Changes

==Etymology==

==Etymology==

The word "nature" is derived from the Latin word ''natura'', or "the course of things, natural character."[http://www.etymonline.com/index.php?term=nature] ''Natura'' was a Latin translation of the Greek word ''[[physis]]'' (φύσις), which originally related to the intrinsic characteristics that plants, animals, and other features of the world develop of their own accord. A useful though somewhat erratically presented account of the pre-Socratic use of the concept of φύσις may be found in Naddaf, Gerard ''The Greek Concept of Nature'', SUNY Press, 2006. The word φύσις, while first used in connection with a plant in Homer (as we have seen), occurs very early in Greek philosophy, and in several senses. Generally, these senses match rather well the current senses in which the English word ''nature'' is used, as confirmed by Guthrie, W.K.C. ''Presocratic Tradition from Parmenides to Democritus'' (volume 2 of his ''History of Greek Philosophy'', Cambridge UP, 1965). This is shown in the first written use of the word φύσις, in connection with a plant. The first known use of ''physis'' was by [[Homer]] in reference to the intrinsic qualities of a plant: ὣς ἄρα φωνήσας πόρε φάρμακον ἀργεϊφόντης ἐκ γαίης ἐρύσας, καί μοι '''φύσιν''' αὐτοῦ ἔδειξε. (So saying, Argeiphontes [=Hermes] gave me the herb, drawing it from the ground, and showed me its '''nature'''.) ''[[Odyssey]]'' 10.302-3 (ed. A.T. Murray). (The word is dealt with thoroughly in Liddell and Scott's ''[http://archimedes.fas.harvard.edu/pollux Greek Lexicon]''.) For later but still very early Greek uses of the term, see related below. The concept of nature as a whole, the physical [[universe]], is one of several expansions of the original notion; it began with certain core applications of the word φύσις by pre-Socratic philosophers, and has steadily gained currency ever since. This usage was confirmed during the advent of modern [[scientific method]] in the last several centuries. Isaac Newton's [[Philosophiae Naturalis Principia Mathematica]] (1687), for example, is translated "Mathematical Principles of Natural Philosophy", and reflects the then-current use of the words "[[natural philosophy]]", akin to "systematic study of nature." The etymology of the word "physical" shows its use as a synonym for "natural" in about the mid-15th century: [http://www.etymonline.com/index.php?term=physical]  

The word "nature" is derived from the Latin word ''natura'', or "the course of things, natural character."[https://www.etymonline.com/index.php?term=nature] ''Natura'' was a Latin translation of the Greek word ''[[physis]]'' (φύσις), which originally related to the intrinsic characteristics that plants, animals, and other features of the world develop of their own accord. A useful though somewhat erratically presented account of the pre-Socratic use of the concept of φύσις may be found in Naddaf, Gerard ''The Greek Concept of Nature'', SUNY Press, 2006. The word φύσις, while first used in connection with a plant in Homer (as we have seen), occurs very early in Greek philosophy, and in several senses. Generally, these senses match rather well the current senses in which the English word ''nature'' is used, as confirmed by Guthrie, W.K.C. ''Presocratic Tradition from Parmenides to Democritus'' (volume 2 of his ''History of Greek Philosophy'', Cambridge UP, 1965). This is shown in the first written use of the word φύσις, in connection with a plant. The first known use of ''physis'' was by [[Homer]] in reference to the intrinsic qualities of a plant: ὣς ἄρα φωνήσας πόρε φάρμακον ἀργεϊφόντης ἐκ γαίης ἐρύσας, καί μοι '''φύσιν''' αὐτοῦ ἔδειξε. (So saying, Argeiphontes [=Hermes] gave me the herb, drawing it from the ground, and showed me its '''nature'''.) ''[[Odyssey]]'' 10.302-3 (ed. A.T. Murray). (The word is dealt with thoroughly in Liddell and Scott's ''[https://archimedes.fas.harvard.edu/pollux Greek Lexicon]''.) For later but still very early Greek uses of the term, see related below. The concept of nature as a whole, the physical [[universe]], is one of several expansions of the original notion; it began with certain core applications of the word φύσις by pre-Socratic philosophers, and has steadily gained currency ever since. This usage was confirmed during the advent of modern [[scientific method]] in the last several centuries. Isaac Newton's [[Philosophiae Naturalis Principia Mathematica]] (1687), for example, is translated "Mathematical Principles of Natural Philosophy", and reflects the then-current use of the words "[[natural philosophy]]", akin to "systematic study of nature." The etymology of the word "physical" shows its use as a synonym for "natural" in about the mid-15th century: [https://www.etymonline.com/index.php?term=physical]  

Within the various uses of the word today, "nature" may refer to the general realm of various types of living plants and animals, and in some cases to the processes associated with inanimate objects; the way that particular types of things exist and change of their own accord, such as the [[weather]] and [[geology]] of the Earth, and the [[matter]] and [[energy]] of which all these things are composed.  It is often taken to mean the "[[natural environment]]" or [[wilderness]]; wild animals, rocks, forest, beaches, and in general those things that have not been substantially altered by human intervention, or which persist despite human intervention.  This more traditional concept of natural things which can still be found today implies a distinction between the natural and the [[artificial]], with the latter being understood as that which has been brought into being by a [[human]] or human-like [[consciousness]] or [[mind]].

