History of Earth 2068726 225971441 2008-07-16T07:25:08Z Ckatz 831038 Reverted edits by [[Special:Contributions/198.54.202.74|198.54.202.74]] ([[User talk:198.54.202.74|talk]]) to last version by Rjwilmsi {{dablink|This article focuses on scientific information about the Earth. For religious beliefs about the Earth, see [[creation myth]]. For the history of modern humans, see [[History of the world]].}} [[Image:Geologic clock.jpg|thumb|350px|Geological time put in a diagram called a [[geological clock]], showing the relative lengths of the [[eon]]s of the Earth's history.]] The '''history of Earth''' covers approximately [[Age of the Earth|4.6 billion years]] (4,567,000,000 years), from [[Earth]]’s formation out of the [[solar nebula]] to the present. This article presents a broad overview, summarizing the leading, most current scientific theories. ==Origin== [[Image:Protoplanetary-disk.jpg|left|thumb|250px|An artist's impression of [[protoplanetary disk]].]] {{main|Formation and evolution of the Solar System}} The Earth formed as part of the birth of the [[Solar System]]: what eventually became the solar system initially existed as a large, rotating cloud of [[dust]], [[Rock (geology)|rocks]], and [[gas]]. It was composed of [[hydrogen]] and [[helium]] produced in the [[Big Bang]], as well as heavier [[chemical element|elements]] ejected by [[supernova]]s. Then, as one theory suggests, about 4.6 billion years ago a nearby [[star]] was destroyed in a [[supernova]] and the explosion sent a [[shock wave]] through the [[solar nebula]], causing it to gain [[angular momentum]]. As the cloud began to accelerate its [[rotation]], [[Gravitation|gravity]] and [[inertia]] flattened it into a [[protoplanetary disk]] oriented perpendicularly to its axis of rotation. Most of the mass concentrated in the middle and began to heat up, but small [[Perturbation (astronomy)|perturbations]] due to collisions and the angular momentum of other large debris created the means by which [[protoplanet]]s began to form. The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of [[kinetic energy|kinetic heat]] at the center. Its inability to transfer that energy away through any other process at a rate capable of relieving the build-up resulted in the disk's center heating up. Ultimately, [[nuclear fusion]] of [[hydrogen]] into [[helium]] began, and eventually, after contraction, a [[T Tauri star]] ignited to create the [[Sun]]. Meanwhile, as gravity caused [[matter]] to condense around the previously perturbed objects outside of the new sun's gravity grasp, dust particles and the rest of the [[protoplanetary disk]] began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately destined to become protoplanets.<ref>{{cite web | last = Chaisson | first = Eric J. | year = 2005 | url = http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_plan_1.html | title = Solar System Modeling | work = [http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/splash.html Cosmic Evolution] | publisher = [[Tufts University]] | accessdate = 2006-03-27 }}</ref> These included one collection approximately 150 million [[kilometer]]s from the center: Earth. The [[solar 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. <div style="clear: both"></div> ==Moon== [[Image:Big Slash.gif|200px|right|thumb|Animation (not to scale) of [[Theia (planet)|Theia]] forming in Earth’s [[Lagrangian point#L4 and L5|L<sub>5</sub> point]] and then, perturbed by gravity, colliding to help form the moon. The animation progresses in one-year steps making Earth appear not to move. The view is of the south pole.]] {{main|Moon#Origin and geologic evolution|l1=Origin and geologic evolution|Giant impact hypothesis}} The origin of the [[Moon]] is still uncertain, although much evidence exists for the giant impact hypothesis. Earth may not have been the only planet forming 150 million kilometers from the Sun. It is hypothesized that another collection occurred 150 million kilometers from both the Sun and the Earth, at their fourth or fifth [[Lagrangian point]]. This planet, named [[Theia (planet)|Theia]], is thought to have been smaller than the current Earth, probably about the size and mass of [[Mars]]. Its orbit may at first have been stable, but destabilized as Earth increased its mass by the accretion of more and more material. Theia swung back and forth relative to Earth until, finally, an estimated 4.533 billion years ago,<ref>{{cite journal | first = Carsten | last = Münker | coauthors = Jörg A. Pfänder, Stefan Weyer, Anette Büchl, Thorsten Kleine, Klaus Mezger | date = [[July 4]], [[2003]] | title = Evolution of Planetary Cores and the Earth-Moon System from Nb/Ta Systematics | journal = [[Science (journal)|Science]] | volume = 301 | issue = 5629 | pages = 84–87 | doi = 10.1126/science.1084662 | url = http://sciencemag.org/cgi/content/abstract/301/5629/84 | pmid = 12843390 }}</ref> it collided at a low, oblique angle. The low speed and angle were not enough to destroy Earth, but a large portion of its crust was ejected into space. Heavier elements from Theia sank to Earth’s core, while the remaining material and ejecta condensed into a single body within a couple of weeks. Under the influence of its own gravity, this became a more spherical body: the Moon.<ref>{{cite web | last = Taylor | first = G. Jeffrey | date = [[April 26]], [[2004]] | url = http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=446 | title = Origin of the Earth and Moon | publisher = [[NASA]] | accessdate = 2006-03-27 }}</ref> The impact is also thought to have changed Earth’s axis to produce the large 23.5° [[axial tilt]] that is responsible for Earth’s seasons. (A simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons.) It may also have sped up Earth’s rotation and initiated the planet’s [[plate tectonics]]. <div style="clear: both"></div> ==The Hadean eon==<!-- This section is linked from [[Photosynthesis]] --> {{main|Hadean}} [[Image:Volcano q.jpg|thumb|left|200px|Volcanic eruptions would have been common in Earth's early days.]] The early Earth, during the very early [[Hadean]] eon, was very different from the world known today. There were no oceans and no oxygen in the atmosphere. It was bombarded by planetoids and other material left over from the formation of the solar system. This bombardment, combined with heat from radioactive breakdown, residual heat, and heat from the pressure of contraction, caused the planet at this stage to be fully molten. During the [[iron catastrophe]] heavier elements sank to the center while lighter ones rose to the surface producing the layered [[structure of the Earth]] and also setting up the formation of [[Earth's magnetic field]]. Earth's early atmosphere would have comprised surrounding material from the solar nebula, especially light gases such as [[hydrogen]] and [[helium]], but the [[solar wind]] and Earth's own heat would have driven off this atmosphere. This changed when Earth was about 40% its present radius, and gravitational attraction allowed the retention of an atmosphere which included water. Temperatures plummeted and the crust of the planet was accumulated on a solid surface, with areas melted by large impacts on the scale of decades to hundreds of years between impacts. Large impacts would have caused localized melting and partial differentiation, with some lighter elements on the surface or released to the moist atmosphere.