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For other uses, see Life (disambiguation) and Life on Earth (disambiguation).

Life (Biota / Vitae / Eobionti)

Ruwenpflanzen.pg

!lants in the Rwenzori "ountains, #ganda

$%ientifi% %lassifi%ation e

&omains and 'ingdoms

Life on Earth

 on*%ellular life+note -+note -

Viruses+note -

Viroids

0ellular life

!ro'ar1otes

&omain Ba%teria

&omain 2r%haea

&omain Eu'ar1a

3ingdom !rotista

3ingdom !lantae

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3ingdom Fungi

3ingdom 2nimalia

4allpaper of a barren and inhospitable alien lands%ape.pg

5his arti%le is one of a series on

Life in the #ni6erse

2strobiolog1

Life in the $olar $1stem

Life on Venus

Life on Earth

Life on "ars

Life on Europa

Life on 5itan

Life outside the $olar $1stem

$E57

E8oplanetolog1

!lanetar1 habitabilit1

0ir%umstellar habitable zone

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6 t e

Life is a %hara%teristi% distinguishing ph1si%al entities ha6ing biologi%al pro%esses, su%h as

signaling and self*sustaining pro%esses, from those that do not, either be%ause su%h fun%tions

ha6e %eased, or be%ause the1 ne6er had su%h fun%tions and are %lassified as inanimate. Various

forms of life e8ist su%h as plants, animals, fungi, protists, ar%haea, and ba%teria. 5he %riteria %an

at times be ambiguous and ma1 or ma1 not define 6iruses, 6iroids or potential artifi%ial life as

li6ing. Biolog1 is the primar1 s%ien%e %on%erned with the stud1 of life, although man1 other

s%ien%es are in6ol6ed.

5he definition of life is %ontro6ersial. 5he %urrent definition is that organisms maintain

homeostasis, are %omposed of %ells, undergo metabolism, %an grow, adapt to their en6ironment,

respond to stimuli, and reprodu%e. 9owe6er, man1 other biologi%al definitions ha6e been

 proposed, and there are also some borderline %ases, su%h as 6iruses. Bioph1si%ists ha6e also

 proposed some definitions, man1 being based on %hemi%al s1stems. 5here are also some li6ing

s1stems theories, su%h as the :aia h1pothesis, the idea that the Earth is ali6e; the former first

de6eloped b1

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s%ientifi% h1pothesis that li6ing organisms possess a =life for%e= or =6ital spar'=. 2biogenesis is

the natural pro%ess of life arising from non*li6ing matter, su%h as simple organi% %ompounds.

Life on Earth arose .>*?. billion 1ears ago. 7t is widel1 a%%epted that %urrent life on Earth

des%ended from a R2 world, but R2 based life ma1 not ha6e been the first. 5he me%hanism

 b1 whi%h life began on Earth is un'nown, although man1 h1potheses ha6e been formulated, most

 based on the "iller@#re1 e8periment.

$in%e appearing, life on Earth has %hanged its en6ironment on a geologi% time s%ale. 5o sur6i6e

in most e%os1stems, life %an adapt and thri6e in a wide range of %onditions. $ome organisms,

%alled e8tremophiles, %an thri6e in ph1si%all1 or geo%hemi%all1 e8treme %onditions that are

detrimental to most other life on Earth. !roperties %ommon to all organisms are the need for

%ertain %ore %hemi%al elements needed for bio%hemi%al fun%tioning. 2ristotle was the first person

to %lassif1 organisms. Later, 0arl Linnaeus introdu%ed his s1stem of binomial nomen%lature for

the %lassifi%ation of spe%ies. Fungi was later %lassified as its on 'ingdom. E6entuall1 new groups

of life were re6ealed, su%h as %ells and mi%roorganisms, and e6en non*%ellular reprodu%ing

agents, su%h as 6iruses and 6iroids. 0ells are the smallest units of life, often %alled the =building

 blo%'s of life=. 5here are two 'ind of %ells, pro'ar1otes and eu'ar1otes. 0ells %onsist of

%1toplasm en%losed within a membrane, whi%h %ontains man1 biomole%ules su%h as proteins and

nu%lei% a%ids. 0ells reprodu%e through a pro%ess of %ell di6ision in whi%h the parent %ell di6ides

into two or more daughter %ells.

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5hough onl1 %onfirmed on Earth, man1 belie6e in the e8isten%e of e8traterrestrial life. 2rtifi%ial

life is a %omputer simulation of an1 aspe%t of life, whi%h is used to e8amine s1stems related to

life. &eath is the permanent termination all biologi%al fun%tions that sustain an organism, and as

su%h, the end of its life. E8tin%tion is the pro%ess b1 whi%h a group of ta8a, normall1 a spe%ies,

dies out. Fossils are the preser6ed remains or tra%es of organisms.

0ontents +hide-

&efinitions

. Biolog1

.. 2lternati6e definitions

.. Viruses

. Bioph1si%s

. Li6ing s1stems theories

.. :aia h1pothesis

.. onfra%tionabilit1

.. Life as a propert1 of e%os1stems

..? 0omple8 s1stems biolog1

..A &arwinian d1nami%

.. Cperator theor1

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9istor1 of stud1

. "aterialism

. 91lomorphism

. $pontaneous generation

.? Vitalism

Crigin

? En6ironmental %onditions

?. Range of toleran%e

?. E8tremophiles

?. 0hemi%al elements

A 0lassifi%ation

0ells

D E8traterrestrial life

> 2rtifi%ial life

&eath

. E8tin%tion

. Fossils

$ee also

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otes

Referen%es

Further reading

? E8ternal lin's

&efinitions

7t is a %hallenge for s%ientists and philosophers to define life.+-+-+-+?-+A- 5his is partiall1

 be%ause life is a pro%ess, not a substan%e.+-+D-+>- 2n1 definition must be general enough to both

en%ompass all 'nown life and an1 un'nown life that ma1 be different from life on Earth.+-+-