Within the various uses of the word today, "nature" may refer to the general realm of various types of living plants and animals, and in some cases to the processes associated with inanimate objects; the way that particular types of things exist and change of their own accord, such as the [[weather]] and [[geology]] of the Earth, and the [[matter]] and [[energy]] of which all these things are composed.  It is often taken to mean the "[[natural environment]]" or [[wilderness]]; wild animals, rocks, forest, beaches, and in general those things that have not been substantially altered by human intervention, or which persist despite human intervention.  This more traditional concept of natural things which can still be found today implies a distinction between the natural and the [[artificial]], with the latter being understood as that which has been brought into being by a [[human]] or human-like [[consciousness]] or [[mind]].

'''Earth''' (or, "the earth") is the fifth largest [[planet]] in the [[solar system]], third in order of distance from the [[Sun]]. It is the largest of its [[planetary system]]'s [[terrestrial planet]]s and the only place in the [[universe]] known to support life.  

'''Earth''' (or, "the earth") is the fifth largest [[planet]] in the [[solar system]], third in order of distance from the [[Sun]]. It is the largest of its [[planetary system]]'s [[terrestrial planet]]s and the only place in the [[universe]] known to support life.  

The most prominent features of the earth's climate are its two large polar regions, two relatively narrow [[temperate]] zones, and a wide [[equator]]ial [[tropical]] to [[subtropical]] region. An excellent summary description of global climate can be found at:[http://www.blueplanetbiomes.org/climate.htm] [[precipitation (meteorology)|Precipitation]] patterns vary widely according to location, ranging from several [[metre]]s of water per year to less than a [[millimetre]]. About 70 percent of the surface is covered by salt-water oceans. The remainder consists of continents and islands, with the vast majority of the inhabited land in the [[Northern Hemisphere]].  

The most prominent features of the earth's climate are its two large polar regions, two relatively narrow [[temperate]] zones, and a wide [[equator]]ial [[tropical]] to [[subtropical]] region. An excellent summary description of global climate can be found at:[https://www.blueplanetbiomes.org/climate.htm] [[precipitation (meteorology)|Precipitation]] patterns vary widely according to location, ranging from several [[metre]]s of water per year to less than a [[millimetre]]. About 70 percent of the surface is covered by salt-water oceans. The remainder consists of continents and islands, with the vast majority of the inhabited land in the [[Northern Hemisphere]].  

Earth has evolved through geological and biological processes that have left traces of the original conditions. The [[Crust (geology)|outer surface]] is divided into several [[tectonic plate]]s that gradually migrate across the surface over geologic time spans, which at least several times have changed relatively quickly. The interior of the planet remains active, with a thick layer of molten [[Earth mantle]] and an iron-filled core that generates a [[magnetic field]].

Earth has evolved through geological and biological processes that have left traces of the original conditions. The [[Crust (geology)|outer surface]] is divided into several [[tectonic plate]]s that gradually migrate across the surface over geologic time spans, which at least several times have changed relatively quickly. The interior of the planet remains active, with a thick layer of molten [[Earth mantle]] and an iron-filled core that generates a [[magnetic field]].

The [[atmosphere|atmospheric]] conditions have been significantly altered from the original conditions by the presence of life forms, [http://www.sciencedaily.com/releases/2005/09/050911103921.htm] which create an ecological balance that stabilizes the surface conditions. Despite the wide regional variations in climate by [[latitude]] and other geographic factors, the long-term average global climate is quite stable during interglacial periods,[http://www.epa.gov/climatechange/science/pastcc.html] and variations of a degree or two of average global temperature have historically had major effects on the ecological balance, and on the actual geography of the Earth.[http://vathena.arc.nasa.gov/curric/land/global/climchng.html] [http://www.aip.org/history/climate/]  

The [[atmosphere|atmospheric]] conditions have been significantly altered from the original conditions by the presence of life forms, [https://www.sciencedaily.com/releases/2005/09/050911103921.htm] which create an ecological balance that stabilizes the surface conditions. Despite the wide regional variations in climate by [[latitude]] and other geographic factors, the long-term average global climate is quite stable during interglacial periods,[https://www.epa.gov/climatechange/science/pastcc.html] and variations of a degree or two of average global temperature have historically had major effects on the ecological balance, and on the actual geography of the Earth.[https://vathena.arc.nasa.gov/curric/land/global/climchng.html] [https://www.aip.org/history/climate/]  

===Historical perspective===

===Historical perspective===

Based on the available evidence, scientists have reconstructed detailed information about the planet's past. Earth is estimated to have formed approximately 4.55 billion years ago out of the [[solar nebula]], along with the Sun and other planets. (The Age of the Earth, ISBN 0-8047-1569-6) The moon formed relatively soon afterwards (roughly 20 million years later, or 4.53 billion years ago).

Based on the available evidence, scientists have reconstructed detailed information about the planet's past. Earth is estimated to have formed approximately 4.55 billion years ago out of the [[solar nebula]], along with the Sun and other planets. (The Age of the Earth, ISBN 0-8047-1569-6) The moon formed relatively soon afterwards (roughly 20 million years later, or 4.53 billion years ago).

