Changes

== Life ==  

== Life ==  

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 | year = 2006 | url = http://www.calacademy.org/exhibits/xtremelife/what_is_life.php | title = Definition of Life | publisher = California Academy of Sciences | accessdate = 2007-01-07 }}</ref> Life may also be said to be simply the characteristic state of [[organism]]s.

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]].[http://www.calacademy.org/exhibits/xtremelife/what_is_life.php] Life may also be said to be simply the characteristic state of [[organism]]s.

Properties common to terrestrial organisms ([[plant]]s, [[animal]]s, [[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.

Properties common to terrestrial organisms ([[plant]]s, [[animal]]s, [[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 air, land, surface rocks and water — within which life occurs, and which [[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).  Currently the entire Earth contains over 75 billion tons (150 ''trillion'' pounds or about 6.8&nbsp;x&nbsp;10¹³&nbsp;[[kilogram]]s) 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 |year=1999 |chapter=How Meat-centred Eating Patterns Affect Food Security and the Environment |chapterurl=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 |id=ISBN 0-88936-882-1}}, which takes global average weight as 60 kg.), the total human biomass is the average weight multiplied by the current human population of approximately 6.5 billion (see, ''e.g.'', {{cite web |title=World Population Information |url=http://www.census.gov/ipc/www/world.html |accessmonthday=September 28 |accessyear=2006 |publisher=U.S. Census Bureau}}): Assuming 60–70 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 billion (390×10⁹) and 455 billion kg (between 845 billion and 975 billion lb, or about 423 million-488 million [[short ton]]s). The total biomass of all kinds on earth is estimated to be in excess of 6.8&nbsp;x&nbsp;10¹³&nbsp;kg (75 billion short tons). By these calculations, the portion of total biomass accounted for by humans would be very roughly 0.6%.</ref>

The [[biosphere]] is the part of Earth's outer shell — including air, land, surface rocks and water — within which life occurs, and which [[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).  Currently the entire Earth contains over 75 billion tons (150 ''trillion'' pounds or about 6.8&nbsp;x&nbsp;10¹³&nbsp;[[kilogram]]s) of [[biomass (ecology)|biomass]] (life), which lives within various environments within the biosphere. The figure "about one-half of one percent" takes into account the following (See, e.g., [http://www.idrc.ca/en/ev-30610-201-1-DO_TOPIC.html] ISBN 0-88936-882-1), which takes global average weight as 60 kg.), the total human biomass is the average weight multiplied by the current human population of approximately 6.5 billion (see, ''e.g.'', [http://www.census.gov/ipc/www/world.html] Assuming 60–70 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 billion (390×10⁹) and 455 billion kg (between 845 billion and 975 billion lb, or about 423 million-488 million [[short ton]]s). The total biomass of all kinds on earth is estimated to be in excess of 6.8&nbsp;x&nbsp;10¹³&nbsp;kg (75 billion short tons). By these calculations, the portion of total biomass accounted for by humans would be very roughly 0.6%.

