaccretion from thesolar nebula.[4][5][6] Volcanicoutgassing probably created the primordialatmosphere and then the ocean, but the early atmosphere contained almost nooxygen. Much of 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 namedTheia is thought to have formed the Moon. Over time, Earth cooled, causing the formation of a solidcrust, and allowing liquid water on the surface.
TheHadean eon represents the time before a reliable (fossil) record of life; it began with the formation of the planet and ended 4.0 billion years ago. The followingArchean andProterozoic eons produced thebeginnings of life on Earth and its earliestevolution. The succeeding eon is thePhanerozoic, divided into three eras: thePalaeozoic, an era of arthropods, fishes, and the first life on land; theMesozoic, which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs; and theCenozoic, which saw the rise of mammals. Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale.
Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen.Life remained mostly small and microscopic until about580 million years ago, when complexmulticellular life arose, developed over time, and culminated in theCambrian Explosion about 538.8 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,[16] have goneextinct.[17][18] Estimates on the number of Earth's currentspecies range from 10 million to 14 million,[19] of which about 1.2 million are documented, but over 86 percent have not been described.[20]
Earth's crust has constantly changed since its formation, as has life since its first appearance. Species continue toevolve, taking on new forms, splitting into daughter species, or going extinct in the face of ever-changing physical environments. The process ofplate tectonics continues to shape Earth's continents and oceans and the life they harbor.
Eons
Ingeochronology, time is generally measured inmya (million years ago), each unit representing the period of approximately 1,000,000 years in the past. The history of Earth is divided into four greateons, 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.
Earth is formed out of debris around the solarprotoplanetary disk. There is no life. Temperatures are extremely hot, with frequent volcanic activity and hellish-looking environments (hence the eon's name, which comes fromHades). The atmosphere is nebular. Possible early oceans or bodies of liquid water. The Moon is formed around this time probably due to aprotoplanet's collision into Earth.
Prokaryote life, the first form of life, emerges at the very beginning of this eon, in a process known asabiogenesis. The continents ofUr,Vaalbara andKenorland may have existed around this time. The atmosphere is composed of volcanic and greenhouse gases.
The name of this eon means "early life".Eukaryotes, a more complex form of life, emerge, including some forms ofmulticellular organisms.Bacteria begin producing oxygen, shaping the third and current of Earth's atmospheres. Plants, later animals and possibly earlier forms of fungi form around this time. The early and late phases of this eon may have undergone "Snowball Earth" periods, in which all of the planet suffered below-zero temperatures. The early continents ofColumbia,Rodinia andPannotia, in that order, may have existed in this eon.
Complex life, includingvertebrates, begin to dominate Earth's ocean in a process known as theCambrian explosion.Pangaea forms and later dissolves intoLaurasia andGondwana, which in turn dissolve into the current continents. Gradually, life expands to land and familiar forms of plants, animals and fungi begin appearing, including annelids, insects and reptiles, hence the eon's name, which means "visible life". Severalmass extinctions occur, among which birds, the descendants of non-avian dinosaurs, and more recently mammals emerge. Modern animals—including humans—evolve at the most recent phases of this eon.
The history of Earth can be organized chronologically according to thegeologic time scale, which is split into intervals based onstratigraphic analysis.[2][21]
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The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of Earth to the present, but this gives little space for the most recent eon. 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, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline.
(Horizontal scale is millions of years for the above timelines; thousands of years for the timeline below)
The standard model for the formation of theSolar System (includingEarth) is thesolar nebula hypothesis.[22] In this model, the Solar System formed from a large, rotating cloud of interstellar dust and gas called thesolar nebula. It was composed ofhydrogen andhelium createdshortly after theBig Bang 13.8 Ga (billion years ago) and heavierelements ejected bysupernovae. About 4.5 Ga, the nebula began a contraction that may have been triggered by theshock wave from a nearbysupernova.[23] A shock wave would have also made the nebula rotate. As the cloud began to accelerate, itsangular momentum,gravity, andinertia flattened it into aprotoplanetary disk perpendicular to its axis of rotation. Smallperturbations due to collisions and the angular momentum of other large debris created the means by which kilometer-sizedprotoplanets began to form, orbiting the nebular center.[24]
The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it untilnuclear fusion of hydrogen into helium began. After more contraction, aT Tauri star ignited and evolved into theSun. Meanwhile, in the outer part of the nebula gravity causedmatter to condense around density perturbations and dust particles, and the rest of the protoplanetary disk began separating into rings. In a process known as runawayaccretion, successively larger fragments of dust and debris clumped together to form planets.[24] Earth formed in this manner about 4.54 billion years ago (with anuncertainty of 1%)[25][26][4] and was largely completed within 10–20 million years.[27] In June 2023, scientists reported evidence that the planet Earth may have formed in just three million years, much faster than the 10−100 million years thought earlier.[28][29] Nonetheless, thesolar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies. The same process is expected to produceaccretion disks around virtually all newly forming stars in the universe, some of which yieldplanets.[30]
The proto-Earth grew by accretion until its interior was hot enough to melt the heavy,siderophilemetals. Having higherdensities than the silicates, these metals sank. This so-callediron catastrophe resulted in the separation of aprimitive mantle and a (metallic) core only 10 million years after Earth began to form, producing thelayered structure of Earth and setting up the formation ofEarth's magnetic field.[31] J.A. Jacobs was the first to suggest thatEarth's inner core—a solid center distinct from the liquidouter core—isfreezing and growing out of the liquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius per billion years[32]).[33]
Artist's conception ofHadean Eon Earth, when it was much hotter and inhospitable to all forms of life.