<ref name="Alfven1976">{{cite book | last = Alfvén | first = Hannes | coauthors = Gustaf Arrhenius | title = Evolution of the Solar System | url = http://history.nasa.gov/SP-345/sp345.htm | accessdate = 2006-08-22 | year = 1976 | publisher = National Aeronautics and Space Administration | location = Washington, D.C. | chapter = ORIGIN OF THE EARTH'S OCEAN AND ATMOSPHERE | chapterurl = http://history.nasa.gov/SP-345/ch26.htm }}</ref> The surface cooled quickly, forming the solid [[Crust (geology)|crust]] within 150 million years;<ref name="zircon">{{cite journal | last = Wilde | first = Simon A. | coauthors = John W. Valley, William H. Peck, and Colin M. Graham | date = [[January 11]], [[2001]] | title = Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago | journal = [[Nature (journal)|Nature]] | volume = 409 | pages = 175–178 | url = http://www.geology.wisc.edu/zircon/Wilde_et_al.PDF | doi = 10.1038/35051550 |format=PDF}} <small>(PDF).</small></ref> although new research<ref name= "Early Earth with Crust">{{cite journal | title = Early Earth Likely Had Continents And Was Habitable | date = [[2005-11-17]] | url = http://www.colorado.edu/news/releases/2005/438.html }}</ref> suggests that the actual number is 100 million years based on the level of [[hafnium]] found in the geology at [[Jack Hills]] in Western Australia. From 4 to 3.8 billion years ago, Earth underwent a period of [[Late Heavy Bombardment|heavy asteroidal bombardment]].<ref name="space.com-bombardment">{{cite web | last = Britt | first = Robert Roy | date = [[2002-07-24]] | url = http://www.space.com/scienceastronomy/planetearth/earth_bombarded_020724.html | title = Evidence for Ancient Bombardment of Earth | publisher = [[Space.com]] | accessdate = 2006-04-15 }}</ref> Steam [[outgassing|escaped]] from the crust while more gases were released by volcanoes, completing the second [[Earth's atmosphere|atmosphere]]. Additional water was imported by [[Meteoroid#bolide|bolide]] collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity. The planet cooled. Clouds formed. Rain [[Origin of the world's oceans|gave rise to the oceans]] within 750 million years (3.8 billion years ago), but probably earlier. Recent evidence suggests the [[Origin of water on Earth|oceans may have begun]] forming by 4.2 billion years ago<ref name="Cavosie_etal_2005">{{cite journal | last = Cavosie | first = A. J. | coauthors = J. W. Valley, S. A., Wilde, and E.I.M.F. | date = [[July 15]], [[2005]] | title = Magmatic δ<sup>18</sup>O in 4400-3900 Ma detrital zircons: A record of the alteration and recycling of crust in the Early Archean | journal = Earth and Planetary Science Letters | volume = 235 | issue = 3-4 | pages = 663–681 | doi = 10.1016/j.epsl.2005.04.028 | url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4GDKB05-3&_coverDate=07%2F15%2F2005&_alid=382434001&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=5801&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=be47e49c535d059be188b66c6e596dd5 }}</ref>.<ref name="Executive Summary: University of California, Los Angeles"> {{cite journal | last = Young | first = Edward | date = [[July 4]], [[2005]] | title = Executive Summary 2005 | url = http://nai.arc.nasa.gov/team/index.cfm?page=execsumm&teamid=26&year=7 }}</ref> 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. [[Volcano|Volcanic]] activity was intense and, without an [[ozone layer]] to hinder its entry, [[ultraviolet radiation]] flooded the surface. <div style="clear: both"></div> == Life == [[Image:Dna-split.png|thumb|right|150px|The replicator in virtually all known life is [[deoxyribonucleic acid]]. DNA is far more complex than the original replicator and its replication systems are highly elaborate.]] {{main|Origin of life}} The details of the origin of life are unknown, though the broad principles have been established. Two schools of thought regarding the origin of life have been proposed. The first suggests that organic components may have arrived on [[Earth]] from space (see “[[Panspermia]]”), while the other argues for terrestrial origins. The mechanisms by which life would initially arise are nevertheless held to be similar.<ref name="Scientific-American-panspermia">{{cite journal | last = Warmflash | first = David | coauthors = Benjamin Weiss | year = 2005 | month = November | title = Did Life Come From Another World? | journal = [[Scientific American]] | pages = 64–71 | url = http://www.sciam.com/article.cfm?articleID=00073A97-5745-1359-94FF83414B7F0000&pageNumber=1&catID=2 }}</ref> If life arose on Earth, the timing of this event is highly speculative—perhaps it arose around 4 billion years ago.<ref>{{cite web | last = Chaisson | first = Eric J. | year = 2005 | url = http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_chem_2.html | title = Chemical Evolution | work = [http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/splash.html Cosmic Evolution] | publisher = [[Tufts University]] | accessdate = 2006-03-27 }}</ref> In the energetic chemistry of early Earth, a molecule (or even something else) gained the ability to make copies of itself–the replicator. The nature of this molecule is unknown, its function having long since been superseded by life’s current replicator, [[DNA]]. In making copies of itself, the replicator did not always perform accurately: some copies contained an “error.” If the change destroyed the copying ability of the molecule, there could be no more copies, and the line would “die 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.” 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.<ref name="Dawkins-Ancestors-563">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 563–578 | chapter = Canterbury }}</ref> Several different 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, [[phospholipid]]s, [[crystal]]s,<ref name="Dawkins-Watchmaker-150">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title = [[The Blind Watchmaker]] | origyear = 1986 | year = 1996 | publisher = W. W. Norton & Company | location = New York | id = ISBN 0-393-31570-3 | pages = 150–157 | chapter = Origins and miracles }}</ref> or even quantum systems.<ref name="Davies">{{cite journal | last = Davies | first = Paul | authorlink = Paul Davies | date = [[October 6]], [[2005]] | title = A quantum recipe for life | journal = [[Nature (journal)|Nature]] | volume = 437 | issue = 7060 | pages = 819 | url = http://www.nature.com/nature/journal/v437/n7060/full/437819a.html | doi = 10.1038/437819a }} <small>(subscription required).</small></ref> There is currently no method of determining which of these models, if any, 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]].<ref name="Fortey-38">{{cite book | last = Fortey | first = Richard | authorlink = Richard Fortey | title = Life: A Natural History of the First Four Billion Years of Life on Earth | origyear = 1997 | year = 1999 | month = September | publisher = Vintage Books | location = New York | id = ISBN 0-375-70261-X | pages = 38 | chapter = Dust to Life }}</ref> Among these were many of the relatively simple [[Organic chemistry|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.