Nature

==Life==
{{Life timeline}}
[[File:Malards in Golden Gate Park.jpg|thumb|left|upright|Female mallard and ducklings – [[reproduction]] is essential for continuing life.]]
{{Main|Life|Biology|Biosphere}}
Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life is characterized by [[Organism|organization]], [[metabolism]], [[cell growth|growth]], [[adaptation]], response to [[stimulus (physiology)|stimuli]], and [[reproduction]].<ref>{{cite web|date=2006 |url=http://www.calacademy.org/exhibits/xtremelife/what_is_life.php |title=Definition of Life |publisher=California Academy of Sciences |access-date=January 7, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070208220940/http://www.calacademy.org/exhibits/xtremelife/what_is_life.php |archive-date=February 8, 2007 }}</ref> Life may also be said to be simply the characteristic state of [[organism]]s.

Properties common to terrestrial organisms (plants, animals, [[Fungus|fungi]], [[protist]]s, [[archaea]], and bacteria) are that they are cellular, carbon-and-water-based with complex organization, having a metabolism, a capacity to grow, respond to stimuli, and reproduce. An entity with these properties is generally considered life. However, not every definition of life considers all of these properties to be essential. Human-made [[Artificial life|analogs of life]] may also be considered to be life.

The [[biosphere]] is the part of Earth's outer shell—including land, surface rocks, water, air and the atmosphere—within which life occurs, and which [[Biology|biotic]] processes in turn alter or transform. From the broadest [[Geophysiology|geophysiological]] point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the [[lithosphere]] (rocks), [[hydrosphere]] (water), and [[atmosphere]] (air). The entire Earth contains over 75&nbsp;billion tons (150 ''trillion'' pounds or about 6.8×10<sup>13</sup>&nbsp;kilograms) of [[biomass (ecology)|biomass]] (life), which lives within various environments within the biosphere.<ref>The figure "about one-half of one percent" takes into account the following (See, e.g., {{cite book |last=Leckie |first=Stephen |date=1999 |chapter=How Meat-centred Eating Patterns Affect Food Security and the Environment |chapter-url=http://www.idrc.ca/en/ev-30610-201-1-DO_TOPIC.html |title=For hunger-proof cities: sustainable urban food systems |publisher=International Development Research Centre |location=Ottawa |isbn=978-0-88936-882-8 |archive-url=https://web.archive.org/web/20101113020336/http://www.idrc.ca/en/ev-30610-201-1-DO_TOPIC.html |archive-date=November 13, 2010 }}, which takes global average weight as 60&nbsp;kg.), the total human biomass is the average weight multiplied by the current human population of approximately 6.5&nbsp;billion (see, ''e.g.'', {{cite web |title=World Population Information |url=https://www.census.gov/ipc/www/world.html |access-date=September 28, 2006 |publisher=U.S. Census Bureau }}{{Dead link|date=February 2019 |bot=InternetArchiveBot |fix-attempted=yes }}): Assuming 60–70&nbsp;kg to be the average human mass (approximately 130–150&nbsp;[[pound (mass)|lb]] on the average), an approximation of total global human mass of between 390&nbsp;billion (390×10<sup>9</sup>) and 455&nbsp;billion kg (between 845&nbsp;billion and 975&nbsp;billion lb, or about 423&nbsp;million–488&nbsp;million [[short ton]]s). The total biomass of all kinds on earth is estimated to be in excess of 6.8&nbsp;x&nbsp;10<sup>13</sup>&nbsp;kg (75&nbsp;billion short tons). By these calculations, the portion of total biomass accounted for by humans would be very roughly 0.6%.</ref>