The firsteon in Earth's history, theHadean, begins with Earth's formation and is followed by theArchean eon at 3.8 Ga.[2]: 145 The oldest rocks found on Earth date to about 4.0 Ga, and the oldestdetritalzircon crystals in rocks to about 4.4 Ga,[34][35][36] soon after the formation of Earth'scrust and Earth itself. Thegiant 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 themantle and crust into space and created the Moon.[37][38][39]
Fromcrater counts on other celestial bodies, it is inferred that a period of intense meteorite impacts, called theLate Heavy Bombardment, began about 4.1 Ga, and concluded around 3.8 Ga, at the end of the Hadean.[40] In addition, volcanism was severe due to the largeheat flow andgeothermal gradient.[41] Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that Earth already had oceans or seas at that time.[34]
By the beginning of the Archean, Earth had cooled significantly. Present life forms could not have survived at Earth's surface, because the Archean atmosphere lackedoxygen hence had noozone layer to block ultraviolet light. Nevertheless, it is believed that primordial life began to evolve by the early Archean, with candidatefossils dated to around 3.5 Ga.[42] Some scientists even speculate that life could have begun during the early Hadean, as far back as 4.4 Ga, surviving the possible Late Heavy Bombardment period inhydrothermal vents below Earth's surface.[43]
Artist's impression of the enormous collision that probably formed the Moon
Earth's onlynatural satellite, the Moon, is larger relative to its planet than any other satellite in the Solar System.[nb 1] During theApollo 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,[46] formed at least 30 million years after the Solar System.[47] 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.[48]
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 Earth[49]) and a small metallic core. Second, Earth and Moon have the same oxygenisotopic signature (relative abundance of the oxygen isotopes). Of the theories proposed to account for these phenomena, one is widely accepted: Thegiant impact hypothesis proposes that the Moon originated after a body the size ofMars (sometimes namedTheia[47]) struck the proto-Earth a glancing blow.[1]: 256 [50][51]
The collision released about 100 million times more energy than the more recentChicxulub impact that is believed to have caused the extinction of the non-avian dinosaurs. It was enough to vaporize some of Earth's outer layers and melt both bodies.[50][1]: 256 A portion of the mantle material wasejected into orbit around Earth. The giant impact hypothesis predicts that the Moon was depleted of metallic material,[52] explaining its abnormal composition.[53] The ejecta in orbit around Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.[54]
Artist's impression of a Hadean landscape with the relatively newly formed Moon still looming closely over Earth and both bodies sustaining strongvolcanism.
First continents
Geologic map of North America, color-coded by age. From most recent to oldest, age is indicated by yellow, green, blue, and red. The reds and pinks indicate rock from theArchean.