<ref name="Fortey-39">{{cite book | last = Fortey | first = Richard | authorlink = Richard Fortey | title = Life: A Natural History of the First Four Billion Years of Life on Earth | origyear = 1997 | year = 1999 | month = September | publisher = Vintage Books | location = New York | id = ISBN 0-375-70261-X | pages = 39 | chapter = Dust to Life }}</ref> The presence of certain molecules could [[Catalyst|speed up]] a chemical reaction. All this continued for a very long time, with reactions occurring more or less at random, until by chance there arose a new molecule: the [[replicator]]. This had the bizarre property of promoting the chemical reactions which produced a copy of itself, and [[evolution]] began properly. Other theories posit a different replicator. In any case, DNA took over the function of the replicator at some point; all known life (with the exception of some viruses and [[prions]]) use DNA as their replicator, in an almost identical manner (see ''[[genetic code]]''). <div style="clear: both"></div> == Cells == [[Image:CellMembraneDrawing.jpg|thumb|left|200px|A small section of a cell membrane. This modern cell membrane is far more sophisticated than the original simple phospholipid bilayer (the small blue spheres with two tails). Proteins and [[carbohydrate]]s serve various functions in regulating the passage of material through the membrane and in reacting to the environment.]] Modern life has its replicating material packaged neatly inside a [[cellular membrane]]. It is easier to understand the origin of the cell membrane than the origin of the replicator, since the [[phospholipid]] molecules that make up a cell membrane will often form a [[bilayer]] spontaneously when placed in water. Under certain conditions, many such spheres can be formed (see “[[Origin of life#Bubble Theory|The bubble theory]]”).<ref name="Fortey-40">{{cite book | last = Fortey | first = Richard | authorlink = Richard Fortey | title = Life: A Natural History of the First Four Billion Years of Life on Earth | origyear = 1997 | year = 1999 | month = September | publisher = Vintage Books | location = New York | id = ISBN 0-375-70261-X | pages = 40 | chapter = Dust to Life }}</ref> It is not known whether this process preceded or succeeded the origin of the replicator (or perhaps it ''was'' the replicator). The prevailing theory is that the replicator, perhaps [[RNA]] by this point (the [[RNA world]] hypothesis), along with its replicating apparatus and maybe other biomolecules, had already evolved. Initial [[protocell]]s may have simply burst when they grew too large; the scattered contents may then have recolonized other “bubbles.” [[Protein]]s 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 since it can both store genetic information and [[catalyze]] reactions. At some point [[DNA]] took over the genetic storage role from RNA, and [[protein]]s known as [[enzyme]]s took over the catalysis role, leaving RNA to transfer information and modulate the process. There is increasing belief that these early cells may have evolved in association with underwater volcanic vents known as “[[black smoker]]s”.<ref name="Fortey-42">{{cite book | last = Fortey | first = Richard | authorlink = Richard Fortey | title = Life: A Natural History of the First Four Billion Years of Life on Earth | origyear = 1997 | year = 1999 | month = September | publisher = Vintage Books | location = New York | id = ISBN 0-375-70261-X | pages = 42–44 | chapter = Dust to Life }}</ref> or even hot, deep rocks.<ref name="Dawkins-Ancestors-580">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 580 | chapter = Canterbury }}</ref> However, it is believed that out of this multiplicity of cells, or protocells, only one survived. Current evidence suggests that the [[last universal common ancestor]] lived during the early [[Archean]] eon, perhaps roughly 3.5 billion years ago or earlier.<ref name="Penny-LUCA">{{cite journal | last = Penny | first = David | coauthors = Anthony Poole | year = 1999 | month = December | title = The nature of the last universal common ancestor | journal = Current Opinions in Genetics and Development | volume = 9 | issue = 6 | pages = 672–677 | pmid = 1060760 | url = http://awcmee.massey.ac.nz/people/dpenny/pdf/Penny_Poole_1999.pdf | doi = 10.1016/S0959-437X(99)00020-9 |format=PDF}} <small>(PDF)</small></ref><sup>,</sup><ref name="Munster">{{cite web | year = 2003 | url = http://www.uni-muenster.de/GeoPalaeontologie/Palaeo/Palbot/seite1.html | title = Earliest Life | publisher = [[University of Münster]] | accessdate = 2006-03-28 }}</ref> This “LUCA” cell is the ancestor of all cells and hence all life on Earth. It was probably a [[prokaryote]], possessing a cell membrane and probably [[ribosome]]s, but lacking a [[cell nucleus|nucleus]] or membrane-bound [[organelle]]s such as [[mitochondrion|mitochondria]] or [[chloroplast]]s. Like all modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and [[enzyme]]s 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]].<ref name="Penny-LUCA" /> <div style="clear: both"></div> ==Photosynthesis and oxygen== [[Image:Crepuscular1.jpg|thumb|right|200px|The harnessing of the [[sun]]’s energy led to several major changes in life on Earth.]] It is likely that the initial cells were all [[heterotroph]]s, using surrounding organic molecules (including those from other cells) as raw material and an energy source.<ref name="Dawkins-Ancestors-564">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 564–566 | chapter = Canterbury }}</ref> 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 billion years ago<ref name="De-Marais-photosynthesis">{{cite journal | last = De Marais | first = David J. | date = [[September 8]], [[2000]] | title = Evolution: When Did Photosynthesis Emerge on Earth? | journal = [[Science (journal)|Science]] | volume = 289 | issue = 5485 | pages = 1703–1705 | pmid = 11001737 | url = http://www.sciencemag.org/cgi/content/summary/289/5485/1703 }}<small> ([http://edmall.gsfc.nasa.gov/aacps/news/Photosynthesis.html full text])</small></ref>, 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 ([[carbohydrate]]s). 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 oceans would have turned to a green color while oxygen was reacting with minerals. When the reactions stopped, oxygen could finally enter 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.<ref name="Fortey-50">{{cite book | last = Fortey | first = Richard | authorlink = Richard Fortey | title = Life: A Natural History of the First Four Billion Years of Life on Earth | origyear = 1997 | year = 1999 | month = September | publisher = Vintage Books | location = New York | id = ISBN 0-375-70261-X | pages = 50–51 | chapter = Dust to Life }}</ref> Among the oldest examples of oxygen-producing lifeforms are fossil [[Stromatolite| Stromatolites]]. This, then, is 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:<ref name="cosmic-evolution-bio1">{{cite web | last = Chaisson | first = Eric J. | year = 2005 | url = http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_bio_1.html | title = Early Cells | work = [http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/splash.html Cosmic Evolution] | publisher = [[Tufts University]] | accessdate = 2006-03-29 }}</ref> without the ozone layer, ultraviolet radiation bombarding the surface would have caused unsustainable levels of [[mutation]] in exposed cells. Besides making large amounts of energy available to life-forms and blocking ultraviolet radiation, the effects of photosynthesis had a third, major, and world-changing impact. Oxygen was toxic; probably much life on Earth died out as its levels rose (the “[[Oxygen Catastrophe]]”).