Initially molten, the outer layer of the planet cooled, resulting in the solid crust. Outgassing and [[Volcano|volcanic]] activity produced the primordial atmosphere. Condensing [[water vapor]], augmented by [[ice]] delivered by [[comet]]s, [[Origin of the world's oceans|produced the oceans]].[http://adsabs.harvard.edu/abs/2000M&PS...35.1309M] The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago. [http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=76]

Initially molten, the outer layer of the planet cooled, resulting in the solid crust. Outgassing and [[Volcano|volcanic]] activity produced the primordial atmosphere. Condensing [[water vapor]], augmented by [[ice]] delivered by [[comet]]s, [[Origin of the world's oceans|produced the oceans]].[https://adsabs.harvard.edu/abs/2000M&PS...35.1309M] The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago. [https://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=76]

Continents formed, then broke up and re-formed as the surface of Earth reshaped itself over the course of hundreds of millions of years, occasionally combining to make a [[supercontinent]]. Roughly 750 million years ago, the earliest known supercontinent [[Rodinia]], began to break apart. The continents later recombined to form [[Pannotia]] which broke apart about 540 million years ago, then finally [[Pangaea]], which broke apart about 180 million years ago.[http://www.americanscientist.org/template/AssetDetail/assetid/34004]

Continents formed, then broke up and re-formed as the surface of Earth reshaped itself over the course of hundreds of millions of years, occasionally combining to make a [[supercontinent]]. Roughly 750 million years ago, the earliest known supercontinent [[Rodinia]], began to break apart. The continents later recombined to form [[Pannotia]] which broke apart about 540 million years ago, then finally [[Pangaea]], which broke apart about 180 million years ago.[https://www.americanscientist.org/template/AssetDetail/assetid/34004]

There is significant evidence, still being discussed among scientists, that a severe [[Glacier|glacial]] action during the [[Neoproterozoic]] era covered much of the planet in a sheet of ice. 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 million years ago.[http://www.gps.caltech.edu/~jkirschvink/pdfs/firstsnowball.pdf] ISBN 0-521-36615-1

There is significant evidence, still being discussed among scientists, that a severe [[Glacier|glacial]] action during the [[Neoproterozoic]] era covered much of the planet in a sheet of ice. 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 million years ago.[https://www.gps.caltech.edu/~jkirschvink/pdfs/firstsnowball.pdf] ISBN 0-521-36615-1

Since the [[Cambrian explosion]] there have been five distinctly identifiable [[Extinction event|mass extinctions]]. The last mass extinction occurred some 65 million years ago, when a meteorite collision probably triggered the extinction of the [[avian|non-avian]] [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s, which then resembled [[shrew]]s. Over the past 65 million years, mammalian life diversified. ISBN 0-684-81326-2

Since the [[Cambrian explosion]] there have been five distinctly identifiable [[Extinction event|mass extinctions]]. The last mass extinction occurred some 65 million years ago, when a meteorite collision probably triggered the extinction of the [[avian|non-avian]] [[dinosaur]]s and other large reptiles, but spared small animals such as [[mammal]]s, which then resembled [[shrew]]s. Over the past 65 million years, mammalian life diversified. ISBN 0-684-81326-2

Several million years ago, a species of small African ape gained the ability to stand upright.  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 [[Oxygen Catastrophe]], produced by the proliferation of algae during the [[Siderian]] period, required about 300 million years to culminate.)

Several million years ago, a species of small African ape gained the ability to stand upright.  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 [[Oxygen Catastrophe]], produced by the proliferation of algae during the [[Siderian]] period, required about 300 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. 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 years. [http://www.springerlink.com/index/D85T53513002564V.pdf] The extent of the current extinction event is still being researched, debated and calculated by biologists.(See, e.g. [http://park.org/Canada/Museum/extinction/holmass.html], [http://park.org/Canada/Museum/extinction/extincmenu.html], [http://park.org/Canada/Museum/extinction/patterns.html])

The present era is classified as part of a mass extinction event, the [[Holocene extinction event]], the fastest ever to have occurred. 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 years. [https://www.springerlink.com/index/D85T53513002564V.pdf] The extent of the current extinction event is still being researched, debated and calculated by biologists.(See, e.g. [https://park.org/Canada/Museum/extinction/holmass.html], [https://park.org/Canada/Museum/extinction/extincmenu.html], [https://park.org/Canada/Museum/extinction/patterns.html])

===Atmosphere, climate and weather===

===Atmosphere, climate and weather===

The atmosphere of the Earth serves as a key factor in sustaining the planetary [[Nature#Ecosystems|ecosystem]]. The thin layer of [[gas]]es that envelops the Earth is held in place by the planet's gravity. Dry [[air]] consists of 78% [[nitrogen]], 21% [[oxygen]], 1% [[argon]] and other [[inert gases]], carbon dioxide, etc.; but air also contains a variable amount of [[water vapor]].  The atmospheric pressure declines steadily with altitude, and has a [[scale height]] of about 8 [[kilometre]]s at the Earth's surface: the height at which the atmospheric pressure has declined by a factor of ''[[E (mathematical constant)|e]]'' (a mathematical constant

The atmosphere of the Earth serves as a key factor in sustaining the planetary [[Nature#Ecosystems|ecosystem]]. The thin layer of [[gas]]es that envelops the Earth is held in place by the planet's gravity. Dry [[air]] consists of 78% [[nitrogen]], 21% [[oxygen]], 1% [[argon]] and other [[inert gases]], carbon dioxide, etc.; but air also contains a variable amount of [[water vapor]].  The atmospheric pressure declines steadily with altitude, and has a [[scale height]] of about 8 [[kilometre]]s at the Earth's surface: the height at which the atmospheric pressure has declined by a factor of ''[[E (mathematical constant)|e]]'' (a mathematical constant

equal to 2.71...).[http://www.grc.nasa.gov/WWW/K-12/Numbers/Math/Mathematical_Thinking/ideal_gases_under_constant.htm] [http://www.pnas.org/cgi/content/full/99/suppl_1/2546] The [[ozone layer]] of the Earth's atmosphere plays an important role in depleting the amount of [[ultraviolet]] (UV) radiation that reaches the surface. As [[DNA]] is readily damaged by UV light, this serves to protect life at the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes.   

