Nature

===Evolution===
[[File:Amazon Manaus forest.jpg|thumb|An area of the [[Amazon Rainforest]] shared between [[Colombia]] and [[Brazil]]. The [[tropical rainforest]]s of South America contain the largest [[biodiversity|diversity]] of species on [[Earth]].<ref>{{cite web|url=http://earthobservatory.nasa.gov/Newsroom/view.php?id=28907 |title=Why the Amazon Rainforest is So Rich in Species: News |publisher=Earthobservatory.nasa.gov |date=December 5, 2005 |access-date=May 14, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110225204348/http://earthobservatory.nasa.gov/Newsroom/view.php?id=28907 |archive-date=February 25, 2011 }}</ref><ref>{{cite web|url=http://earthobservatory.nasa.gov/Newsroom/view.php?id=28907 |title=Why The Amazon Rainforest Is So Rich in Species |publisher=Sciencedaily.com |date=December 5, 2005 |access-date=May 14, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110225204348/http://earthobservatory.nasa.gov/Newsroom/view.php?id=28907 |archive-date=February 25, 2011 }}</ref>]]
{{Main|Evolution}}
The [[origin of life]] on Earth is not well understood, but it is known to have occurred at least 3.5&nbsp;billion years ago,<ref name="Origin1">{{cite journal|last1=Schopf|first1=J. William|last2=Kudryavtsev|first2=Anatoliy B.|last3=Czaja|first3=Andrew D.|last4=Tripathi|first4=Abhishek B.|title=Evidence of Archean life: Stromatolites and microfossils|journal=Precambrian Research|volume=158|issue=3–4|year=2007|pages=141–155|doi=10.1016/j.precamres.2007.04.009|bibcode=2007PreR..158..141S}}</ref><ref name="Origin2">{{cite journal | last1 = Schopf | first1 = JW | year = 2006 | title = Fossil evidence of Archaean life | doi = 10.1098/rstb.2006.1834 | journal = Philos Trans R Soc Lond B Biol Sci | volume = 361 | issue = 1470| pages = 869–885 | pmid=16754604 | pmc=1578735}}</ref><ref name="RavenJohnson2002">{{cite book|author1=Raven, Peter Hamilton |author2=Johnson, George Brooks |title=Biology|url=https://archive.org/details/biologyrave00rave|url-access=registration|access-date=July 7, 2013|date=2002|publisher=McGraw-Hill Education|isbn=978-0-07-112261-0|page=[https://archive.org/details/biologyrave00rave/page/68 68]|df=mdy-all}}</ref> during the [[hadean]] or [[archean]] eons on a [[Early Earth|primordial Earth]] that had a substantially different environment than is found at present.<ref name=Line>{{cite journal|author = Line, M.|title = The enigma of the origin of life and its timing|journal = Microbiology|volume = 148|issue = Pt 1|pages = 21–27|date=January 1, 2002| pmid = 11782495 | doi = 10.1099/00221287-148-1-21 |doi-access = free}}</ref> These life forms possessed the basic traits of self-replication and inheritable traits. Once life had appeared, the process of [[evolution]] by [[natural selection]] resulted in the development of ever-more diverse life forms.

Species that were unable to adapt to the changing environment and competition from other life forms became extinct. However, the [[fossil]] record retains evidence of many of these older species. Current fossil and [[DNA]] evidence shows that all existing species can trace a continual ancestry back to the first primitive life forms.<ref name=Line/>

When basic forms of plant life developed the process of [[photosynthesis]] the sun's energy could be harvested to create conditions which allowed for more complex life forms.<ref>{{Cite web|url=https://phys.org/news/2016-03-photosynthesis-ancient-thought.html|title=Photosynthesis more ancient than thought, and most living things could do it|website=Phys.org|access-date=2019-01-19|archive-url=https://web.archive.org/web/20190120043127/https://phys.org/news/2016-03-photosynthesis-ancient-thought.html|archive-date=January 20, 2019|url-status=live|df=mdy-all}}</ref> The resultant [[oxygen]] accumulated in the atmosphere and gave rise to the [[ozone layer]]. The incorporation of smaller cells within larger ones resulted in the [[endosymbiotic theory|development of yet more complex cells]] called [[eukaryotes]].<ref>{{cite journal |first=L. V. |last=Berkner |author2=L. C. Marshall |date=May 1965 |title=On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere |journal=Journal of the Atmospheric Sciences |volume=22 |issue=3 |pages=225–261|doi=10.1175/1520-0469(1965)022<0225:OTOARO>2.0.CO;2 |bibcode=1965JAtS...22..225B |doi-access=free }}</ref> Cells within colonies became increasingly specialized, resulting in true multicellular organisms. With the ozone layer absorbing harmful [[ultraviolet radiation]], life colonized the surface of Earth.