<ref name="cosmic-evolution-bio1" /> 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. <div style="clear: both"></div> ==Endosymbiosis and the three domains of life== {{main|Endosymbiotic theory}} [[Image:Endosymbiosis.PNG|thumb|left|200px|Some of the pathways by which the various [[endosymbiont]]s might have arisen.]] Modern [[taxonomy]] classifies life into [[three-domain system|three domains]]. The time of the origin of these domains are 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 billion years ago,<ref name="SciAm-eukaryote">{{cite journal | last = Woese | first = Carl | authorlink = Carl Woese | coauthors = J. Peter Gogarten | date = [[October 21]], [[1999]] | title = When did eukaryotic cells evolve? What do we know about how they evolved from earlier life-forms? | journal = [[Scientific American]] | url = http://www.sciam.com/askexpert_question.cfm?articleID=000C32DD-60E1-1C72-9EB7809EC588F2D7&pageNumber=1&catID=3 }}</ref> 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 period a [[proto-mitochondrion|bacterial cell]] related to today’s ''[[Rickettsia]]''<ref name="Andersson">{{cite journal | last = Andersson | first = Siv G. E. | coauthors = Alireza Zomorodipour, Jan O. Andersson, Thomas Sicheritz-Pontén, U. Cecilia M. Alsmark, Raf M. Podowski, A. Kristina Näslund, Ann-Sofie Eriksson, Herbert H. Winkler, & Charles G. Kurland | date = [[November 12]], [[1998]] | title = The genome sequence of ''Rickettsia prowazekii'' and the origin of mitochondria | journal = [[Nature (journal)|Nature]] | volume = 396 | issue = 6707 | pages = 133–140 | pmid = 9823893, {{doi|10.1038/24094}} | url = http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v396/n6707/full/396133a0_fs.html | doi = 10.1038/24094}} | url = http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v396/n6707/full/396133a0_fs.html | doi_brokendate = 2008-06-24 }}</ref> 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). Perhaps the smaller cell attempted 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, and soon a stable [[symbiosis|symbiotic]] relationship developed. 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. [[Symbiosis]] developed between the larger cell and the population of smaller cells inside it to the extent that they are considered to have become a single [[organism]], the smaller cells being classified as [[organelle]]s called [[mitochondrion|mitochondria]]. A similar event took place with [[photosynthetic]] [[cyanobacteria]]<ref name="Bergland">{{cite journal | last = Berglsand | first = Kristin J. | coauthors = Robert Haselkorn | year = 1991 | month = June | title = Evolutionary Relationships among the Eubacteria, Cyanobacteria, and Chloroplasts: Evidence from the ''rpoC1'' Gene of ''Anabaena'' sp. Strain PCC 7120 | journal = Journal of Bacteriology | volume = 173 | issue = 11 | pages = 3446–3455 | pmid = 1904436 | url = http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=207958&blobtype=pdf }} <small>(PDF)</small></ref> entering larger [[heterotrophic]] cells and becoming [[chloroplast]]s.<ref name="Dawkins-Ancestors-536">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 536–539 | chapter = The Great Historic Rendezvous }}</ref><sup>,</sup><ref name="Fortey-60">{{cite book | last = Fortey | first = Richard | authorlink = Richard Fortey | title = Life: A Natural History of the First Four Billion Years of Life on Earth | origyear = 1997 | year = 1999 | month = September | publisher = Vintage Books | location = New York | id = ISBN 0-375-70261-X | pages = 60–61 | chapter = Dust to Life }}</ref> Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes some time before one billion years ago. There were probably several such inclusion events, as the figure at left 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]], [[spirochete]]s gave rise to [[cilia]] and [[flagella]], and that perhaps a [[DNA virus]] gave rise to the [[cell nucleus]],<ref name="takemura">{{cite journal | last = Takemura | first = Masaharu | year = 2001 | month = May | title = Poxviruses and the origin of the eukaryotic nucleus. | journal = Journal of Molecular Evolution | volume = 52 | issue = 5 | pages = 419–425 | pmid = 11443345 }}</ref><sup>,</sup><ref name="bell">{{cite journal | last = Bell | first = Philip J | year = 2001 | month = September | title = Viral eukaryogenesis: was the ancestor of the nucleus a complex DNA virus? | journal = Journal of Molecular Evolution | volume = 53 | issue = 3 | pages = 251–256 | pmid = 11523012 | doi = 10.1007/s002390010215 }}</ref> though none of these theories are generally accepted.<ref name="peroxisome">{{cite journal | last = Gabaldón | first = Toni | coauthors = Berend Snel, Frank van Zimmeren, Wieger Hemrika, Henk Tabak, and Martijn A. Huynen | date = [[March 23]], [[2006]] | title = Origin and evolution of the peroxisomal proteome. | journal = Biology Direct | volume = 1 | issue = 1 | pages = 8 | pmid = 16556314 | url = http://www.biology-direct.com/content/pdf/1745-6150-1-8.pdf | doi = 10.1186/1745-6150-1-8 |format=PDF}} <small>(PDF)</small></ref> During this period, the [[supercontinent]] [[Columbia (supercontinent)|Columbia]] is believed to have existed, probably from around 1.8 to 1.5 billion years ago; it is the oldest hypothesized supercontinent.<ref name="bbc-columbia">{{cite web | last = Whitehouse | first = David | year = 2002 | url = http://news.bbc.co.uk/2/hi/science/nature/1892869.stm | title = Ancient supercontinent proposed | publisher = [[BBC]] | accessdate = 2006-04-16 }}</ref> <div style="clear: both"></div> ==Multicellularity== [[Image:Volvox aureus.jpg|thumb|right|200px|''[[Volvox]] aureus'' is believed to be similar to the first multicellular plants.]] 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 billion years ago, the [[supercontinent]] [[Rodinia]] was assembling.<ref name="hanson-rodinia">{{cite journal | last = Hanson | first = Richard E. | coauthors = James L. Crowley, Samuel A. Bowring, Jahandar Ramezani, Wulf A. Gose, et al. | date = [[May 21]], [[2004]] | title = Coeval Large-Scale Magmatism in the Kalahari and Laurentian Cratons During Rodinia Assembly | journal = [[Science (journal)|Science]] | volume = 304 | issue = 5674 | pages = 1126–1129 | doi = 10.1126/science.1096329 | url = http://www.sciencemag.org/cgi/content/abstract/304/5674/1126 | pmid = 15105458 }}</ref> 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,<ref name="cosmic-evolution-bio2">{{cite web | last = Chaisson | first = Eric J. | year = 2005 | url = http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_bio_2.html | title = Ancient Fossils | work = [http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/splash.html Cosmic Evolution] | publisher = [[Tufts University]] | accessdate = 2006-03-31 }}</ref> the first [[multicellular]] plants emerged, probably [[green algae]].