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 |accessmonthday=September 23|accessyear=2006}}</ref> More than 2 million species of plant and animal life have been identified to date,<ref>{{cite web |last=Pidwirny |first=Michael |year=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 |accessmonthday=September 23|accessyear=2006}}</ref> and estimates of the actual number of existing species range from several million to well over 50 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 |accessmonthday=September 23|accessyear=2006}}</ref><ref>"Animal." World Book Encyclopedia. 16 vols. Chicago: World Book, 2003. This source gives an estimate of from 2-50 million.</ref><ref>{{cite web |url=http://www.sciencedaily.com/releases/2003/05/030526103731.htm |title=Just How Many  Species Are There, Anyway? |publisher=Science Daily |year=2003 |month=May |accessmonthday=September 26|accessyear=2006 }}</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. |coauthors=et al. |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 |year=1998 |accessmonthday=September 26|accessyear=2006 }} Website based on the contents of the book: {{cite book |editor=Sisk, T.D., ed. |year=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=http://www.sciencedaily.com/releases/2002/04/020425072847.htm |year=2002 |month=April |publisher=Science Daily |accessmonthday=September 27|accessyear=2006 }}</ref> The total number of species is presently in rapid decline.<ref>{{cite journal |last=Bunker |first=Daniel E. |coauthors=et al. |title=Species Loss and Aboveground Carbon Storage in a Tropical Forest |url=http://www.sciencemag.org/cgi/content/abstract/310/5750/1029 |journal=Science |year=2005 |month=November |volume=310 |issue=5750 |pages=pp. 1029-31 |id={{doi|10.1126/science.1117682}} }}</ref><ref>{{cite journal |last=Wilcox |first=Bruce A. |title=Amphibian Decline: More Support for Biocomplexity as a Research Paradigm |journal=EcoHealth |year=2006 |month=March |volume=3 |issue=1  |url=http://www.ecohealth.net/pdfs/Vol3/ECH_Editorial_3_1.pdf |id={{doi|10.1007/s10393-005-0013-5}} |pages=pp.1-2 }}</ref><ref>{{cite book |editor=Clarke, Robin, Robert Lamb, Dilys Roe Ward eds. |year=2002 |title= Global environment outlook 3 : past, present and future perspectives |chapter=Decline and loss of species |chapterurl=http://www.grida.no/geo/geo3/english/221.htm |publisher=Nairobi, Kenya : UNEP |location=London; Sterling, VA |id=ISBN 92-807-2087-2}}</ref>

Over nine-tenths of the total biomass on Earth is plant life, on which animal life depends very heavily for its existence.[http://www.biologie.uni-hamburg.de/b-online/e54/54c.htm] More than 2 million species of plant and animal life have been identified to date, [http://www.physicalgeography.net/fundamentals/9h.html] and estimates of the actual number of existing species range from several million to well over 50 million.[http://faculty.plattsburgh.edu/thomas.wolosz/howmanysp.htm] [http://www.sciencedaily.com/releases/2003/05/030526103731.htm] 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.[http://biology.usgs.gov/luhna/chap4.html] Website based on the contents of the book: Perspectives on the land use history of North America: a context for understanding our changing environment [http://www.sciencedaily.com/releases/2002/04/020425072847.htm]  The total number of species is presently in rapid decline. [http://www.sciencemag.org/cgi/content/abstract/310/5750/1029] [http://www.ecohealth.net/pdfs/Vol3/ECH_Editorial_3_1.pdf] [http://www.grida.no/geo/geo3/english/221.htm] ISBN 92-807-2087-2)

===Evolution===

===Evolution===

Life, as we understand it, is currently only known to exist on the planet Earth. The [[origin of life]] is still a poorly understood process, but it is thought to have occurred about 3.9 to 3.5 billion years ago during the [[hadean]] or [[archean]] eons on a 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 | url=http://mic.sgmjournals.org/cgi/content/full/148/1/21?view=long&pmid=11782495 | journal = Microbiology | volume = 148 | issue = Pt 1 | pages = 21-7 | year = 2002 | id = PMID 11782495}}</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 formation of ever-more diverse life forms.

Life, as we understand it, is currently only known to exist on the planet Earth. The [[origin of life]] is still a poorly understood process, but it is thought to have occurred about 3.9 to 3.5 billion years ago during the [[hadean]] or [[archean]] eons on a primordial earth that had a substantially different environment than is found at present.[http://mic.sgmjournals.org/cgi/content/full/148/1/21?view=long&pmid=11782495] 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 formation 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/>

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.

The advent of [[photosynthesis]] in very basic forms of plant life worldwide allowed the sun's energy to be harvested to create conditions allowing for more complex life. 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 |coauthors=L. C. Marshall |year=1965 |month=May |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=pp. 225–261 |url=http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0469(1965)022%3C0225:OTOARO%3E2.0.CO%3B2 }}</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.