equal to 2.71...).[https://www.grc.nasa.gov/WWW/K-12/Numbers/Math/Mathematical_Thinking/ideal_gases_under_constant.htm] [https://www.pnas.org/cgi/content/full/99/suppl_1/2546] The [[ozone layer]] of the Earth's atmosphere plays an important role in depleting the amount of [[ultraviolet]] (UV) radiation that reaches the surface. As [[DNA]] is readily damaged by UV light, this serves to protect life at the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes.   

Terrestrial weather occurs almost exclusively in the [[troposphere|lower part of the atmosphere]], and serves as a convective system for redistributing heat. Ocean currents are another important factor in determining climate, particularly the major underwater [[thermohaline circulation]] which distributes heat energy from the equatorial oceans to the polar regions. These currents help to moderate the differences in temperature between winter and summer in the temperate zones. Also, without the redistributions of heat energy by the ocean currents and atmosphere, the tropics would be much hotter, and the polar regions much colder.

Terrestrial weather occurs almost exclusively in the [[troposphere|lower part of the atmosphere]], and serves as a convective system for redistributing heat. Ocean currents are another important factor in determining climate, particularly the major underwater [[thermohaline circulation]] which distributes heat energy from the equatorial oceans to the polar regions. These currents help to moderate the differences in temperature between winter and summer in the temperate zones. Also, without the redistributions of heat energy by the ocean currents and atmosphere, the tropics would be much hotter, and the polar regions much colder.

The climate of a region depends on a number of factors, especially [[latitude]]. A latitudinal band of the surface with similar climatic attributes forms a climate region. There are a number of such regions, ranging from the [[tropical climate]] at the equator to the [[polar climate]] in the northern and southern extremes. Weather is also influenced by the [[seasons]], which result from the [[Earth]]'s [[axis of rotation|axis]] being [[axial tilt|tilted]] relative to its [[orbital plane (astronomy)|orbital plane]]. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the [[sun]]. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the [[northern hemisphere|northern]] and [[southern hemisphere|southern]] hemispheres experience opposite seasons.

The climate of a region depends on a number of factors, especially [[latitude]]. A latitudinal band of the surface with similar climatic attributes forms a climate region. There are a number of such regions, ranging from the [[tropical climate]] at the equator to the [[polar climate]] in the northern and southern extremes. Weather is also influenced by the [[seasons]], which result from the [[Earth]]'s [[axis of rotation|axis]] being [[axial tilt|tilted]] relative to its [[orbital plane (astronomy)|orbital plane]]. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the [[sun]]. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the [[northern hemisphere|northern]] and [[southern hemisphere|southern]] hemispheres experience opposite seasons.

Weather is a [[Chaos theory|chaotic system]] that is readily modified by small changes to the [[Natural environment|environment]], so accurate [[Meteorology|weather forecasting]] is currently limited to only a few days. Overall, two things are currently happening worldwide: (1) temperature is increasing on the average; and (2) regional climates have been undergoing noticeable changes.[http://www.sciencedaily.com/releases/2001/04/010406073554.htm]

Weather is a [[Chaos theory|chaotic system]] that is readily modified by small changes to the [[Natural environment|environment]], so accurate [[Meteorology|weather forecasting]] is currently limited to only a few days. Overall, two things are currently happening worldwide: (1) temperature is increasing on the average; and (2) regional climates have been undergoing noticeable changes.[https://www.sciencedaily.com/releases/2001/04/010406073554.htm]

== 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]].[http://www.calacademy.org/exhibits/xtremelife/what_is_life.php]  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]].[https://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 x 10¹³ [[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 [[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 x 10¹³ kg (75 billion short tons). By these calculations, the portion of total biomass accounted for by humans would be very roughly 0.6%.

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 x 10¹³ [[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., [https://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.'', [https://www.census.gov/ipc/www/world.html] Assuming 60–70 kg to be the average human mass (approximately 130–150 [[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 x 10¹³ 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.[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)

Over nine-tenths of the total biomass on Earth is plant life, on which animal life depends very heavily for its existence.[https://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, [https://www.physicalgeography.net/fundamentals/9h.html] and estimates of the actual number of existing species range from several million to well over 50 million.[https://faculty.plattsburgh.edu/thomas.wolosz/howmanysp.htm] [https://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.[https://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 [https://www.sciencedaily.com/releases/2002/04/020425072847.htm]  The total number of species is presently in rapid decline. [https://www.sciencemag.org/cgi/content/abstract/310/5750/1029] [https://www.ecohealth.net/pdfs/Vol3/ECH_Editorial_3_1.pdf] [https://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.[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.

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.[https://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.

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]].[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.

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]].[https://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===

The first form of life to develop on the Earth were microbes, and they remained the only form of life on the planet until about a billion years ago when multi-cellular organisms began to appear.[http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=44277&blobtype=pdf] Microorganisms are single-celled organisms that are generally smaller than the human eye can see. They include Bacteria, Fungi, Archaea and Protista.