<ref name="bhattacharya">{{cite journal | last = Bhattacharya | first = Debashish | coauthors = Linda Medlin | year = 1998 | title = Algal Phylogeny and the Origin of Land Plants | journal = Plant Physiology | volume = 116 | pages = 9–15 | url = http://www.iib.unsam.edu.ar/IIB-INTECH/html/docencia/BioVegetal/Evolucion03.pdf | doi = 10.1104/pp.116.1.9 | format = {{dead link|date=June 2008}} &ndash; <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3ABhattacharya+intitle%3AAlgal+Phylogeny+and+the+Origin+of+Land+Plants&as_publication=Plant+Physiology&as_ylo=1998&as_yhi=1998&btnG=Search Scholar search]</sup> }} <small>(PDF)</small></ref> Possibly by around 900 million years ago,<ref name="Dawkins-Ancestors-488">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 488 | chapter = Choanoflagellates }}</ref> true multicellularity had also evolved in animals. At first it probably somewhat resembled that of today’s [[sponge]]s, where all cells were [[totipotent]] and a disrupted organism could reassemble itself.<ref name="Dawkins-Ancestors-483">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 483–487 | chapter = Sponges }}</ref> 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. Many scientists believe that a very severe ice age began around 770 million years ago, so severe that the surface of all the oceans completely froze ([[Snowball Earth]]). Eventually, after 20 million years, enough carbon dioxide escaped through volcanic outgassing; the resulting greenhouse effect raised global temperatures.<ref name="hoffman-science">{{cite journal | last = Hoffman | first = Paul F. | coauthors = Alan J. Kaufman, Galen P. Halverson, & Daniel P. Schrag | date = 1998-08-28 | title = A Neoproterozoic Snowball Earth | journal = [[Science (journal)|Science]] | volume = 281 | issue = 5381 | pages = 1342–1346 | doi = 10.1126/science.281.5381.1342 | url = http://www.sciencemag.org/cgi/content/abstract/281/5381/1342?ijkey=ceb9fa6933b922f34f231d8ee123250301a2b541&keytype2=tf_ipsecsha | accessdate = 2006-04-16 | pmid = 9721097 }} <small>(abstract)</small></ref> By around the same time, 750 million years ago,<ref name="torsvik">{{cite journal | last = Torsvik | first = Trond H. | date = [[May 30]], [[2003]] | title = The Rodinia Jigsaw Puzzle | journal = [[Science (journal)|Science]] | volume = 300 | issue = 5624 | pages = 1379–1381 | doi = 10.1126/science.1083469 | url = http://www.sciencemag.org/cgi/content/full/300/5624/1379?ijkey=fYKdIXStamWxU&keytype=ref&siteid=sci | pmid = 12775828 }}</ref> Rodinia began to break up. <div style="clear: both"></div> ==Colonization of land== [[Image:Mars Twin Peaks (1024px).jpg|thumb|left|200px|For most of Earth’s history, there were no multicellular organisms on land. Parts of the surface may have vaguely resembled this view of [[Mars]], one of Earth’s neighboring planets.{{Fact|date=February 2007}}]] 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 likely colonized the land as early as 2.6 billion years ago<ref name="pisani">{{cite journal | last = Pisani | first = Davide | coauthors = Laura L. Poling, Maureen Lyons-Weiler, & S. Blair Hedges | date = [[January 19]], [[2004]] | title = The colonization of land by animals: molecular phylogeny and divergence times among arthropods | journal = BMC Biology | volume = 2 | issue = 1 | doi = 10.1186/1741-7007-2-1 | url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=333434#B17 | pages = 1 | pmid = 14731304 }}</ref> even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent [[Pannotia]] formed around 600 million years ago and then broke apart a short 50 million years later.<ref name="liebermean">{{cite journal | last = Lieberman | first = Bruce S. | year = 2003 | title = Taking the Pulse of the Cambrian Radiation | journal = Integrative and Comparative Biology | volume = 43 | issue = 1 | pages = 229–237 | doi = 10.1093/icb/43.1.229 | url = http://icb.oxfordjournals.org/cgi/content/full/43/1/229 }}</ref> [[Fish]], the [[prehistoric fish|earliest]] [[vertebrates]], evolved in the oceans around 530 million years ago.<ref name="Dawkins-Ancestors-354">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 354 | chapter = Lampreys and Hagfish }}</ref> A major [[Cambrian-Ordovician extinction events|extinction event]] occurred near the end of the Cambrian period,<ref name="bbc-cambrian">{{cite web | url = http://www.bbc.co.uk/education/darwin/exfiles/cambrian.htm | title = The Mass Extinctions: The Late Cambrian Extinction | publisher = [[BBC]] | accessdate = 2006-04-09 }}</ref> which ended 488 million years ago<ref name="landing">{{cite journal | last = Landing | first = E. | coauthors = S. A. Bowring, K. L. Davidek, [[Richard Fortey|R. A. Fortey]], & W. A. P. Wimbledon | year = 2000 | title = Cambrian–Ordovician boundary age and duration of the lowest Ordovician Tremadoc Series based on U–Pb zircon dates from Avalonian Wales | journal = Geological Magazine | volume = 137 | issue = 5 | pages = 485–494 | doi = 10.1017/S0016756800004507 | url = http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=60617 }} <small>(abstract)</small></ref>. Several hundred million years ago, plants (probably resembling [[algae]]) and fungi started growing at the edges of the water, and then out of it.<ref name="Fortey-138">{{cite book | last = Fortey | first = Richard | authorlink = Richard Fortey | title = Life: A Natural History of the First Four Billion Years of Life on Earth | origyear = 1997 | year = 1999 | month = September | publisher = Vintage Books | location = New York | id = ISBN 0-375-70261-X | pages = 138–140 | chapter = Landwards }}</ref> The oldest fossils of land fungi and plants date to 480–460 million years ago, though molecular evidence suggests the fungi may have colonized the land as early as 1000 million years ago and the plants 700 million years ago.<ref name="heckman">{{cite journal | last = Heckman | first = D. S. | coauthors = D. M. Geiser, B. R. Eidell, R. L. Stauffer, N. L. Kardos, & S. B. Hedges | date= [[August 10]], [[2001]] | title = Molecular evidence for the early colonization of land by fungi and plants. | journal = [[Science (journal)|Science]] | volume = 10 | issue = 293 | pages = 1129–1133 | pmid = 11498589, {{doi|10.1126/science.1061457}} | url = http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=11498589 | doi = 10.1126/science.1061457}} | url = http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve }} <small>(abstract)</small></ref> 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 [[arthropod]]s on land around 450 million years ago<ref name="johnson">{{cite journal | last = Johnson | first = E. W. | coauthors = D. E. G. Briggs, R. J. Suthren, J. L. Wright, & S. P. Tunnicliff | year = 1994 | month = May | title = Non-marine arthropod traces from the subaereal Ordivician Borrowdale volcanic group, English Lake District | journal = Geological Magazine | volume = 131 | issue = 3 | pages = 395–406 | url = http://geolmag.geoscienceworld.org/cgi/content/abstract/131/3/395 }} <small>(abstract)</small></ref>, 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 million years ago<ref name="macnaughton">{{cite journal | last = MacNaughton | first = Robert B. | coauthors = Jennifer M. Cole, Robert W. Dalrymple, Simon J. Braddy, Derek E. G. Briggs, & Terrence D. Lukie | year = 2002 | title = First steps on land: Arthropod trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada | journal = Geology | volume = 30 | issue = 5 | pages = 391–394 | doi = 10.1130/0091-7613(2002)030<0391:FSOLAT>2.0.CO;2 | url = http://www.gsajournals.org/gsaonline/?request=get-abstract&issn=0091-7613&volume=30&page=391 }} <small>(abstract)</small></ref>. At the end of the [[Ordovician]] period, 440 million years ago, additional [[Ordovician-Silurian extinction events|extinction events]] occurred, perhaps due to a concurrent [[ice age]].