The advent of [[photosynthesis]] in very basic forms of plant life worldwide allowed the sun's energy to be harvested to create conditions allowing for more complex life. 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]].[http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0469(1965)022%3C0225:OTOARO%3E2.0.CO%3B2]  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===

===Microbes===

These life forms are found in almost every location on the Earth where there is liquid water, including the interior of rocks within the planet.<ref>{{cite journal | author = Szewzyk U, Szewzyk R, Stenström T | title = Thermophilic, anaerobic bacteria isolated from a deep borehole in granite in Sweden. | url=http://www.pnas.org/cgi/reprint/91/5/1810 | journal = Proc Natl Acad Sci U S A | volume = 91 | issue = 5 | pages = 1810-3 | year = 1994 | id = PMID 11607462}}</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 = 233-43 | year = 2003 | id = 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 | year = 1981 | id = PMID 11541716}}</ref>  They form an essential part of the planetary ecosystem. However some microorganisms are [[pathogenic]] and can post health risk to other organisms.

 

These life forms are found in almost every location on the Earth where there is liquid water, including the interior of rocks within the planet.[http://www.pnas.org/cgi/reprint/91/5/1810]

Their reproduction is both rapid and profuse. The combination of a high mutation rate and a [[horizontal gene transfer]] They form an essential part of the planetary ecosystem. However some microorganisms are [[pathogenic]] and can post health risk to other organisms.

===Plants and animals===

===Plants and animals===

The distinction between plant and animal life is not sharply drawn, with some categories of life that stand between or across the two. Originally [[Aristotle]] divided all living things between plants, which generally do not move, and animals. In [[Carolus Linnaeus|Linnaeus]]' system, these became the [[kingdom (biology)|Kingdoms]] [[Vegetabilia]] (later [[Plantae]]) 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 |accessmonthday=September 27|accessyear=2006 }}</ref><ref>{{cite book |year=1998 |title=Status and Trends of the Nation's Biological Resources |chapter=Glossary |chapterurl=http://biology.usgs.gov/s+t/SNT/noframe/zy198.htm#F |publisher=Department of the Interior, Geological Survey |location=Reston, VA |id=SuDocs No. I 19.202:ST 1/V.1-2}}</ref> and some classifications use the term ''bacterial flora'' separately from ''plant flora''.

The distinction between plant and animal life is not sharply drawn, with some categories of life that stand between or across the two. Originally [[Aristotle]] divided all living things between plants, which generally do not move, and animals. In [[Carolus Linnaeus|Linnaeus]]' system, these became the [[kingdom (biology)|Kingdoms]] [[Vegetabilia]] (later [[Plantae]]) 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,[http://webster.com/cgi-bin/dictionary?va=flora] [http://biology.usgs.gov/s+t/SNT/noframe/zy198.htm#F] 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 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]].

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 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]].

[[Animals]] as a category have several characteristics that generally set them apart from other living things, though not traced by scientists to having legs or wings instead of roots and leaves.  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 [[plant]]s and [[alga]]e. They are also distinguished from plants, algae, and [[fungus|fungi]] by lacking [[cell wall]]s.

[[Animals]] as a category have several characteristics that generally set them apart from other living things, though not traced by scientists to having legs or wings instead of roots and leaves.  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 [[plant]]s 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 [[sea sponge|sponge]]s (Phylum Porifera), animals have bodies differen­tiated into separate [[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|shell]]s, [[bone]]s, and [[spicule]]s, a framework upon which cells can move about and be reorganized during development and maturation, and which supports the complex anatomy required for mobility.  

With a few exceptions, most notably the [[sea sponge|sponge]]s (Phylum Porifera), animals have bodies differen­tiated into separate [[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|shell]]s, [[bone]]s, and [[spicule]]s, a framework upon which cells can move about and be reorganized during development and maturation, and which supports the complex anatomy required for mobility.

 

==Ecosystems==

==Ecosystems==