The first form of life to develop on the Earth were microbes, and they remained the only form of life on the planet until about a billion years ago when multi-cellular organisms began to appear.[https://www.pubmedcentral.nih.gov/picrender.fcgi?artid=44277&blobtype=pdf] Microorganisms are single-celled organisms that are generally smaller than the human eye can see. They include Bacteria, Fungi, Archaea and Protista.

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]

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.[https://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.

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,[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''.

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,[https://webster.com/cgi-bin/dictionary?va=flora] [https://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]].

==Ecosystems==

==Ecosystems==

[[Loch Lomond]] in Scotland forms a relatively isolated ecosystem. The fish community of this lake has remained unchanged over a very long period. [http://cat.inist.fr/?aModele=afficheN&cpsidt=3302548]  

[[Loch Lomond]] in Scotland forms a relatively isolated ecosystem. The fish community of this lake has remained unchanged over a very long period. [https://cat.inist.fr/?aModele=afficheN&cpsidt=3302548]  

All forms of life interact with the environment in which they exist, and also with other life forms.  In the 20th century this premise gave rise to the concept of '''ecosystems''', which can be defined as any situation where there is interaction between organisms and their environment.

All forms of life interact with the environment in which they exist, and also with other life forms.  In the 20th century this premise gave rise to the concept of '''ecosystems''', which can be defined as any situation where there is interaction between organisms and their environment.

Ecosystems are composed of a variety of [[abiotic]] and [[biotic]] components that function in an interrelated way.[http://www.physicalgeography.net/fundamentals/9j.html] The structure and composition is determined by various environmental factors that are interrelated. Variations of these factors will initiate dynamic modifications to the ecosystem. Some of the more important components are: soil, atmosphere, radiation from the sun, water, and living organisms.

Ecosystems are composed of a variety of [[abiotic]] and [[biotic]] components that function in an interrelated way.[https://www.physicalgeography.net/fundamentals/9j.html] The structure and composition is determined by various environmental factors that are interrelated. Variations of these factors will initiate dynamic modifications to the ecosystem. Some of the more important components are: soil, atmosphere, radiation from the sun, water, and living organisms.

Each living organism has a continual relationship with every other element that makes up its environment. Within the ecosystem, species are connected and dependent upon one another in the [[food chain]], and exchange [[energy]] and [[matter]] between themselves as well as with their environment.[http://www.physicalgeography.net/fundamentals/9d.html]  

Each living organism has a continual relationship with every other element that makes up its environment. Within the ecosystem, species are connected and dependent upon one another in the [[food chain]], and exchange [[energy]] and [[matter]] between themselves as well as with their environment.[https://www.physicalgeography.net/fundamentals/9d.html]  

Every species has limits of tolerance to factors that affect its survival, reproductive success and ability to continue to thrive and interact sustainably with the rest of its environment, which in turn may have effects on these factors for many other species or even on the whole of life.[http://www.physicalgeography.net/fundamentals/9e.html]  esp. section on "Abiotic Factors and Tolerance Limits." The concept of an ecosystem is thus an important subject of study, as such study provides information needed to make decisions about how human life may interact in a way that allows the various ecosystems to be sustained for future use rather than used up or otherwise rendered ineffective. For the purpose of such study, a unit of smaller size is called a ''[[microecosystem]]''. For example, an ecosystem can be a stone and all the life under it. A ''macroecosystem'' might involve a whole [[ecoregion]], with its [[drainage basin]].[http://www.fs.fed.us/institute/news_info/Identifying_ecoregion_boundaries.pdf]  

Every species has limits of tolerance to factors that affect its survival, reproductive success and ability to continue to thrive and interact sustainably with the rest of its environment, which in turn may have effects on these factors for many other species or even on the whole of life.[https://www.physicalgeography.net/fundamentals/9e.html]  esp. section on "Abiotic Factors and Tolerance Limits." The concept of an ecosystem is thus an important subject of study, as such study provides information needed to make decisions about how human life may interact in a way that allows the various ecosystems to be sustained for future use rather than used up or otherwise rendered ineffective. For the purpose of such study, a unit of smaller size is called a ''[[microecosystem]]''. For example, an ecosystem can be a stone and all the life under it. A ''macroecosystem'' might involve a whole [[ecoregion]], with its [[drainage basin]].[https://www.fs.fed.us/institute/news_info/Identifying_ecoregion_boundaries.pdf]  

The following ecosystems are examples of the kinds currently under intensive study:

The following ecosystems are examples of the kinds currently under intensive study:

* [[oceanic]] ecosystems.

* [[oceanic]] ecosystems.

Another classification can be made by reference to its communities, such as in the case of a [[human ecosystem]]. Regional groupings of distinctive plant and animals best adapted to the region's physical [[natural environment]], latitude, altitude, and terrain are known as [[biome]]s. The broadest classification, today under wide study and analysis, and also subject to widespread arguments about its nature and validity, is that of the entire sum of life seen as analogous to a self-sustaining organism; a theory studied as [[Earth science|earth system science]] (less formally known as [[Gaia theory]]).[http://www.sciencemag.org/cgi/content/summary/292/5524/1965] [http://www.schumachercollege.org.uk/articles/college-articles/stephan/earthsystemscience.html]

Another classification can be made by reference to its communities, such as in the case of a [[human ecosystem]]. Regional groupings of distinctive plant and animals best adapted to the region's physical [[natural environment]], latitude, altitude, and terrain are known as [[biome]]s. The broadest classification, today under wide study and analysis, and also subject to widespread arguments about its nature and validity, is that of the entire sum of life seen as analogous to a self-sustaining organism; a theory studied as [[Earth science|earth system science]] (less formally known as [[Gaia theory]]).[https://www.sciencemag.org/cgi/content/summary/292/5524/1965] [https://www.schumachercollege.org.uk/articles/college-articles/stephan/earthsystemscience.html]