<ref name="bbc-ordovician">{{cite web | url = http://www.bbc.co.uk/education/darwin/exfiles/ordovician.htm| title = The Mass Extinctions: The Late Ordovician Extinction | publisher = [[BBC]] | accessdate = 2006-05-22 }}</ref> Around 380 to 375 million years ago, the first [[tetrapod]]s evolved from fish.<ref name="clack-sa">{{cite journal | last = Clack | first = Jennifer A. | year = 2005 | month = December | title = Getting a Leg Up on Land | journal = [[Scientific American]] | url = http://www.sciam.com/article.cfm?articleID=000DC8B8-EA15-137C-AA1583414B7F0000&sc=I100322 }}</ref> 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.<ref name="clack-sa" /> 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 [[amphibian]]s. About 365 million years ago, another [[Late Devonian extinction|period of extinction]] occurred, perhaps as a result of global cooling.<ref name="bbc-devonian">{{cite web | url = http://www.bbc.co.uk/education/darwin/exfiles/devonian.htm | title = The Mass Extinctions: The Late Devonian Extinction | publisher = [[BBC]] | accessdate = 2006-04-04 }}</ref> Plants evolved [[seed]]s, which dramatically accelerated their spread on land, around this time (by approximately 360 million years ago).<ref name="willis">{{cite book | last = Willis | first = K. J. | coauthors = J. C. McElwain | title = The Evolution of Plants | year = 2002 | publisher = Oxford University Press | location = Oxford | id = ISBN 0-19-850065-3 | pages = 93 }}</ref><sup>,</sup><ref name="waikato">{{cite web | url = http://sci.waikato.ac.nz/evolution/plantEvolution.shtml | title = Plant Evolution | publisher = [[University of Waikato]] | accessdate = 2006-04-07 }}</ref> [[Image:Pangaea continents.png|thumb|right|200px|[[Pangaea]], the most recent supercontinent, existed from 300 to 180 million years ago. The outlines of the modern continents and other land masses are indicated on this map.]] Some 20 million years later (340 million years ago<ref name="Dawkins-Ancestors-293">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 293–296 | chapter = Amphibians }}</ref>), the [[amniotic egg]] evolved, which could be laid on land, giving a survival advantage to [[tetrapod]] embryos. This resulted in the divergence of [[amniote]]s from [[amphibian]]s. Another 30 million years (310 million years ago<ref name="Dawkins-Ancestors-254">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 254–256 | chapter = Sauropsids }}</ref>) saw the divergence of the [[synapsid]]s (including [[mammal]]s) from the [[sauropsid]]s (including [[bird]]s and non-avian, non-mammalian [[reptile]]s). 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 [[Permian-Triassic extinction event|most severe extinction event]] to date took place 250 million years ago, at the boundary of the [[Permian]] and [[Triassic]] periods; 95% of life on Earth died out,<ref name="bbc-permian-triassic">{{cite web | year = 2002 | url = http://www.bbc.co.uk/science/horizon/2002/dayearthdied.shtml | title = The Day the Earth Nearly Died | work = Horizon | publisher = [[BBC]] | accessdate = 2006-04-09 }}</ref> 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.<ref name="bbc-antarctic-crater">{{cite web | date = [[3 June]] [[2006]] | url = http://news.bbc.co.uk/2/hi/science/nature/5045024.stm | title = Big crater seen beneath ice sheet | work = &nbsp; | publisher = BBC News | accessdate = 2006-11-15 }}</ref> But life persevered, and around 230 million years ago <ref name="bbc-new blood">{{cite episode | title = New Blood | episodelink = Walking with Dinosaurs#"New Blood" | series = Walking with Dinosaurs | serieslink = Walking with Dinosaurs | airdate = 1999 | writers = [[BBC]] }} <small>([http://www.bbc.co.uk/sn/prehistoric_life/dinosaurs/chronology/220mya1.shtml description])</small></ref>, [[dinosaur]]s split off from their reptilian ancestors. An extinction event between the [[Triassic-Jurassic extinction event|Triassic and Jurassic periods]] 200 million years ago spared many of the dinosaurs,<ref name="bbc-triassic">{{cite web | url = http://www.bbc.co.uk/education/darwin/exfiles/triassic.htm | title = The Mass Extinctions: The Late Triassic Extinction | publisher = [[BBC]] | accessdate = 2006-04-09 }}</ref> 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 [[shrew]]s.<ref name="Dawkins-Ancestors-169">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 169 | chapter = The Great Cretaceous Catastrophe }}</ref> By 180 million years ago, 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 [[bird]]s, lived around 150 million years ago.<ref name="archaeopteryx">{{cite web | year = 1996 | url = http://www.ucmp.berkeley.edu/diapsids/birds/archaeopteryx.html | title = ''Archaeopteryx'': An Early Bird | publisher = [[University of California, Berkeley]] Museum of Paleontology | accessdate = 2006-04-09 }}</ref> The earliest evidence for the [[Flowering plant|angiosperms]] evolving [[flower]]s is during the [[Cretaceous]] period, some 20 million years later (132 million years ago)<ref name="tol-angiosperms">{{cite web | last = Soltis | first = Pam | coauthors = Doug Soltis, & Christine Edwards | year = 2005 | url = http://tolweb.org/tree?group=Angiosperms&contgroup=Spermatopsida | title = Angiosperms | work = [http://tolweb.org The Tree of Life Project] | accessdate = 2006-04-09 }}</ref> Competition with birds drove many [[pterosaur]]s to extinction, and the dinosaurs were probably already in decline for various reasons<ref name="bbc-death-dynasty">{{cite episode | title = Death of a Dynasty| episodelink = Walking with Dinosaurs#"Death of a Dynasty" | series = Walking with Dinosaurs | serieslink = Walking with Dinosaurs | airdate = 1999 | writers = [[BBC]] }} <small>([http://www.bbc.co.uk/sn/prehistoric_life/dinosaurs/chronology/65mya1.shtml description])</small></ref> when, 65 million years ago, a 10-kilometer [[meteorite]] likely struck Earth just off the [[Yucatán Peninsula]], ejecting vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. Most large animals, including the non-avian dinosaurs, [[Cretaceous-Tertiary extinction event|became extinct]].<ref name="cosmic-evolution-bio4">{{cite web | last = Chaisson | first = Eric J. | year = 2005 | url = http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_bio_4.html | title = Recent Fossils | work = [http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/splash.html Cosmic Evolution] | publisher = [[Tufts University]] | accessdate = 2006-04-09 }}</ref> 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 million years ago), the last common ancestor of [[primate]]s lived.<ref name="Dawkins-Ancestors-160">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 160 | chapter = Lemurs, Bushbabies and their Kin }}</ref> By the late [[Eocene]] epoch, 34 million years ago, some terrestrial mammals had returned to the oceans to become animals such as ''[[Basilosaurus]]'' which later gave rise to [[dolphin]]s and [[whale]]s.<ref name="bbc-whale-killer">{{cite episode | title = Whale Killer| episodelink = Walking with Beasts#"Whale Killer" | series = Walking with Beasts | serieslink = Walking with Beasts| airdate = 2001 | writers = [[BBC]] }}</ref> ==Humanity== [[Image:Austrolopithecus africanus.jpg|thumb|left|200px|''[[Australopithecus]] africanus'', an early [[Hominidae|hominid]].]] {{main|Human evolution}} 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 [[bonobo]]s, and [[chimpanzee]]s.