Plate tectonics is driven bymantle convection, the flow of heated rock from Earth's interior to the surface.[55]: 2 The rising mantle extruded atmid-oceanic ridges builds up rigidtectonic plates, which are eventually shifted tosubduction zones where they sink back into the mantle. During the early Archean (about 3.0 Ga) the mantle was probably around 1,600 °C (2,910 °F) hotter than today,[56]: 82 so convection in the mantle was faster, leading to a faster tectonic process during the Hadean and Archean. Subduction zones were more common and tectonic plates smaller.[1]: 258 [57]
The initial crust, which formed when 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 thought the original crust wasbasaltic like today'soceanic crust, because little crustal differentiation had yet taken place.[1]: 258 The first larger pieces ofcontinental crust, formed of lighter elements which float upward duringpartial melting in the lower crust, appeared at the end of the Hadean, about 4.0 Ga. What is left of these first small continents are calledcratons; they form the cores around which today's continents grew.[58]
Theoldest rocks on Earth are found in theNorth American craton ofCanada. They aretonalites from about 4.0 Ga. They show traces ofmetamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing that rivers and seas existed then.[59] Cratons consist primarily of two alternating types ofterranes. The first are so-calledgreenstone belts, consisting of low-grade metamorphosed sedimentary rocks. These "greenstones" are similar to the sediments today found inoceanic trenches, above subduction zones, suggesting the start of subduction during the Archean. The second type is a complex offelsicmagmatic rocks, mostly tonalite,trondhjemite orgranodiorite, similar togranite: such terranes are called TTG-terranes. TTG-complexes are seen as therelicts of the first continental crust, formed by partial melting in basalt.[60]: Chapter 5
Earth is often described as having had three atmospheres. The first, 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, and today's atmosphere is depleted of these elements compared to cosmic abundances.[61] After the impact which created the Moon, the molten Earth released volatile gases; and later more gases were released byvolcanoes, completing a second atmosphere rich ingreenhouse gases but poor in oxygen.[1]: 256 Finally, the third atmosphere, rich in oxygen, emerged when bacteriabegan to produce oxygen about 2.8 Ga.[62]: 83–84, 116–117
In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing ofvolatiles from Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known asimpact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would, therefore, have started to form even as Earth formed.[66] The new atmosphere probably containedwater vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.[67]
Planetesimals at a distance of 1 astronomical unit (AU), the distance of Earth from the Sun, probably did not contribute any water to Earth because the solar nebula was too hot for ice to form and the hydration of rocks by water vapor would have taken too long.[66][68] The water must have been supplied by meteorites from the outer asteroid belt and some large planetary embryos from beyond 2.5 AU.[66][69] Comets may also have contributed. Though most comets are today in orbits farther away from the Sun thanNeptune, computer simulations show that they were originally far more common in the inner parts of the Solar System.[59]: 130–132
As Earth cooled,clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4.4 Ga.[34] By the start of the Archean eon, they already covered much of Earth. This early formation has been difficult to explain because of a problem known as thefaint young Sun paradox. Stars are known to get brighter as they age, and the Sun has become 30% brighter since its formation 4.5 billion years ago.[70] Many models indicate that the early Earth should have been covered in ice.[71][66] A likely solution is that there was enough carbon dioxide and methane to produce agreenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. It is hypothesized that there also existed an organic haze created from the products of methane photolysis that caused ananti-greenhouse effect as well.[72] Another greenhouse gas,ammonia, would have been ejected by volcanos but quickly destroyed by ultraviolet radiation.[62]: 83
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.[73][74]
The first step in the emergence of life may have been chemical reactions that produced many of the simplerorganic compounds, includingnucleobases andamino acids, that are the building blocks of life. Anexperiment in 1952 byStanley Miller andHarold 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 oflightning.[75] 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.[76]Computer simulations show thatextraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of Earth.[77]
Additional complexity could have been reached from at least three possible starting points:self-replication, an organism's ability to produce offspring that are similar to itself;metabolism, its ability to feed and repair itself; and externalcell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances.[78]
Even the simplest members of thethree modern domains of life useDNA to record their "recipes" and a complex array ofRNA andprotein molecules to "read" these instructions and use them for growth, maintenance, and self-replication.
The discovery that a kind of RNA molecule called aribozyme cancatalyze both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA.[79] They could have formed anRNA world in which there were individuals but nospecies, asmutations andhorizontal gene transfers would have meant that the offspring in each generation were quite likely to have differentgenomes from those that their parents started with.[80] 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.[81] Ribozymes remain as the main components ofribosomes, the "protein factories" of modern cells.[82]
Although short, self-replicating RNA molecules have been artificially produced in laboratories,[83] doubts have been raised about whether natural non-biological synthesis of RNA is possible.[84][85][86] The earliest ribozymes may have been formed of simplernucleic acids such asPNA,TNA orGNA, which would have been replaced later by RNA.[87][88] Otherpre-RNA replicators have been posited, includingcrystals[89]: 150 and even quantum systems.[90]
In 2003 it was proposed that porous metal sulfideprecipitates would assist RNA synthesis at about 100 °C (212 °F) and at ocean-bottom pressures nearhydrothermal vents. In this hypothesis, the proto-cells would be confined in the pores of the metal substrate until the later development of lipid membranes.[91]
Metabolism first: iron–sulfur world
The replicator in virtually all known life isdeoxyribonucleic acid. DNA is far more complex than the original replicator and its replication systems are highly elaborate.