==Human interrelationship==

==Human interrelationship==

Although humans currently comprise only about one-half of one percent of the total living biomass on Earth. For an example of a range of opinions, see: [http://www.wilderness.org/Library/Documents/Beauty_Quotes.cfm] and [[Ralph Waldo Emerson]]'s analysis of the subject: [http://www.emersoncentral.com/beauty.htm] the human effect on nature is disproportionately large. Because of the extent of human influence, the boundaries between what we regard as nature and "made environments" is not clear cut except at the extremes. Even at the extremes, the amount of natural environment that is free of discernible human influence is presently diminishing at an increasingly rapid pace, or, according to some, has already disappeared.

Although humans currently comprise only about one-half of one percent of the total living biomass on Earth. For an example of a range of opinions, see: [https://www.wilderness.org/Library/Documents/Beauty_Quotes.cfm] and [[Ralph Waldo Emerson]]'s analysis of the subject: [https://www.emersoncentral.com/beauty.htm] the human effect on nature is disproportionately large. Because of the extent of human influence, the boundaries between what we regard as nature and "made environments" is not clear cut except at the extremes. Even at the extremes, the amount of natural environment that is free of discernible human influence is presently diminishing at an increasingly rapid pace, or, according to some, has already disappeared.

The development of technology by the human race has allowed the greater exploitation of natural resources and has helped to alleviate some of the risk from natural hazards. In spite of this progress, however, the fate of human [[civilization]] remains closely linked to changes in the environment. There exists a highly complex feedback-loop between the use of advanced technology and changes to the environment that are only slowly becoming understood.[http://www.lbl.gov/Science-Articles/Archive/ESD-feedback-loops.html] Manmade threats to the Earth's natural environment include [[pollution]], [[deforestation]], and disasters such as oil spills. Humans have contributed to the [[extinction]] of many plants and animals.

The development of technology by the human race has allowed the greater exploitation of natural resources and has helped to alleviate some of the risk from natural hazards. In spite of this progress, however, the fate of human [[civilization]] remains closely linked to changes in the environment. There exists a highly complex feedback-loop between the use of advanced technology and changes to the environment that are only slowly becoming understood.[https://www.lbl.gov/Science-Articles/Archive/ESD-feedback-loops.html] Manmade threats to the Earth's natural environment include [[pollution]], [[deforestation]], and disasters such as oil spills. Humans have contributed to the [[extinction]] of many plants and animals.

Humans employ nature for both leisure and economic activities. The acquisition of natural resources for industrial use remains a primary component of the world's economic system. Some activities, such as [[hunting]] and [[fishing]], are used for both sustenance and leisure, often by different people. [[Agriculture#History|Agriculture]] was first adopted around the [[9th millennium BCE]]. Ranging from food production to [[energy]], nature influences economic wealth.

Humans employ nature for both leisure and economic activities. The acquisition of natural resources for industrial use remains a primary component of the world's economic system. Some activities, such as [[hunting]] and [[fishing]], are used for both sustenance and leisure, often by different people. [[Agriculture#History|Agriculture]] was first adopted around the [[9th millennium BCE]]. Ranging from food production to [[energy]], nature influences economic wealth.

Although early humans gathered uncultivated plant materials for food and employed the medicinal properties of vegetation for healing,[http://www.nps.gov/plants/medicinal/plants.htm] most modern human use of plants is through agriculture. The clearance of large tracts of land for crop growth has led to a significant reduction in the amount available of forestation and wetlands, resulting in the loss of habitat for many plant and animal species as well as increased erosion.[http://www.eh-resources.org/philosophy.html]  

Although early humans gathered uncultivated plant materials for food and employed the medicinal properties of vegetation for healing,[https://www.nps.gov/plants/medicinal/plants.htm] most modern human use of plants is through agriculture. The clearance of large tracts of land for crop growth has led to a significant reduction in the amount available of forestation and wetlands, resulting in the loss of habitat for many plant and animal species as well as increased erosion.[https://www.eh-resources.org/philosophy.html]  

===Wilderness===

===Wilderness===

==Beauty in nature==

==Beauty in nature==

Beauty in nature is has long been a common theme in life and in art, and books emphasizing beauty in nature fill large sections of libraries and bookstores. That nature has been depicted and celebrated by so much [[art]], [[photography]], [[poetry]] and other literature shows the strength with which many people associate nature and beauty. Why this association exists, and what the association consists of, is studied by the branch of philosophy called [[aesthetics]].  Beyond certain basic characteristics that many philosophers agree about to explain what is seen as beautiful, the opinions are virtually endless. For an example of a range of opinions, see: [http://www.wilderness.org/Library/Documents/Beauty_Quotes.cfm] [http://www.emersoncentral.com/beauty.htm]

Beauty in nature is has long been a common theme in life and in art, and books emphasizing beauty in nature fill large sections of libraries and bookstores. That nature has been depicted and celebrated by so much [[art]], [[photography]], [[poetry]] and other literature shows the strength with which many people associate nature and beauty. Why this association exists, and what the association consists of, is studied by the branch of philosophy called [[aesthetics]].  Beyond certain basic characteristics that many philosophers agree about to explain what is seen as beautiful, the opinions are virtually endless. For an example of a range of opinions, see: [https://www.wilderness.org/Library/Documents/Beauty_Quotes.cfm] [https://www.emersoncentral.com/beauty.htm]