<ref name="Dawkins-Ancestors-100">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 100–101 | chapter = Chimpanzees }}</ref> 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 [[bipedal|walk upright]].<ref name="Dawkins-Ancestors-95">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 95–99 | chapter = Ape-Men }}</ref> [[Brain]] size increased rapidly, and by 2 million years ago, the very first animals classified in the genus ''[[Homo (genus)|Homo]]'' had appeared.<ref name="Fortey-300">{{cite book | last = Fortey | first = Richard | authorlink = Richard Fortey | title = Life: A Natural History of the First Four Billion Years of Life on Earth | origyear = 1997 | year = 1999 | month = September | publisher = Vintage Books | location = New York | id = ISBN 0-375-70261-X | pages = 38 | chapter = Humanity }}</ref> 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.<ref name="Dawkins-Ancestors-100" /> The ability to control [[fire]] likely began in ''[[Homo erectus]]'' (or ''[[Homo ergaster]]''), probably at least 790,000 years ago<ref name="goren-inbar">{{cite journal | last = Goren-Inbar | first = Naama | coauthors = Nira Alperson, Mordechai E. Kislev, Orit Simchoni, Yoel Melamed, Adi Ben-Nun, & Ella Werker | date = 2004-04-30 | title = Evidence of Hominin Control of Fire at Gesher Benot Ya`aqov, Israel | journal = [[Science (journal)|Science]] | volume = 304 | issue = 5671 | pages = 725–727 | doi = 10.1126/science.1095443 | url = http://www.sciencemag.org/cgi/content/abstract/304/5671/725 | accessdate = 2006-04-11 | pmid = 15118160 }} <small>(abstract)</ref> but perhaps as early as 1.5 million years ago.<ref name="Dawkins-Ancestors-67">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 67 | chapter = Ergasts }}</ref> 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]]''<ref name="McClellan">{{cite book | author=McClellan|url=http://books.google.com/books?id=aJgp94zNwNQC&printsec=frontcover#PPA11 | title=Science and Technology in World History: An Introduction| location=Baltimore, Maryland | publisher=JHU Press | year=2006 | id=ISBN 0801883601}} [http://books.google.com/books?id=aJgp94zNwNQC&printsec=frontcover#PPA8 Page 8-12 ]</ref> However 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''.<ref name="Dawkins-Ancestors-67-71">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title =[[The Ancestor's Tale]]: A Pilgrimage to the Dawn of Life | year = 2004 | publisher = Houghton Mifflin Company | location = Boston | id = ISBN 0-618-00583-8 | pages = 67–71 | chapter = Ergasts }}</ref> 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 [[Neuroplasticity|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.<ref name="McNeill-7">{{cite book | last = McNeill | first = Willam H. | authorlink = William McNeill | title = A World History | origyear = 1967 | edition = 4th ed. | year = 1999 | publisher = Oxford University Press | location = New York | id = ISBN 0-19-511615-1 | pages = 7 | chapter = In The Beginning }}</ref> Anatomically modern humans — ''[[Homo sapiens]]'' — are believed to have originated somewhere around 200,000 years ago or earlier [[single-origin hypothesis|in Africa]]; the oldest fossils date back to around 160,000 years ago.<ref name="gibbons">{{cite journal | last = Gibbons | first = Ann | date = 2003-06-13 | title = Oldest Members of ''Homo sapiens'' Discovered in Africa | journal = [[Science (journal)|Science]] | volume = 300 | issue = 5626 | pages = 1641 | doi = 10.1126/science.300.5626.1641 | url = http://www.sciencemag.org/cgi/content/summary/300/5626/1641 | accessdate = 2006-04-11 | pmid = 12805512 }} <small>(abstract)</small></ref> The first humans to show evidence of [[spirituality]] are the [[Neanderthal]]s (usually classified as a separate species with no surviving descendants); they buried their dead, often apparently with food or tools.<ref name="hopfe">{{cite book | last = Hopfe | first = Lewis M. | title = Religions of the World | origyear = 1976 | edition = 4th ed. | id = ISBN 0-02-356930-1 | year = 1987 | publisher = MacMillan Publishing Company | location = New York | pages = 17 | chapter = Characteristics of Basic Religions }}</ref> However, evidence of more sophisticated beliefs, such as the early [[Cro-Magnon]] [[cave painting]]s (probably with magical or religious significance)<ref name="hopfe-19">{{cite book | last = Hopfe | first = Lewis M. | title = Religions of the World | origyear = 1976 | edition = 4th ed. | id = ISBN 0-02-356930-1 | year = 1987 | publisher = MacMillan Publishing Company | location = New York | pages = 17–19 | chapter = Characteristics of Basic Religions }}</ref> did not appear until some 32,000 years ago.<ref>{{cite web | url = http://www.metmuseum.org/toah/hd/chav/hd_chav.htm | title = Chauvet Cave | publisher = [[Metropolitan Museum of Art]] | accessdate = 2006-04-11 }}</ref> Cro-Magnons also left behind stone figurines such as [[Venus of Willendorf]], probably also signifying religious belief.<ref name="hopfe-19" /> By 11,000 years ago, ''Homo sapiens'' had reached the southern tip of [[South America]], the last of the uninhabited continents.<ref name="oxford-atlas">{{cite book | editor = Patrick K. O’Brien, ed. | title = Atlas of World History | origyear = 2002 | edition = concise edition | year = 2003 | publisher = [[Oxford University Press]] | location = New York | id = ISBN 0-19-521921-X | pages = 16 | chapter = The Human Revolution }}</ref> Tool use and language continued to improve; interpersonal relationships became more complex. <div style="clear: both"></div> ==Civilization== {{main|History of the world}} [[Image:Da Vinci Vitruve Luc Viatour.jpg|thumb|right|200px|[[Vitruvian Man]] by [[Leonardo da Vinci]] epitomizes the advances in art and science seen during the Renaissance.]] Throughout more than 90% of its history, ''Homo sapiens'' lived in small bands as nomadic [[hunter-gatherer]]s.<ref name="McNeill-hunter-gatherer">{{cite book | last = McNeill | first = Willam H. | authorlink = William McNeill | title = A World History | origyear = 1967 | edition = 4th ed. | year = 1999 | publisher = Oxford University Press | location = New York | id = ISBN 0-19-511615-1 | pages = 8 | chapter = In The Beginning }}</ref> As language became more complex, the ability to remember and transmit information resulted in a new sort of replicator: the [[meme]].<ref name="dawkins-sg">{{cite book | last = Dawkins | first = Richard | authorlink = Richard Dawkins | title = The Selfish Gene | origyear = 1976 | edition = 2nd ed. | year = 1989 | publisher = Oxford University Press | location = Oxford | id = ISBN 0-19-286092-5 | pages = 189–201 | chapter = Memes: the new replicators }}</ref> Ideas could be rapidly exchanged and passed down the generations. [[Cultural evolution]] quickly outpaced [[biological evolution]], and history proper began. Somewhere between 8500 and 7000 [[Before Christ|BC]], humans in the [[Fertile Crescent]] in [[Middle East]] began the systematic husbandry of plants and animals: [[agriculture]].<ref name="Tudge">{{cite book | last = Tudge | first = Colin | authorlink = Colin Tudge | title = [[Neanderthals, Bandits and Farmers|Neanderthals, Bandits and Farmers: How Agriculture Really Began]] | year = 1998 | publisher = Weidenfeld & Nicolson | location = London | id = ISBN 0-297-84258-7 }}</ref> 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 [[Domestication|domesticable]] plant species, such as [[Australia]].