Life is a characteristic distinguishing physical entities having
biological processes (such as signaling and self-sustaining
processes) from those that do not,[1][2] either because
suchfunctions have ceased (death), or because they lack such
functions and are classified as inanimate.[][!]["] #arious forms of
life e$ist such as plants, animals, fungi, protists, archaea, and
bacteria. %he criteria can at times be ambiguous and may or may not
define viruses, viroids or potential artificial life as living.
&iology is the primary science concerned 'ith the study of
life, although many other sciences are involved.%he smallest
contiguous unit of life is called an organism. (rganisms are
composed of one or more cells, undergo metabolism, maintain
homeostasis, can gro', respond to stimuli, reproduce (either
se$ually or ase$ually) and, through evolution, adapt to their
environment in successive generations.[1] ) diverse array of living
organisms can befound in the biosphere of *arth, and the properties
common to these organisms+plants, animals, fungi, protists,
archaea, and bacteria+are a carbon- and 'ater-basedcellular form
'ith comple$ organi,ation and heritable genetic
information.)biogenesis is the natural process of life arising from
non-living matter, such as simple organic compounds. %he age of the
*arth is about !."! billion years old.[-][.][/] %he earliest life
on *arth arose at least ." billion years ago,[0][11][11] during the
*oarchean *ra 'hen sufficient crust had solidified follo'ing the
molten 2adean *on. %he earliest physical evidence of life on *arth
is biogenic graphite from .. billion-year-old metasedimentary rocks
found in 3estern 4reenland [12] and microbial mat fossils in .!/
billion-year-old sandstonefrom in 3estern )ustralia.[1][1!] 5ome
theories, such as the 6ate 2eavy &ombardment theory, suggest
that life on *arth mayhave started even earlier,[1"] and may have
begun as early as !.2" billion years ago according to one
study,[1-] and even earlier yet, !.! billion years ago, according
to another.[1.] %he mechanism by 'hich life began on *arth is
unkno'n, although many hypotheses have been formulated. 5ince
emerging, life has evolved into a variety of forms, 'hich have been
classified into a hierarchy of ta$a. 6ife can survive and thrive in
a 'ide range of conditions. 7onetheless, more than 00 percent of
all species, amounting to over five billion species,[1/] that ever
lived on *arth are estimated to be e$tinct.[10][21] *stimates on
the number of *arth8s current species range from 11 million to
1!million,[21] of 'hich about 1.2 million have been documented and
over /- percent have not yet been described.[22]%he chemistry
leading to life may have begun shortly after the &ig &ang,
1./ billion years ago, during a habitable epoch 'hen the9niverse
'as only 11:1. million years old.[2][2!][2"] )ccording to the
panspermia hypothesis, microscopic life+distributed bymeteoroids,
asteroids and other small 5olar 5ystem bodies+maye$ist throughout
the universe.[2-] %hough life is confirmed only on the*arth, many
think that e$traterrestrial life is not only plausible, but
probable or inevitable.[2.][2/] (ther planets and moons[20] in
the5olar 5ystem and other planetary systems are being e$amined for
evidence of having once supported simple life, and pro;ects such
as5*%< are trying to detect radio transmissions from possible
alien civili,ations.%he meaning of life+its significance, origin,
purpose, and ultimate fate+is a central concept and =uestion in
philosophy and religion. &oth philosophy and religion have
offered interpretations as to ho' life relates to e$istence and
consciousness, and on related issues such as life stance, purpose,
conception of a god or gods, a soul or an afterlife. >ifferent
cultures throughout history have had 'idely varying approaches to
these issues.Contents[hide] 1*arly theorieso 1.1?aterialismo
1.22ylomorphismo 1.#italism 2>efinitionso 2.1&iologyo
2.26iving systems theories (rigin !*nvironmental conditionso
!.1@ange of tolerance "Aorm and function -Blassification
.*$traterrestrial lifeo ..1@esearch />eath 0)rtificial life
11Chanero,oic *ono 11.1Caleo,oic *rao 11.2?eso,oic *rao 11.Beno,oic
*ra 115ee also 127otes 1@eferences 1!Aurther reading 1"*$ternal
linksEarly theoriesMaterialismClant gro'th in the 2oh
@ainforest2erds of ,ebra and impala gathering on the ?aasai ?ara
plain)n aerial photo of microbial mats around the 4rand Crismatic
5pring ofDello'stone 7ational Cark5ome of the earliest theories of
life 'ere materialist, holding that all that e$ists is matter, and
that life is merely a comple$ form or arrangement of matter.