Another long-standing hypothesis is that the first life was composed of protein molecules. Amino acids, the building blocks ofproteins, are easily synthesized in plausible prebiotic conditions, as are smallpeptides (polymers of amino acids) that make good catalysts.[92]: 295–297 A series of experiments starting in 1997 showed that amino acids and peptides could form in the presence ofcarbon monoxide andhydrogen sulfide withiron sulfide andnickel 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 hydrothermal vents.[93]
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.[94]
Membranes first: Lipid world
It has been suggested that double-walled "bubbles" oflipids like those that form the external membranes of cells may have been an essential first step.[95] Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously formliposomes, double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject tonatural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside.[96]
Someclays, notablymontmorillonite, 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 ofnatural selection (as the clay "species" that grows fastest in a particular environment rapidly becomes dominant), and can catalyze the formation of RNA molecules.[97] Although this idea has not become the scientific consensus, it still has active supporters.[98]: 150–158 [89]
Research in 2003 reported that montmorillonite could also accelerate the conversion offatty 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 earliestcells may have been aided by similar processes.[99]
A similar hypothesis presents self-replicating iron-rich clays as the progenitors ofnucleotides, lipids and amino acids.[100]
It is believed that of this multiplicity of protocells, only oneline survived. Currentphylogenetic evidence suggests that thelast universal ancestor (LUA) lived during the earlyArchean eon, perhaps 3.5 Ga or earlier.[101][102] This LUA cell is the ancestor of all life on Earth today. It was probably aprokaryote, possessing a cell membrane and probably ribosomes, but lacking anucleus or membrane-boundorganelles such asmitochondria orchloroplasts. Like modern cells, it used DNA as its genetic code, RNA for information transfer andprotein synthesis, and enzymes tocatalyze reactions. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes bylateral gene transfer.[103]
Artist's impression of Earth during the later Archean, the largely cooledplanetary crust and water-rich barrensurface, marked byvolcanoes andcontinents, features alreadyroundmicrobialites. The Moon, still orbiting Earth much closer than today and still dominating Earth's sky, produced strongtides.[104]
The Proterozoic eon lasted from 2.5 Ga to 538.8 Ma (million years) ago.[105] 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 intoeukaryotes and multicellular forms. The Proterozoic saw a couple of severe ice ages calledSnowball Earths. After the last Snowball Earth about 600 Ma, the evolution of life on Earth accelerated. About 580 Ma, theEdiacaran biota formed the prelude for theCambrian Explosion.[citation needed]
The earliest cells absorbed energy and food from the surrounding environment. They usedfermentation, 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 ananaerobic (oxygen-free) environment. The evolution ofphotosynthesis made it possible for cells to derive energy from the Sun.[106]: 377
Most of the life that covers the surface of 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.[107] Some organisms, includingpurple bacteria andgreen sulfur bacteria, use ananoxygenic form of photosynthesis that uses alternatives to hydrogen stripped from water aselectron donors; examples are hydrogen sulfide, sulfur and iron. Suchextremophile organisms are restricted to otherwise inhospitable environments such as hot springs and hydrothermal vents.[106]: 379–382 [108]
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.[107] The latter "probably increased global productivity by at least two or three orders of magnitude".[109][110] Among the oldest remnants of oxygen-producing lifeforms are fossilstromatolites.[109][110][111]
At first, the released oxygen was bound up withlimestone,iron, and other minerals. The oxidized iron appears as red layers in geological strata calledbanded iron formations that formed in abundance during theSiderian period (between 2500 Ma and 2300 Ma).[2]: 133 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 time transformed Earth's atmosphere to its current state. This was Earth's third atmosphere.[112]: 50–51 [62]: 83–84, 116–117
Some oxygen was stimulated by solar ultraviolet radiation to formozone, 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: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells.[113][59]: 219–220
Graph showing range of estimatedpartial pressure of atmospheric oxygen through geologic time[111]
Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died out as its levels rose in what is known as theoxygen 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.[113]
Artist's rendition of an oxinated fully-frozenSnowball Earth with no remaining liquid surface water.
Thenatural evolution of the Sun made it progressively moreluminous during the Archean and Proterozoic eons; the Sun's luminosity increases 6% every billion years.[59]: 165 As a result, Earth began to receive more heat from the Sun in the Proterozoic eon. However, 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 onpaleomagnetic evidence, they must have been located near the equator. Thus, this glaciation, known as theHuronian glaciation, may have been global. Some scientists suggest this was so severe that Earth was frozen over from the poles to the equator, a hypothesis called Snowball Earth.[114]
The Huronian ice age might have been caused by theincreased 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.[59]: 172 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.[115]
However, the term Snowball Earth is more commonly used to describe later extreme ice ages during theCryogenian period. There were four periods, each lasting about 10 million years, between 750 and 580 million years ago, when Earth is thought to have been covered with ice apart from the highest mountains, and average temperatures were about −50 °C (−58 °F).[116] The snowball may have been partly due to the location of the supercontinentRodinia straddling theEquator. 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 Cryogenian 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 methanegas hydrates. According to the alternativeSlushball Earth theory, even at the height of the ice ages there was still open water at the Equator.[117][118]