Looked at through the lens of the [[visual arts]], nature and wildness have been important subjects in various epochs of world history.  An early tradition of [[landscape art]] began in [[China]] during the [[Tang Dynasty art|Tang Dynasty]] (618-907).  The tradition of representing nature ''as it is'' became one of the aims of [[Chinese painting]] and was a significant influence in Asian art.  Artists learned to depict mountains and rivers "from the perspective of nature as a whole and on the basis of their understanding of the laws of nature … as if seen through the eyes of a bird."  In the 13th century, the [[Song Dynasty]] artist [[Shi Erji]] listed "scenes lacking any places made inaccessible by nature," as one of the 12 things to avoid in painting.[http://www.asia-art.net/chinese_tech_brush.html Chinese brush painting]  

Looked at through the lens of the [[visual arts]], nature and wildness have been important subjects in various epochs of world history.  An early tradition of [[landscape art]] began in [[China]] during the [[Tang Dynasty art|Tang Dynasty]] (618-907).  The tradition of representing nature ''as it is'' became one of the aims of [[Chinese painting]] and was a significant influence in Asian art.  Artists learned to depict mountains and rivers "from the perspective of nature as a whole and on the basis of their understanding of the laws of nature … as if seen through the eyes of a bird."  In the 13th century, the [[Song Dynasty]] artist [[Shi Erji]] listed "scenes lacking any places made inaccessible by nature," as one of the 12 things to avoid in painting.[https://www.asia-art.net/chinese_tech_brush.html Chinese brush painting]  

In the [[Western culture|Western world]] the idea of wilderness having intrinsic value emerged in the 1800s, especially in the works of the [[Romantic movement]].  [[Kingdom of Great Britain|British]] artists [[John Constable]] and [[JMW Turner]] turned their attention to capturing the beauty of the natural world in their paintings. Before that, paintings had been primarily of religious scenes or of human beings. [[William Wordsworth|William Wordsworth’s]] poetry described the wonder of the natural world, which had formerly been viewed as a threatening place. Increasingly the valuing of nature became an aspect of Western culture.[http://www.spacesfornature.org/greatspaces/conservation.html] This artistic movement also coincided with the [[Transcendentalism|Transcendentalist movement]] in the Western world.

In the [[Western culture|Western world]] the idea of wilderness having intrinsic value emerged in the 1800s, especially in the works of the [[Romantic movement]].  [[Kingdom of Great Britain|British]] artists [[John Constable]] and [[JMW Turner]] turned their attention to capturing the beauty of the natural world in their paintings. Before that, paintings had been primarily of religious scenes or of human beings. [[William Wordsworth|William Wordsworth’s]] poetry described the wonder of the natural world, which had formerly been viewed as a threatening place. Increasingly the valuing of nature became an aspect of Western culture.[https://www.spacesfornature.org/greatspaces/conservation.html] This artistic movement also coincided with the [[Transcendentalism|Transcendentalist movement]] in the Western world.

Many scientists, who study nature in more specific and organized ways, also share the conviction that nature is beautiful; the French mathematician, [[Henri Poincaré|Jules Henri Poincaré]] (1854-1912) said:  

Many scientists, who study nature in more specific and organized ways, also share the conviction that nature is beautiful; the French mathematician, [[Henri Poincaré|Jules Henri Poincaré]] (1854-1912) said:  

Some fields of [[science]] see nature as matter in motion, obeying certain laws of nature which science seeks to understand. For this reason the most fundamental science is generally understood to be "[[physics]]"—the name for which is still recognizable as meaning that it is the study of nature.  

Some fields of [[science]] see nature as matter in motion, obeying certain laws of nature which science seeks to understand. For this reason the most fundamental science is generally understood to be "[[physics]]"—the name for which is still recognizable as meaning that it is the study of nature.  

Matter is commonly defined as the substance of which physical objects are composed. It constitutes the [[observable universe]]. According to the theory of [[special relativity]], there is no unchangeable distinction between matter and [[energy]], because matter can be converted to energy (see [[annihilation]]), and vice versa (see [[matter creation]]).  The visible components of the universe are now believed to compose only 4 percent of the total mass. The remainder is believed to consist of 23 percent cold [[dark matter]] and 73 percent [[dark energy]].[http://map.gsfc.nasa.gov/m_mm/mr_limits.html] The exact nature of these components is still unknown and is currently under intensive investigation by physicists.

Matter is commonly defined as the substance of which physical objects are composed. It constitutes the [[observable universe]]. According to the theory of [[special relativity]], there is no unchangeable distinction between matter and [[energy]], because matter can be converted to energy (see [[annihilation]]), and vice versa (see [[matter creation]]).  The visible components of the universe are now believed to compose only 4 percent of the total mass. The remainder is believed to consist of 23 percent cold [[dark matter]] and 73 percent [[dark energy]].[https://map.gsfc.nasa.gov/m_mm/mr_limits.html] The exact nature of these components is still unknown and is currently under intensive investigation by physicists.

The behavior of matter and energy throughout the [[observable universe]] appears to follow well-defined [[physical law]]s. These laws have been employed to produce [[Physical cosmology|cosmological]] models that successfully explain the structure and the evolution of the universe we can observe. The mathematical expressions of the laws of physics employ a set of twenty [[physical constant]]s [http://www.physics.nist.gov/cuu/Constants/introduction.html] that appear to be static across the observable universe.[http://arxiv.org/abs/physics/0004062] The values of these constants have been carefully measured, but the reason for their specific values remains a mystery.