*mpedocles (!1 &B) argued that every thing in the universe is
made up of a combination of four eternal EelementsE or Eroots of
allEF earth, 'ater, air, and fire. )ll change is e$plained by the
arrangement and rearrangement of these four elements. %he various
forms of life are caused by an appropriate mi$ture of
elements.[1]>emocritus (!-1 &B) thought that the essential
characteristic of life is having a soul (psyche). 6ike other
ancient 'riters, he 'as attempting to e$plain 'hat makes something
a living thing. 2is e$planation 'as that fiery atoms make a soul in
e$actly the same 'ay atoms and void account for any other thing. 2e
elaborates on fire because of the apparent connection bet'een life
and heat, and because fire moves.[1]Clato8s 'orld of eternal and
unchanging Aorms, imperfectly represented in matter by a divine
)rtisan, contrasts sharply 'ith the various mechanistic
3eltanschauungen, of 'hich atomism 'as, by the fourth century at
least, the most prominent... %his debate persisted throughout the
ancient 'orld. )tomistic mechanism got a shot in the arm from
*picurus... 'hile the 5toics adopted a divine teleology... %he
choice seems simpleF either sho' ho' a structured, regular 'orld
could arise out of undirected processes, or in;ect intelligence
into the system.[2]+@. G. 2ankinson, Cause and Explanation in
Ancient Greek Thought%he mechanistic materialism that originated in
ancient 4reece 'as revived and revised by the Arench philosopher
@enH >escartes, 'ho held that animals and humans 'ere
assemblages of parts that together functioned as a machine.
ar'inian evolution.[-"](thers take a systemic vie'point that does
not necessarily depend on molecular chemistry. (ne systemic
definition of life is that living things are self-organi,ing and
autopoietic(self-producing). #ariationsof this definition include
5tuart Kauffman8s definition as an autonomous agent or a
multi-agent system capable of reproducing itself or themselves, and
of completing at least one thermodynamic 'ork
cycle.[--]Viruses*lectron micrograph of adenovirus'ith a cartoon to
demonstrate its icosahedral structure#iruses are most often
considered replicators rather than forms of life. %hey have been
described as Eorganisms at the edge of life,E[-.]since they possess
genes, evolve by natural selection,[-/][-0] andreplicate by
creating multiple copies of themselves through self-assembly.
2o'ever, viruses do not metaboli,e and they re=uire a host cell to
make ne' products. #irus self-assembly 'ithin host cells has
implications for the study of the origin of life, as it may support
the hypothesis that life could have started as self-assembling
organic molecules.[.1][.1][.2]Li!ing systems theories%he idea that
the *arth is alive is found in philosophy and religion, but the
first scientific discussion of it 'as by the 5cottish scientist
Games 2utton. 7), the e$ception being sulfur. %he latter is a
component of the amino acids cysteine and methionine. %he most
biologically abundant of these elements is carbon, 'hich has the
desirable attribute of forming multiple, stable covalent bonds.
