Nature

===Historical perspective===
{{Main|History of the Earth|Evolution}}
[[File:Pangea animation 03.gif|thumb|300px|An animation showing the movement of the continents from the separation of [[Pangaea]] until the present day]]
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Earth is estimated to have formed 4.54&nbsp;billion years ago from the [[solar nebula]], along with the [[Sun]] and other [[planet]]s.<ref>{{cite book |first=G. Brent |last=Dalrymple |date=1991 |title=The Age of the Earth |publisher=Stanford University Press |location=Stanford |isbn=978-0-8047-1569-0}}</ref> The Moon formed roughly 20&nbsp;million years later. Initially molten, the outer layer of the Earth cooled, resulting in the solid crust. Outgassing and [[Volcano|volcanic]] activity produced the primordial atmosphere. Condensing [[water vapor]], most or all of which came from [[ice]] delivered by [[comet]]s, [[Origin of water on Earth|produced the oceans]] and other water sources.<ref>
{{cite journal|first=A.|last=Morbidelli|display-authors=etal|date=2000|bibcode=2000M&PS...35.1309M|title=Source Regions and Time Scales for the Delivery of Water to Earth|journal=Meteoritics & Planetary Science|volume=35|issue=6|pages=1309–1320|doi=10.1111/j.1945-5100.2000.tb01518.x|doi-access=free}}</ref> The highly energetic chemistry is believed to have produced a self-replicating molecule around 4&nbsp;billion years ago.<ref>
{{cite news|title=Earth's Oldest Mineral Grains Suggest an Early Start for Life|publisher=NASA Astrobiology Institute|date=December 24, 2001|url=http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=76|access-date=May 24, 2006|url-status=dead|archive-url=https://web.archive.org/web/20060928231649/http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=76|archive-date=September 28, 2006}}</ref>
[[File:Hyperia.jpg|thumb|left|[[Plankton]] inhabit oceans, seas and lakes, and have existed in various forms for at least 2&nbsp;billion years.<ref name="Margulis1995">{{cite book|last=Margulis|first=Lynn|author-link=Lynn Margulis|author2=Dorian Sagan|date=1995|title=What is Life?|publisher=Simon & Schuster|location=New York|isbn=978-0-684-81326-4|url=https://archive.org/details/isbn_9780684810874}}</ref>]]
Continents formed, then broke up and reformed as the surface of Earth reshaped over hundreds of millions of years, occasionally combining to make a [[supercontinent]]. Roughly 750&nbsp;million years ago, the earliest known supercontinent [[Rodinia]], began to break apart. The continents later recombined to form [[Pannotia]] which broke apart about 540&nbsp;million years ago, then finally [[Pangaea]], which broke apart about 180&nbsp;million years ago.<ref>{{cite journal |first=J.B. |last=Murphy |author2=R.D. Nance |date=2004 |url=http://www.americanscientist.org/issues/page2/how-do-supercontinents-assemble |title=How do supercontinents assemble? |journal=American Scientist |volume=92 |issue=4 |doi=10.1511/2004.4.324 |page=324 |access-date=August 23, 2010 |archive-url=https://web.archive.org/web/20110128224011/http://www.americanscientist.org/issues/page2/how-do-supercontinents-assemble |archive-date=January 28, 2011 |url-status=live |df=mdy-all }}</ref>

During the [[Neoproterozoic]] era, freezing temperatures covered much of the Earth in [[glacier]]s and ice sheets. This hypothesis has been termed the "[[Snowball Earth]]", and it is of particular interest as it precedes the [[Cambrian explosion]] in which multicellular life forms began to proliferate about 530–540&nbsp;million years ago.<ref>{{cite book |first=J.L. |last=Kirschvink |date=1992 |chapter=Late Proterozoic Low-Latitude Global Glaciation: The Snowball Earth |chapter-url=http://www.gps.caltech.edu/~jkirschvink/pdfs/firstsnowball.pdf |title=The Proterozoic Biosphere |editor=J.W. Schopf |editor2=C. Klein |publisher=Cambridge University Press |location=Cambridge |pages=51–52 |isbn=978-0-521-36615-1}}</ref>