The behavior of matter and energy throughout the [[observable universe]] appears to follow well-defined [[physical law]]s. These laws have been employed to produce [[Physical cosmology|cosmological]] models that successfully explain the structure and the evolution of the universe we can observe. The mathematical expressions of the laws of physics employ a set of twenty [[physical constant]]s [https://www.physics.nist.gov/cuu/Constants/introduction.html] that appear to be static across the observable universe.[https://arxiv.org/abs/physics/0004062] The values of these constants have been carefully measured, but the reason for their specific values remains a mystery.

==Nature beyond Earth==

==Nature beyond Earth==

Outer space is certainly spacious, but it is far from empty.  Outer space is sparsely filled with several dozen types of [[organic chemistry|organic]] [[molecule]]s discovered to date by [[rotational spectroscopy|microwave spectroscopy]],  [[cosmic microwave background radiation|blackbody radiation]] left over from the [[big bang]] and the origin of the universe, and [[cosmic ray]]s, which include [[ion]]ized [[atomic nucleus|atomic nuclei]] and various [[subatomic particle]]s. There is also some gas, [[Plasma (physics)|plasma]] and [[dust]], and small [[meteor]]s.  Additionally, there are signs of human life in outer space today, such as material left over from previous manned and unmanned launches which are a potential hazard to spacecraft. Some of this [[space debris|debris]] re-enters the atmosphere periodically.

Outer space is certainly spacious, but it is far from empty.  Outer space is sparsely filled with several dozen types of [[organic chemistry|organic]] [[molecule]]s discovered to date by [[rotational spectroscopy|microwave spectroscopy]],  [[cosmic microwave background radiation|blackbody radiation]] left over from the [[big bang]] and the origin of the universe, and [[cosmic ray]]s, which include [[ion]]ized [[atomic nucleus|atomic nuclei]] and various [[subatomic particle]]s. There is also some gas, [[Plasma (physics)|plasma]] and [[dust]], and small [[meteor]]s.  Additionally, there are signs of human life in outer space today, such as material left over from previous manned and unmanned launches which are a potential hazard to spacecraft. Some of this [[space debris|debris]] re-enters the atmosphere periodically.

Although the planet Earth is currently the only known body within the solar system to support life, current evidence suggests that in the distant past the planet [[Mars]] possessed bodies of liquid water on the surface. For a brief period in Mars' history, it may have also been capable of forming life. At present though, most of the water remaining on Mars is frozen. If life exists at all on Mars, it is most likely to be located underground where liquid water can still exist.[http://www.msnbc.msn.com/id/7129347/]

Although the planet Earth is currently the only known body within the solar system to support life, current evidence suggests that in the distant past the planet [[Mars]] possessed bodies of liquid water on the surface. For a brief period in Mars' history, it may have also been capable of forming life. At present though, most of the water remaining on Mars is frozen. If life exists at all on Mars, it is most likely to be located underground where liquid water can still exist.[https://www.msnbc.msn.com/id/7129347/]

Conditions on the other terrestrial planets, [[Mercury (planet)|Mercury]] and [[Venus]], appears to be too harsh to support life as we know it. But it has been conjectured that [[Europa (moon)|Europa]], the fourth-largest moon of [[Jupiter]], may possess a sub-surface ocean of liquid water and could potentially host life.[http://www2.jpl.nasa.gov/galileo/news8.html]  

Conditions on the other terrestrial planets, [[Mercury (planet)|Mercury]] and [[Venus]], appears to be too harsh to support life as we know it. But it has been conjectured that [[Europa (moon)|Europa]], the fourth-largest moon of [[Jupiter]], may possess a sub-surface ocean of liquid water and could potentially host life.[https://www2.jpl.nasa.gov/galileo/news8.html]  

Recently, the team of [[Stéphane Udry]] have discovered a new planet named [[Gliese 581 c]], which is an [[extrasolar]] planet orbiting the [[red dwarf]] [[star]] [[Gliese 581]]. Gliese 581 c appears to lay in the [[habitable zone]] of space surrounding the [[star]], and therefore could possibly host [[life]] as we know it.

Recently, the team of [[Stéphane Udry]] have discovered a new planet named [[Gliese 581 c]], which is an [[extrasolar]] planet orbiting the [[red dwarf]] [[star]] [[Gliese 581]]. Gliese 581 c appears to lay in the [[habitable zone]] of space surrounding the [[star]], and therefore could possibly host [[life]] as we know it.

== External links ==

== External links ==

*[http://naturesmagazine.com/ The Essence of Nature Magazine] - An online magazine dedicated to nature, animals, and the environment.

*[https://naturesmagazine.com/ The Essence of Nature Magazine] - An online magazine dedicated to nature, animals, and the environment.

* [http://www.yesterdays.sk/kategorie.php?id=35 Historical photos of nature]

* [https://www.yesterdays.sk/kategorie.php?id=35 Historical photos of nature]

* [http://www.scribd.com/doc/273950/Exploring-Nature Some nature related activities for kids]

* [https://www.scribd.com/doc/273950/Exploring-Nature Some nature related activities for kids]

[[Category: General Reference]]

[[Category: General Reference]]

[[Category: Biology]]

[[Category: Biology]]