Moderntaxonomy classifies life into three domains. The time of their origin is uncertain. TheBacteria domain probably first split off from the other forms of life (sometimes calledNeomura), but this supposition is controversial. Soon after this, by 2 Ga,[119] the Neomura split into theArchaea and theEukaryota. Eukaryotic cells (Eukaryota) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now becoming known.[120] The earliest fossils possessing features typical offungi date to thePaleoproterozoic era, some 2.4 Ga ago; these multicellularbenthic organisms had filamentous structures capable ofanastomosis.[121]
Around this time, the firstproto-mitochondrion was formed. A bacterial cell related to today'sRickettsia,[122] which had evolved tometabolize 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 toparasitize 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 stablesymbiosis 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 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 singleorganism, and the smaller cells are classified asorganelles called mitochondria.[123]
A similar event occurred withphotosyntheticcyanobacteria[124] entering largeheterotrophic cells and becoming chloroplasts.[112]: 60–61 [125]: 536–539 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. Besides the well-establishedendosymbiotic theory of the cellular origin of mitochondria and chloroplasts, there are theories that cells led toperoxisomes,spirochetes led tocilia andflagella, and that perhaps aDNA virus led to the cell nucleus,[126][127] though none of them are widely accepted.[128]
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. Around 1.1 Ga, theplant,animal, andfungi lines had split, though they still existed as solitary cells. Some of these lived in colonies, and gradually adivision 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[129], the first multicellular plants emerged, probablygreen algae.[130] Possibly by around 900 Ma[125]: 488 true multicellularity had also evolved in animals.[131]
At first, it probably resembled today'ssponges, which havetotipotent cells that allow a disrupted organism to reassemble itself.[125]: 483–487 As the division of labor was completed in the different lineages of multicellular organisms, cells became more specialized and more dependent on each other.[132]
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 andpaleomagnetic poles. No ocean crust dates back further than that, so earlier reconstructions are more difficult. Paleomagnetic poles are supplemented by geologic evidence such asorogenic 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.[133]: 370
Throughout the history of 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.[133]: 370 [134] Rodinia may have been preceded by Early-Middle Proterozoic continents called Nuna and Columbia.[133]: 374 [135][136]
After the break-up of Rodinia about 800 Ma, the continents may have formed another short-lived supercontinent around 550 Ma. The hypothetical supercontinent is sometimes referred to asPannotia or Vendia.[137]: 321–322 The evidence for it is a phase ofcontinental collision known as thePan-African orogeny, which joined the continental masses of current-day Africa, South America, Antarctica and Australia. The existence of Pannotia depends on the timing of the rifting betweenGondwana (which included most of the landmass now in the Southern Hemisphere, as well as theArabian Peninsula and theIndian subcontinent) andLaurentia (roughly equivalent to current-day North America).[133]: 374 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.[138]
Late Proterozoic climate and life
A 580 million year old fossil ofSpriggina floundensi, an animal from theEdiacaran period. Such life forms could have been ancestors to the many new forms that originated in theCambrian Explosion.
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 716.5 and 635 Ma, in theCryogenian period.[139] The intensity and mechanism of both glaciations are still under investigation and harder to explain than the early Proterozoic Snowball Earth.[140]Most paleoclimatologists think the cold episodes were linked to the formation of the supercontinent Rodinia.[141] Because Rodinia was centered on the equator, rates ofchemical weathering increased and carbon dioxide (CO2) was taken from the atmosphere. Because CO2 is an important greenhouse gas, climates cooled globally.[142]
In the same way, during the Snowball Earths most of the continental surface was covered withpermafrost, which decreased chemical weathering again, leading to the end of the glaciations. An alternative hypothesis is that enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures.[141] Increased volcanic activity resulted from the break-up of Rodinia at about the same time.[143]
The Cryogenian period was followed by theEdiacaran period, which was characterized by a rapid development of new multicellular lifeforms.[144] Whether there is a connection between the end of the severe ice ages and the increase in diversity of life is not clear, but it does not seem coincidental. 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.[145] Important developments were the origin of muscular and neural cells. None of the Ediacaran fossils had hard body parts like skeletons. These first appear after the boundary between the Proterozoic andPhanerozoic eons or Ediacaran and Cambrian periods.[146]
The Phanerozoic is the current eon on Earth, which started approximately 538.8 million years ago. It consists of three eras: ThePaleozoic,Mesozoic, andCenozoic,[105] and is the time when multi-cellular life greatly diversified into almost all the organisms known today.[147]
The Paleozoic ("old life") era was the first and longest era of the Phanerozoic eon, lasting from 538.8 to 251.9 Ma.[105] During the Paleozoic, many modern groups of life came into existence. Life colonized the land, first plants, then animals. Two significant 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 supercontinentPangaea in the late Paleozoic.[148]
The Cenozoic ("new life") era began at 66 Ma, and is subdivided into thePaleogene,Neogene, and Quaternary periods. These three periods are further split into seven subdivisions, with the Paleogene composed of ThePaleocene,Eocene, andOligocene, the Neogene divided into theMiocene,Pliocene, and the Quaternary composed of thePleistocene, and Holocene.[151] Mammals, birds, amphibians, crocodilians, turtles, and lepidosaurs survived the Cretaceous–Paleogene extinction event that killed off the non-avian dinosaurs and many other forms of life, and this is the era during which they diversified into their modern forms.[152]
Tectonics, paleogeography and climate
Pangaea was asupercontinent that existed from about 300 to 180 Ma. The outlines of the modern continents and other landmasses are indicated on this map.