Since the Cambrian explosion there have been five distinctly identifiable [[Extinction event|mass extinctions]].<ref>{{cite journal |last=Raup |first=David M. |author2=J. John Sepkoski Jr. |date=March 1982 |title=Mass extinctions in the marine fossil record |journal=Science |volume=215 |issue=4539|pages = 1501–1503 |doi=10.1126/science.215.4539.1501 |pmid=17788674 |bibcode=1982Sci...215.1501R|s2cid=43002817 }}</ref> The last mass extinction occurred some 66 million years ago, when a meteorite collision probably triggered the extinction of the [[Bird|non-avian]] [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s. Over the past 66&nbsp;million years, mammalian life diversified.<ref>{{cite book |last=Margulis |first=Lynn |author2=Dorian Sagan |date=1995 |title=What is Life? |publisher=Simon & Schuster |location=New York |isbn=978-0-684-81326-4 |page=[https://archive.org/details/isbn_9780684810874/page/145 145] |url=https://archive.org/details/isbn_9780684810874/page/145 }}</ref>

Several million years ago, a species of small African [[ape]] gained the ability to stand upright.<ref name="Margulis1995" /> The subsequent advent of human life, and the development of agriculture and further [[civilization]] allowed humans to affect the Earth more rapidly than any previous life form, affecting both the nature and quantity of other organisms as well as global climate. By comparison, the [[Great Oxygenation Event]], produced by the proliferation of [[algae]] during the [[Siderian]] period, required about 300&nbsp;million years to culminate.

The present era is classified as part of a mass [[extinction event]], the [[Holocene extinction]] event, the fastest ever to have occurred.<ref>{{cite journal|author = Diamond J|title = The present, past and future of human-caused extinctions|journal = Philos Trans R Soc Lond B Biol Sci|volume = 325|issue = 1228|pages = 469–476; discussion 476–477|date = 1989|pmid = 2574887|doi = 10.1098/rstb.1989.0100|last2 = Ashmole|first2 = N. P.|last3 = Purves|first3 = P. E.|bibcode = 1989RSPTB.325..469D|doi-access = }}</ref><ref>{{cite journal|author = Novacek M|author2 = Cleland E|title = The current biodiversity extinction event: scenarios for mitigation and recovery|journal = Proc Natl Acad Sci USA|volume = 98|issue = 10|date = 2001|pmid = 11344295|doi = 10.1073/pnas.091093698|pmc = 33235|bibcode = 2001PNAS...98.5466N|pages = 5466–5470|doi-access = free}}</ref> Some, such as [[E. O. Wilson]] of [[Harvard University]], predict that human destruction of the [[biosphere]] could cause the extinction of one-half of all species in the next 100&nbsp;years.<ref>{{cite journal|title=The mid-Holocene extinction of silver fir (Abies alba) in the Southern Alps: a consequence of forest fires? Palaeobotanical records and forest simulations|doi=10.1007/s00334-006-0051-0|date=2006|last1=Wick|first1=Lucia|last2=Möhl|first2=Adrian|journal=Vegetation History and Archaeobotany|volume=15|issue=4|pages=435–444|bibcode=2006VegHA..15..435W |s2cid=52953180|url=http://doc.rero.ch/record/309852/files/334_2006_Article_51.pdf|access-date=November 15, 2018|archive-url=https://web.archive.org/web/20181115113137/http://doc.rero.ch/record/309852/files/334_2006_Article_51.pdf|archive-date=November 15, 2018|url-status=live|df=mdy-all}}</ref> The extent of the current extinction event is still being researched, debated and calculated by biologists.<ref>[http://park.org/Canada/Museum/extinction/holmass.html The Holocene Extinction] {{Webarchive|url=https://web.archive.org/web/20060925041056/http://park.org/Canada/Museum/extinction/holmass.html |date=September 25, 2006 }}. Park.org. Retrieved on November 3, 2016.</ref><ref>[http://park.org/Canada/Museum/extinction/extincmenu.html Mass Extinctions Of The Phanerozoic Menu] {{Webarchive|url=https://web.archive.org/web/20060925030659/http://park.org/Canada/Museum/extinction/extincmenu.html |date=September 25, 2006 }}. Park.org. Retrieved on November 3, 2016.</ref><ref>[http://park.org/Canada/Museum/extinction/patterns.html Patterns of Extinction] {{Webarchive|url=https://web.archive.org/web/20060925024739/http://park.org/Canada/Museum/extinction/patterns.html |date=September 25, 2006 }}. Park.org. Retrieved on November 3, 2016.</ref>
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