<ref name="diamond">{{cite book | last = Diamond | first = Jared | authorlink = Jared Diamond | title = [[Guns, Germs, and Steel]] | origdate = 1999-12-01 | publisher = W. W. Norton & Company | id = ISBN 0-393-31755-2 }}</ref> 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 labour|division of labor]]. This led to Earth’s first [[civilization]] at [[Sumer]] in the [[Middle East]], between 4000 and 3000 BC.<ref name="McNeill-Sumer">{{cite book | last = McNeill | first = Willam H. | authorlink = William McNeill | title = A World History | origyear = 1967 | edition = 4th ed. | year = 1999 | publisher = Oxford University Press | location = New York | id = ISBN 0-19-511615-1 | pages = 15 | chapter = In The Beginning }}</ref> Additional civilizations quickly arose in [[ancient Egypt]], at the [[Indus Valley civilization|Indus River valley]] and in [[Three Sovereigns and Five Emperors|China]]. Starting around 3000 BC, [[Hinduism]], one of the oldest religions still practiced today, began to take form.<ref>{{cite web | url = http://www.bbc.co.uk/religion/religions/hinduism/history/ | title = History of Hinduism | publisher = [[BBC]] | accessdate = 2006-03-27 }}</ref> Others soon followed. The invention of [[writing]] enabled complex societies to arise: record-keeping and [[library|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: [[empire]]s 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.<ref name="McNeill-3">{{cite book | last = McNeill | first = Willam H. | authorlink = William McNeill | title = A World History | origyear = 1967 | edition = 4th ed. | year = 1999 | publisher = Oxford University Press | location = New York | id = ISBN 0-19-511615-1 | pages = 3–6 | chapter = Emergence and Definition of the Major Old World Civilizations to 500 B.C. (introduction) }}</ref> In the [[fourteenth century]], the [[Renaissance]] began in [[Italy]] with advances in religion, art, and science.<ref name="McNeill-317">{{cite book | last = McNeill | first = Willam H. | authorlink = William McNeill | title = A World History | origyear = 1967 | edition = 4th ed. | year = 1999 | publisher = Oxford University Press | location = New York | id = ISBN 0-19-511615-1 | pages = 317–319 | chapter = Europe’s Self-Transformation: 1500–1648 }}</ref> Starting around 1500, European civilization began to undergo changes leading to the [[scientific revolution|scientific]] and [[Industrial Revolution|industrial]] revolutions: that continent began to exert political and cultural [[hegemony|dominance]] over human societies around the planet.<ref name="McNeill-295">{{cite book | last = McNeill | first = Willam H. | authorlink = William McNeill | title = A World History | origyear = 1967 | edition = 4th ed. | year = 1999 | publisher = Oxford University Press | location = New York | id = ISBN 0-19-511615-1 | pages = 295–299 | chapter = The Dominance of the West (introduction) }}</ref> From 1914 to 1918 and 1939 to 1945, nations around the world were embroiled in [[world war]]s. 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 discord, although increased collaboration has resulted as well. <div style="clear: both"></div> ==Recent events== [[Image:Astronaut-EVA.jpg|thumb|left|200px|Four and a half billion years after the planet's formation, one of Earth’s life forms broke free of the [[biosphere]]. For the first time in [[history]], Earth was viewed first hand from the vantage of space.]] Change has continued at a rapid pace from the mid-[[1940s]] to today. Technological developments include [[nuclear weapons]], [[computers]], [[genetic engineering]], and [[nanotechnology]]. 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, [[global warming]] have risen as the world population increases. In [[1957]], the [[Soviet Union]] launched [[Sputnik 1|the first artificial satellite]] into orbit and, soon afterward, [[Yuri Gagarin]] became the first human in space. [[Neil Armstrong]], an [[United States|American]], was the first to set foot on another astronomical object, the Earth's Moon. Unmanned probes have been sent to all the major planets in the solar system, with some (such as [[Voyager]]) having left the solar system. The [[Soviet Union]] and the [[United States of America]] were the primary early leaders in space exploration in the 20th Century. Five space agencies, representing over fifteen countries,<ref>{{cite web | year = 2006 | url = http://www.esa.int/esaHS/partstates.html | title = Human Spaceflight and Exploration — European Participating States | publisher = [[ESA]] | accessdate = 2006-03-27 }}</ref> have worked together to build the [[International Space Station]]. Aboard it, there has been a continuous human presence in space since 2000.<ref>{{cite web | date = [[January 11]], [[2006]] | url = http://www.nasa.gov/mission_pages/station/expeditions/expedition13/exp13_overview.html | title = Expedition 13: Science, Assembly Prep on Tap for Crew | publisher = [[NASA]] | accessdate = 2006-03-27 }}</ref> <div style="clear: both"></div> ==See also== *[[Timeline of the Big Bang]] *[[Geologic time scale]] *[[Timeline of evolution]] *[[Detailed logarithmic timeline]] *[[Natural history]] *[[History of the world]] *[[Risks to civilization, humans and planet Earth|End of civilization]] *[[Timetable of the Precambrian]] *[[Geological history of Earth]] ==References== {{reflist|2}} ==External links== *[http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/splash.html Cosmic Evolution] — a detailed look at events from the origin of the universe to the present *Valley, John W. “[http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0005FA5D-5F7C-1333-9F7C83414B7F0000 A Cool Early Earth?]” ''[[Scientific American]]''. 2005 October 58–65. – discusses the timing of the formation of the oceans and other major events in Earth’s early history. *[[Paul Davies|Davies, Paul]]. “[http://www.guardian.co.uk/science/story/0,3605,1671164,00.html Quantum leap of life]”. ''[[The Guardian]]''. 2005 [[December 20]]. – discusses speculation into the role of quantum systems in the origin of life *[http://www.johnkyrk.com/evolution.html Evolution timeline] (uses [[Adobe Shockwave|Shockwave]]). Animated story of life since about 13,700,000,000 shows everything from the big bang to the formation of the earth and the development of bacteria and other organisms to the ascent of man. * [http://www.sciam.com/article.cfm?chanID=sa006&articleID=0005FA5D-5F7C-1333-9F7C83414B7F0000 Scientific American Magazine (October 2005 Issue) A Cool Early Earth?] * [http://cosmographica.com/gallery/portfolio/portfolio2007/content/442_CoolEarlyEarth_large.html Artist's Conception of Cold Early Earth] {{Nature nav}} [[Category:Earth]] [[Category:History by topic|Earth]] [[Category:Historical geology|*]] {{Link FA|ca}} {{Link FA|vi}} {{Link FA|nl}} [[ar:تاريخ الأرض]] [[ca:Història de la Terra]] [[de:Erdgeschichte]] [[es:Historia de la Tierra]] [[eu:Lurraren Historia]] [[fr:Histoire de la Terre]] [[io:Terala eri]] [[hu:Földtörténet]] [[nl:Geschiedenis van de Aarde]] [[ja:地球史年表]] [[no:Jordhistorie]] [[nds:Eerdhistorie]] [[pl:Historia Ziemi]] [[sv:Jordens historia]] [[tl:Kasaysayan ng Daigdig]] [[vi:Lịch sử Trái Đất]] [[zh:地球歷史]]