%his allo's carbon-based (organic) molecules to form an immense
variety of chemical arrangements.[121] )lternative hypothetical
types of biochemistryhave been proposed that eliminate one or more
of these elements, s'ap out an element for one not on the list, or
change re=uiredchiralities or other chemical
properties.[121][122]Range of tolerane%he inert components of an
ecosystem are the physical and chemical factors necessary for life
+ energy (sunlight or chemical energy), 'ater, temperature,
atmosphere, gravity,nutrients, and ultraviolet solar radiation
protection.[12] uring this time, there are four continentsF
4ond'ana ()frica, 5outh )merica, )ustralia, )ntarctica, 5iberia),
6aurentia (7orth )merica), &altica (7orthern *urope), and
)valonia (3estern *urope). %he recent rise in sea levels provided
many ne' species to thrive in 'ater.[10]3e!onian*ogyrinus (an
amphibian) of the Barboniferous%he >evonian spans from !1"
million years to -1 million years ago. )lso kno'n as E%he )ge of
the AishE, the >evonian features a huge diversification of fish,
including armored fish like >unkleosteus and lobe-finned fish
'hich eventually evolved intothe first tetrapods. (n land, plant
groups diversified incredibly in an event kno'n as the >evonian
*$plosion 'here the first trees evolved, as 'ell as seeds. %his
event also diversified arthropod life. %he first amphibians also
evolved, and the fish 'ere no' at the top of the food chain. 7ear
the end of the >evonian, .1M of all species 'ent e$tinct in an
event kno'n as the 6ate >evonian e$tinction and is the second
mass e$tinction event the 'orld has
seen.[10!]Carboniferous>imetrodon%he Barboniferous spans from -1
million to 11 million years ago. >uring this time, average
global temperatures 'ere e$ceedingly highI the early Barboniferous
averaged at about 21 degrees Belsius(but cooled do'n to 11 degrees
during the ?iddle Barboniferous).[10"]%ropical s'amps dominated the
earth, and the large amounts of trees created much of the carbon
for the coal that is used today (hence the name EBarbon-iferousE).
Cerhaps the most important evolutionary development of the time 'as
the evolution of amniotic eggs, 'hich allo'ed amphibians to head
farther inland and remained the dominant vertebrae throughout the
duration of this period. )lso, the first reptiles and synapsids
evolved in the s'amps. %hroughout the Barboniferous, there 'as a
cooling pattern, 'hich eventually led to the glaciation of 4ond'ana
as much of it 'as situated around the south pole in an event kno'n
as the Cermo-Barboniferous glaciation or theBarboniferous
@ainforest Bollapse.[10-]1ermian%he Cermian spans from 11 million
to 2"1 million years ago and 'as the last period of the Caleo,oic.
)t the beginning, all continents formed together to form the
super-continentCangaea and had one ocean called Canthalassa. %he
earth 'as very dry during this time, 'ith harsh seasons as the
climate of the interior of Cangaea 'asn8t regulated by large bodies
of 'ater. @eptiles and synapsids flourished in the ne' dry climate.
Breatures such as >imetrodon and *daphosaurus ruled the ne'
continent. %he firstconifers evolve, and dominate the terrestrial
landscape. 7earing theend of the Cermian, ho'ever, Cangaea got
drier and drier. %he interior 'as nothing but dry deserts, and ne'
species such as5cutosaurus and 4orgonopsid filled the empty desert.
*ventually,they disappeared, along 'ith 0"M of all life on earth in
an event simply kno'n as Ethe 4reat >yingE, and is the third
mass e$tinction event of the 'orld.[10.][10/]Mesozoi 2raMain
article: Meso/oic)lso kno'n as Ethe )ge of the dinosaursE, the
?eso,oic features the rise of reptiles on their 1"1 million year
con=uest to rule the earth from the seas, the land, and even in the
air. %here areperiods in the ?eso,oicF the %riassic, the Gurassic,
and the Bretaceous.4riassi%he %riassic ranges from 2"1 million to
211 million years ago. %he %riassic is a desolate transitional
state in *arth8s history bet'een the Cermian *$tinction and the
lush Gurassic Ceriod. ying. %emnospondyli evolved during this time
and 'ould be the dominant predator for much of the
%riassic.[211]Clateosaurus (a prosauropod)%he ?iddle %riassic spans
from 2!. million to 2. million years ago.%he ?iddle %riassic
featured the beginnings of the breakup ofCangaea, and the beginning
of the %ethys 5ea. %he ecosystem had recovered from the devastation
that 'as the 4reat >ying. Chytoplankton, coral, and crustaceans
all had recovered, and the reptiles began to get bigger and bigger.
7e' a=uatic reptiles evolved such as