Over nine-tenths of the total biomass on Earth is plant life, on which animal life depends very heavily for its existence.<ref>{{cite web |first=Peter V. |last=Sengbusch |title=The Flow of Energy in Ecosystems – Productivity, Food Chain, and Trophic Level |work=Botany online |publisher=University of Hamburg Department of Biology |url=http://www.biologie.uni-hamburg.de/b-online/e54/54c.htm |access-date=September 23, 2006 |archive-url=https://web.archive.org/web/20110726071651/http://www.biologie.uni-hamburg.de/b-online/e54/54c.htm |archive-date=July 26, 2011 |url-status=live |df=mdy-all }}</ref> More than 2 million species of plant and animal life have been identified to date,<ref>{{cite web |last=Pidwirny |first=Michael |date=2006 |work=Fundamentals of Physical Geography (2nd Edition) |title=Introduction to the Biosphere: Species Diversity and Biodiversity |url=http://www.physicalgeography.net/fundamentals/9h.html |access-date=September 23, 2006 |archive-url=https://web.archive.org/web/20110718040705/http://www.physicalgeography.net/fundamentals/9h.html |archive-date=July 18, 2011 |url-status=live |df=mdy-all }}</ref> and estimates of the actual number of existing species range from several million to well over 50&nbsp;million.<ref>{{cite web |url=http://faculty.plattsburgh.edu/thomas.wolosz/howmanysp.htm |title=How Many Species are There? |work=Extinction Web Page Class Notes |access-date=September 23, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060909194319/http://faculty.plattsburgh.edu/thomas.wolosz/howmanysp.htm |archive-date=September 9, 2006  }}</ref><ref>"Animal." World Book Encyclopedia. 16 vols. Chicago: World Book, 2003. This source gives an estimate of from 2 to 50&nbsp;million.</ref><ref>{{cite web |url=https://www.sciencedaily.com/releases/2003/05/030526103731.htm |title=Just How Many Species Are There, Anyway? |website=Science Daily |date=May 2003 |access-date=September 26, 2006 |archive-url=https://web.archive.org/web/20070211001529/http://www.sciencedaily.com/releases/2003/05/030526103731.htm |archive-date=February 11, 2007 |url-status=live |df=mdy-all }}</ref> The number of individual species of life is constantly in some degree of flux, with new species appearing and others ceasing to exist on a continual basis.<ref>{{cite web |last=Withers |first=Mark A. |display-authors=etal |title=Changing Patterns in the Number of Species in North American Floras |work=Land Use History of North America |url=http://biology.usgs.gov/luhna/chap4.html |date=1998 |access-date=September 26, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060923054200/http://biology.usgs.gov/luhna/chap4.html |archive-date=September 23, 2006  }} Website based on the contents of the book: {{cite book |editor=Sisk, T.D. |date=1998 |title=Perspectives on the land use history of North America: a context for understanding our changing environment |publisher=U.S. Geological Survey, Biological Resources Division |id=USGS/BRD/BSR-1998-0003 |edition=Revised September 1999}}</ref><ref>{{cite web |title=Tropical Scientists Find Fewer Species Than Expected |url=https://www.sciencedaily.com/releases/2002/04/020425072847.htm |date=April 2002 |website=Science Daily |access-date=September 27, 2006 |archive-url=https://web.archive.org/web/20060830182612/http://www.sciencedaily.com/releases/2002/04/020425072847.htm |archive-date=August 30, 2006 |url-status=live |df=mdy-all }}</ref> The total number of species is in rapid decline.<ref>{{cite journal |last=Bunker |first=Daniel E. |display-authors=etal |title=Species Loss and Aboveground Carbon Storage in a Tropical Forest |journal=Science |date=November 2005 |volume=310 |issue=5750 |pages=1029–1031 |doi=10.1126/science.1117682 |pmid=16239439 |bibcode = 2005Sci...310.1029B|citeseerx=10.1.1.465.7559 |s2cid=42696030 }}</ref><ref>{{cite journal |last=Wilcox |first=Bruce A. |title=Amphibian Decline: More Support for Biocomplexity as a Research Paradigm |journal=EcoHealth |date=2006 |volume=3 |issue=1 |doi=10.1007/s10393-005-0013-5|pages = 1–2|s2cid=23011961 }}</ref><ref>{{cite book |editor=Clarke, Robin |editor2=Robert Lamb |editor3=Dilys Roe Ward |date=2002 |title=Global environment outlook 3: past, present and future perspectives |chapter=Decline and loss of species |chapter-url=http://www.unep.org/geo/assessments/global-assessments/global-environment-outlook-3 |publisher=Nairobi, Kenya: UNEP |location=London; Sterling, VA |isbn=978-92-807-2087-7 |url-access=registration |url=https://archive.org/details/globalenvironmen0000unse_z8s0 }}</ref>

===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.

===Microbes===
[[File:Yellow mite (Tydeidae) Lorryia formosa 2 edit.jpg|thumb|upright|A microscopic mite [[Mite|''Lorryia formosa'']]]]
{{Main|Microbe}}
The first form of life to develop on the Earth were microbes, and they remained the only form of life until about a billion years ago when multi-cellular organisms began to appear.<ref>{{cite journal |
author = Schopf J|title = Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic|journal = Proc Natl Acad Sci USA|volume = 91|issue = 15|pages = 6735–42|date = 1994|pmid = 8041691|doi = 10.1073/pnas.91.15.6735 |
pmc = 44277
|bibcode = 1994PNAS...91.6735S|doi-access = free}}</ref> Microorganisms are single-celled organisms that are generally [[microscopic]], and smaller than the human eye can see. They include [[Bacteria]], [[Fungus|Fungi]], [[Archaea]], and [[Protist]]a.