At the end of the Proterozoic, the supercontinent Pannotia had broken apart into the smaller continents Laurentia,Baltica,Siberia and Gondwana.[153] During periods when continents move apart, more oceanic crust is formed by volcanic activity. Because the young volcanic crust is relatively hotter and less dense than the old oceanic crust, the ocean floors rise during such periods. This causes thesea level to rise. Therefore, in the first half of the Paleozoic, large areas of the continents were below sea level.[citation needed]
Early Paleozoic climates were warmer than today, but the end of the Ordovician saw a shortice 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 manybrachiopods, trilobites,Bryozoa andcorals disappeared. These marine species could probably not contend with the decreasing temperature of the sea water.[154]
The continents Laurentia and Baltica collided between 450 and 400 Ma, during theCaledonian Orogeny, to formLaurussia (also known as Euramerica).[155] Traces of the mountain belt this collision caused can be found inScandinavia,Scotland, and the northernAppalachians. In theDevonian period (416–359 Ma)[21] Gondwana and Siberia began to move towards Laurussia. The collision of Siberia with Laurussia caused theUralian Orogeny, the collision of Gondwana with Laurussia is called theVariscan or Hercynian Orogeny in Europe or theAlleghenian Orogeny in North America. The latter phase took place during theCarboniferous period (359–299 Ma)[21] and resulted in the formation of the last supercontinent, Pangaea.[60]
Trilobites first appeared during the Cambrian period and were among the most widespread and diverse groups of Paleozoic organisms.
The rate of the evolution of life as recorded by fossils accelerated in theCambrian period (542–488 Ma).[21] The sudden emergence of many new species,phyla, and forms in this period is called the Cambrian Explosion. It was a form ofadaptive radiation, where vacantniches left by the extinctEdiacaran biota were filled up by the emergence of new phyla.[156] The biological fomenting in the Cambrian Explosion was unprecedented before and since that time.[59]: 229 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. The development of hard body parts such as shells,skeletons orexoskeletons in animals likemolluscs,echinoderms,crinoids andarthropods (a well-known group of arthropods from the lower Paleozoic are thetrilobites) made the preservation andfossilization 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 period than about life in older periods. Some of these Cambrian groups appear complex but are seemingly quite different from modern life; examples areAnomalocaris andHaikouichthys. More recently, however, these seem to have found a place in modern classification.[157]
During the Cambrian, the firstvertebrate animals, among them the firstfishes, had appeared.[125]: 357 A creature that could have been the ancestor of the fishes, or was probably closely related to it, wasPikaia. It had a primitivenotochord, a structure that could have developed into avertebral column later. The first fishes withjaws (Gnathostomata) appeared during the next geological period, theOrdovician. The colonisation of newniches resulted in massive body sizes. In this way, fishes with increasing sizes evolved during the early Paleozoic, such as the titanicplacodermDunkleosteus, which could grow 7 meters (23 ft) long.[158]
The diversity of life forms did not increase significantly because of a series of mass extinctions that define widespread biostratigraphic units calledbiomeres.[159] After each extinction pulse, thecontinental shelf regions were repopulated by similar life forms that may have been evolving slowly elsewhere.[160] By the late Cambrian, the trilobites had reached their greatest diversity and dominated nearly all fossil assemblages.[161]: 34
Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of the Sun'sultraviolet 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 3 Ga[162][163] 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.[164] Fish, theearliest vertebrates, evolved in the oceans around 530 Ma.[125]: 354 A majorextinction event occurred near the end of the Cambrian period,[165] which ended 488 Ma.[166]
Several hundred million years ago, plants (probably resemblingalgae) and fungi started growing at the edges of the water and then out of it.[167]: 138–140 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.[168] 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,[169] perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also unconfirmed evidence that arthropods may have appeared on land as early as 530 Ma.[170]
Tiktaalik, a fish with limb-like fins and a predecessor of tetrapods. Reconstruction from fossils about 375 million years old.
At the end of the Ordovician period, 443 Ma,[21] additionalextinction events occurred, perhaps due to a concurrent ice age.[154] Around 380 to 375 Ma, the firsttetrapods evolved from fish.[171] Fins evolved to become limbs that the first tetrapods used to lift their heads out of the water to breathe air. This would let them live in oxygen-poor water, or pursue small prey in shallow water.[171] 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 theamphibians. About 365 Ma, anotherperiod of extinction occurred, perhaps as a result ofglobal cooling.[172] Plants evolvedseeds, which dramatically accelerated their spread on land, around this time (by approximately 360 Ma).[173][174]
About 20 million years later (340 Ma[125]: 293–296 ), theamniotic egg evolved, which could be laid on land, giving a survival advantage to tetrapod embryos. This resulted in the divergence ofamniotes from amphibians. Another 30 million years (310 Ma[125]: 254–256 ) saw the divergence of thesynapsids (including mammals) from thesauropsids (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.[citation needed]
Dinosaurs were the dominant terrestrial vertebrates throughout most of theMesozoic
After yet another, the most severe extinction of the period (251~250 Ma), around 230 Ma, dinosaurs split off from their reptilian ancestors.[175] TheTriassic–Jurassic extinction event at 200 Ma spared many of the dinosaurs,[21][176] and they soon became dominant among the vertebrates. Though some mammalian lines began to separate during this period, existing mammals were probably small animals resemblingshrews.[125]: 169
The boundary between avian and non-avian dinosaurs is unclear, butArchaeopteryx, traditionally considered one of the first birds, lived around 150 Ma.[177]
The earliest evidence for theangiosperms evolving flowers is during the Cretaceous period, some 20 million years later (132 Ma).[178]
Extinctions
The first of five great mass extinctions was theOrdovician-Silurian extinction. Its possible cause was the intense glaciation of Gondwana, which eventually led to aSnowball Earth. 60% of marine invertebrates became extinct, and 25% of all families.[citation needed]
The second mass extinction was theLate Devonian extinction, probably caused by the evolution of trees, which could have led to the depletion of greenhouse gases (like CO2) or theeutrophication of water. 70% of all species became extinct.[179]
The third mass extinction was the Permian-Triassic, or theGreat Dying, event. The event was possibly caused by some combination of theSiberian Traps volcanic event, an asteroid impact,methane hydrate gasification, sea level fluctuations, and a majoranoxic event. Either the proposedWilkes Land crater[180] in Antarctica orBedout structure off the northwest coast of Australia may indicate an impact connection with the Permian-Triassic extinction. But it remains uncertain whether these or other proposed Permian-Triassic boundary craters are real impact craters or even contemporary with the Permian-Triassic extinction event. This was by far the deadliest extinction ever, with about 57% of allfamilies and 83% of allgenera killed.[181][182]
The fifth and most recent mass extinction was theCretaceous-Paleogene extinction event. In 66 Ma, a 10-kilometer (6.2 mi)asteroid struck Earth just off theYucatán Peninsula—somewhere in the southwestern tip of then Laurasia—where theChicxulub crater is today. This ejected vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. 75% of all life, including the non-avian dinosaurs, became extinct,[184] marking the end of the Cretaceous period and Mesozoic era.[citation needed]
The first true mammals evolved in the shadows of dinosaurs and other large archosaurs that filled the world by the late Triassic. The first mammals were very small, and were probably nocturnal to escape predation. Mammal diversification truly began only after the Cretaceous-Paleogene extinction event.[185] By the early Paleocene Earth recovered from the extinction, and mammalian diversity increased. Creatures likeAmbulocetus took to the oceans to eventually evolve into whales,[186] whereas some creatures, like primates, took to the trees.[187] This all changed during the mid to late Eocene when the circum-Antarctic current formed between Antarctica and Australia which disrupted weather patterns on a global scale. Grasslesssavanna began to predominate much of the landscape, and mammals such asAndrewsarchus rose up to become the largest known terrestrial predatory mammal ever,[188] andearly whales likeBasilosaurus took control of the seas.[citation needed]
The evolution ofgrasses brought a remarkable change to Earth's landscape, and the new open spaces pushed mammals to get bigger and bigger. Grass started to expand in the Miocene, and the Miocene is where many modern- day mammals first appeared. Giantungulates likeParaceratherium andDeinotherium evolved to rule the grasslands. The evolution of grass also broughtprimates down from the trees, and startedhuman evolution. The first big cats evolved during this time as well.[189] TheTethys Sea was closed off by the collision of Africa and Europe.[190]
The formation of Panama was perhaps the most important geological event to occur in the last 60 million years. Atlantic and Pacific currents were closed off from each other, which caused the formation of theGulf Stream, which made Europe warmer. The land bridge allowed the isolated creatures of South America to migrate over to North America and vice versa.[191] Various species migrated south, leading to the presence in South America ofllamas, thespectacled bear,kinkajous andjaguars.[citation needed]
Three million years ago saw the start of the Pleistocene epoch, which featured dramatic climatic changes due to the ice ages. The ice ages led to the evolution and expansion of modern man in Saharan Africa. The mega-fauna that dominated fed on grasslands that, by now, had taken over much of the subtropical world. The large amounts of water held in the ice allowed various water bodies to shrink and sometimes disappear, such as the North Sea and the Bering Strait. It is believed by many that a huge migration took place alongBeringia, which is why, today, there arecamels (which evolved and became extinct in North America),horses (which evolved and became extinct in North America), and Native Americans. The end of the last ice age coincided with the expansion of man and a massive die out of ice age mega-fauna.