These life forms are found in almost every location on the Earth where there is liquid water, including in the Earth's interior.<ref>{{cite journal|author = Szewzyk U|author2 = Szewzyk R|author3 = Stenström T|title = Thermophilic, anaerobic bacteria isolated from a deep borehole in granite in Sweden|doi= 10.1073/pnas.91.5.1810|journal = Proc Natl Acad Sci USA|volume = 91|issue = 5|pages = 1810–1813|date = 1994|pmid = 11607462|pmc = 43253|bibcode = 1994PNAS...91.1810S|doi-access = free}}</ref>
Their reproduction is both rapid and profuse. The combination of a high mutation rate and a [[horizontal gene transfer]]<ref>{{cite journal|author = Wolska K|title = Horizontal DNA transfer between bacteria in the environment|journal = Acta Microbiol Pol|volume = 52|issue = 3|pages = 233–243|date = 2003|pmid = 14743976}}</ref> ability makes them highly adaptable, and able to survive in new environments, including [[outer space]].<ref>{{cite journal|author = Horneck G|title = Survival of microorganisms in space: a review|journal = Adv Space Res|volume = 1|issue = 14|pages = 39–48|date = 1981|pmid = 11541716|doi = 10.1016/0273-1177(81)90241-6}}</ref> They form an essential part of the planetary ecosystem. However, some microorganisms are [[pathogen]]ic and can post health risk to other organisms.

===Plants and animals===
{{Main|Plant|Animal}}
[[File:Diversity of plants (Streptophyta) version 2.png|thumb|left|A selection of diverse [[plant species]]]]
[[File:Animal diversity.png|thumb|A selection of diverse [[animal species]]]]
Originally [[Aristotle]] divided all living things between plants, which generally do not move fast enough for humans to notice, and animals. In [[Carl Linnaeus|Linnaeus]]' system, these became the [[kingdom (biology)|kingdoms]] [[Vegetabilia]] (later [[Plant]]ae) and [[Animal]]ia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the [[fungus|fungi]] and several groups of [[alga]]e were removed to new kingdoms. However, these are still often considered plants in many contexts. Bacterial life is sometimes included in flora,<ref>{{cite web |title=flora |url=http://webster.com/cgi-bin/dictionary?va=flora |work=Merriam-Webster Online Dictionary |publisher=Merriam-Webster |access-date=September 27, 2006 |archive-url=https://web.archive.org/web/20060430072626/http://webster.com/cgi-bin/dictionary?va=flora |archive-date=April 30, 2006 |url-status=dead |df=mdy-all }}</ref><ref>{{cite book |date=1998 |title=Status and Trends of the Nation's Biological Resources |chapter=Glossary |chapter-url=http://biology.usgs.gov/s+t/SNT/noframe/zy198.htm |publisher=Department of the Interior, Geological Survey |location=Reston, VA |id=SuDocs No. I 19.202:ST 1/V.1-2 |url-status=dead |archive-url=https://web.archive.org/web/20070715060359/http://biology.usgs.gov/s+t/SNT/noframe/zy198.htm |archive-date=July 15, 2007  }}</ref> and some classifications use the term ''bacterial flora'' separately from ''plant flora''.

Among the many ways of classifying plants are by regional [[flora]]s, which, depending on the purpose of study, can also include ''fossil flora'', remnants<br /> of plant life from a previous era. People in many regions and countries take great pride in their individual arrays of characteristic flora, which can vary widely across the globe due to differences in climate and [[terrain]].

Regional floras commonly are divided into categories such as ''native flora'' and ''agricultural and garden flora'', the lastly mentioned of which are intentionally grown and cultivated. Some types of "native flora" actually have been introduced centuries ago by people migrating from one region or continent to another, and become an integral part of the native, or natural flora of the place to which they were introduced. This is an example of how human interaction with nature can blur the boundary of what is considered nature.

Another category of plant has historically been carved out for ''weeds''. Though the term has fallen into disfavor among [[Botany|botanists]] as a formal way to categorize "useless" plants, the informal use of the word "weeds" to describe those plants that are deemed worthy of elimination is illustrative of the general tendency of people and societies to seek to alter or shape the course of nature. Similarly, animals are often categorized in ways such as ''domestic'', ''farm animals'', ''wild animals'', ''pests'', etc. according to their relationship to human life.

Animals as a category have several characteristics that generally set them apart from other living things. Animals are [[eukaryote|eukaryotic]] and usually [[multicellular]] (although see [[Myxozoa]]), which separates them from bacteria, [[archaea]], and most [[protist]]s. They are [[heterotroph]]ic, generally digesting food in an internal chamber, which separates them from plants and [[alga]]e. They are also distinguished from plants, algae, and [[fungus|fungi]] by lacking [[cell wall]]s.

With a few exceptions—most notably the two [[Phylum|phyla]] consisting of [[sponge]]s and [[placozoa]]ns—animals have bodies that are differentiated into [[biological tissue|tissues]]. These include [[muscle]]s, which are able to contract and control locomotion, and a [[nervous system]], which sends and processes signals. There is also typically an internal [[digestion|digestive]] chamber. The eukaryotic cells possessed by all animals are surrounded by a characteristic extracellular matrix composed of [[collagen]] and elastic [[glycoprotein]]s. This may be calcified to form structures like [[Animal shell|shells]], [[bone]]s, and [[spicule (sponge)|spicules]], a framework upon which cells can move about and be reorganized during development and maturation, and which supports the complex anatomy required for mobility.