An artist's impression of ice age Earth at glacial maximum.
A small African ape living around 6 Ma was the last animal whose descendants would include both modern humans and their closest relatives, thechimpanzees.[101][125]: 100–101 Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still unclear, apes in one branch developed the ability towalk upright.[125]: 95–99 Brain size increased rapidly, and by 2 Ma, the first animals classified in the genusHomo had appeared.[167]: 300 Around the same time, the other branch split into the ancestors of thecommon chimpanzee and the ancestors of thebonobo as evolution continued simultaneously in all life forms.[125]: 100–101
A reconstruction of human history based on fossil data.[194]
It is more difficult to establish theorigin of language; it is unclear whetherHomo erectus could speak or if that capability had not begun untilHomo sapiens.[125]: 67 As brain size increased, babies were born earlier, before their heads grew too large to pass through thepelvis. As a result, they exhibited moreplasticity, thus possessing an increased capacity to learn and requiring a longer period of dependence. Social skills became more complex, language became more sophisticated, and tools became more elaborate. This contributed to further cooperation and intellectual development.[195]: 7 Modern humans (Homo sapiens) are believed to have originated around 200,000 years ago or earlierin Africa; the oldest fossils date back to around 160,000 years ago.[196]
The first humans to show signs ofspirituality are theNeanderthals (usually classified as a separate species with no surviving descendants); they buried their dead, often with no sign of food or tools.[197]: 17 However, evidence of more sophisticated beliefs, such as the earlyCro-Magnoncave paintings (probably with magical or religious significance)[197]: 17–19 did not appear until 32,000 years ago.[198] Cro-Magnons also left behind stone figurines such asVenus of Willendorf, probably also signifying religious belief.[197]: 17–19 By 11,000 years ago,Homo sapiens had reached the southern tip ofSouth America, the last of the uninhabited continents (except for Antarctica, which remained undiscovereduntil 1820AD).[199] Tool use and communication continued to improve, and interpersonal relationships became more intricate.[citation needed]
Throughout more than 90% of its history,Homo sapiens lived in small bands asnomadichunter-gatherers.[195]: 8 As language became more complex, the ability to remember and communicate information resulted in a new replicator: thememe.[200] Ideas could be exchanged quickly and passed down the generations.Cultural evolution quickly outpacedbiological evolution, andhistory proper began. Between 8500 and 7000BC, humans in theFertile Crescent in theMiddle East began the systematic husbandry of plants and animals:agriculture.[201] This spread to neighboring regions and developed independently elsewhere until mostHomo sapiens lived sedentary lives in permanent settlements as farmers. Not all societies abandoned nomadism, especially those in isolated areas of the globe poor indomesticable plant species, such asAustralia.[202] However, among those civilizations that did adopt agriculture, the relative stability and increased productivity provided by farming allowed the population to expand.[citation needed]
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 increasingdivision of labor. This led to Earth'sfirst civilization atSumer in the Middle East, between 4000 and 3000 BC.[195]: 15 Additional civilizations quickly arose inancient Egypt, at theIndus River valley and in China. The invention ofwriting enabled complex societies to arise: record-keeping andlibraries 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, enabling the first specialized occupations (e.g. craftsmen, merchants, priests, etc.). Curiosity and education drove the pursuit of knowledge and wisdom, and various disciplines, includingscience (in a primitive form), arose. This in turn led to the emergence of increasingly larger and more complex civilizations, such as the first empires, which at times traded with one another, or fought for territory and resources.
By around 500 BC, there were advanced civilizations in the Middle East, Iran, India, China, and Greece, at times expanding, at times entering into decline.[195]: 3 In 221 BC, China became a single polity that would grow to spread its culture throughoutEast Asia, and it has remained the most populous nation in the world. During this period, famousHindu texts known asvedas came in existence inIndus Valley Civilization. This civilization developed inwarfare,arts,science, mathematics andarchitecture.[citation needed] The fundamentals ofWestern civilization were largely shaped inAncient Greece, with the world's firstdemocratic government and major advances in philosophy andscience, and inAncient Rome with advances in law, government, and engineering.[203]
In the 14th century, theRenaissance began inItaly with advances in religion, art, and science.[195]: 317–319 At that time the Christian Church as a political entity lost much of its power. In 1492,Christopher Columbus reached the Americas, initiating great changes to theNew World. European civilization began to change beginning in 1500, leading to theScientific Revolution andIndustrial Revolution. That continent began to exert political and culturaldominance over human societies around the world, a time known as theColonial era(see also:Age of Discovery).[195]: 295–299 In the 18th century a cultural movement known as theAge of Enlightenment further shaped the mentality of Europe and contributed to itssecularization.[citation needed]
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Evolution timeline (usesFlash Player). Animated story of life shows everything from the Big Bang to the formation of Earth and the development of bacteria and other organisms to the ascent of man.
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