+-

Biolog1

$ee also Crganism

$in%e there is no uneGui6o%al definition of life, most %urrent definitions in biolog1 are

des%ripti6e. Life is %onsidered a %hara%teristi% of something that e8hibits all or most of the

following traits+-+-+-+?-+A-+-+D-

9omeostasis regulation of the internal en6ironment to maintain a %onstant state; for e8ample,

sweating to redu%e temperature

Crganization being stru%turall1 %omposed of one or more %ells H the basi% units of life

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"etabolism transformation of energ1 b1 %on6erting %hemi%als and energ1 into %ellular

%omponents (anabolism) and de%omposing organi% matter (%atabolism). Li6ing things reGuire

energ1 to maintain internal organization (homeostasis) and to produ%e the other phenomena

asso%iated with life.

:rowth maintenan%e of a higher rate of anabolism than %atabolism. 2 growing organism

in%reases in size in all of its parts, rather than simpl1 a%%umulating matter.

2daptation the abilit1 to %hange o6er time in response to the en6ironment. 5his abilit1 is

fundamental to the pro%ess of e6olution and is determined b1 the organismIs heredit1, diet, and

e8ternal fa%tors.

Response to stimuli a response %an ta'e man1 forms, from the %ontra%tion of a uni%ellular

organism to e8ternal %hemi%als, to %omple8 rea%tions in6ol6ing all the senses of multi%ellular

organisms. 2 response is often e8pressed b1 motion; for e8ample, the lea6es of a plant turning

toward the sun (phototropism), and %hemota8is.

Reprodu%tion the abilit1 to produ%e new indi6idual organisms, either ase8uall1 from a single

 parent organism, or se8uall1 from two parent organisms.

5hese %omple8 pro%esses, %alled ph1siologi%al fun%tions, ha6e underl1ing ph1si%al and %hemi%al

 bases, as well as signaling and %ontrol me%hanisms that are essential to maintaining life.

2lternati6e definitions

$ee also Entrop1 and life

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2t a higher le6el, li6ing beings are thermod1nami% s1stems with an organized mole%ular

stru%ture+>- that %an reprodu%e itself and e6ol6e as sur6i6al di%tates.+-+-

5hermod1nami%all1, life has been des%ribed as an open s1stem, whi%h ma'es use of gradients in

its surroundings to %reate imperfe%t %opies of itself.+- 9en%e, life is a self*sustained %hemi%al

s1stem %apable of undergoing &arwinian e6olution.+-+-

Cthers ta'e a s1stemi% 6iewpoint that does not ne%essaril1 depend on mole%ular %hemistr1. Cne

s1stemi% definition of life is that li6ing things are self*organizing and autopoieti% (self*

 produ%ing). Variations of this definition in%lude $tuart 3auffmanIs definition as an autonomous

agent or a multi*agent s1stem %apable of reprodu%ing itself or themsel6es, and of %ompleting at

least one thermod1nami% wor' %1%le.+?-

Viruses

2deno6irus with i%osahedral diagram

"ain arti%le Virus

4hether or not 6iruses should be %onsidered as ali6e is a %ontro6ersial Guestion. 5he1 are most

often %onsidered as ust repli%ators rather than forms of life; but sometimes the abilit1 to

repli%ate is %onsidered as the %ru%ial propert1 of life.+A- 5he1 ha6e been des%ribed as

=organisms at the edge of life,=+- sin%e the1 possess genes, e6ol6e b1 natural sele%tion,+D-+>-

and repli%ate b1 %reating multiple %opies of themsel6es through self*assembl1. 9owe6er, 6iruses

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do not metabolize and the1 reGuire a host %ell to ma'e new produ%ts. Virus self*assembl1 within

host %ells has impli%ations for the stud1 of the origin of life, as it ma1 support the h1pothesis that

life %ould ha6e started as self*assembling organi% mole%ules.+-+-+-

Bioph1si%s

5o refle%t the minimum phenomena reGuired, other biologi%al definitions of life ha6e been

 proposed,+- with man1 of these being based upon %hemi%al s1stems. Bioph1si%ists ha6e

%ommented that li6ing things fun%tion on negati6e entrop1.+-+?- 7n other words, li6ing

 pro%esses %an be 6iewed as a dela1 of the spontaneous diffusion or dispersion of the internal

energ1 of biologi%al mole%ules towards more potential mi%rostates.+-+>- 7n more detail,

a%%ording to ph1si%ists su%h as

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of geolog1, but his idea of a li6ing Earth was forgotten in the intense redu%tionism of the th

%entur1.+>- 5he :aia h1pothesis, proposed in the s b1 s%ientist

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() e8plains it, life is a propert1 of an e%ologi%al s1stem rather than a single organism or

spe%ies.+?- 9e argues that an e%os1stemi% definition of life is preferable to a stri%tl1

 bio%hemi%al or ph1si%al one. Robert #lanowi%z () highlights mutualism as the 'e1 to

understand the s1stemi%, order*generating beha6ior of life and e%os1stems.+?D-

0omple8 s1stems biolog1

"ain arti%le 0omple8 s1stems biolog1

$ee also "athemati%al biolog1

0omple8 s1stems biolog1 (0$B) is a field of s%ien%e that studies the emergen%e of %omple8it1 in

fun%tional organisms from the 6iewpoint of d1nami% s1stems theor1.+?>- 5he latter is often

%alled also s1stems biolog1 and aims to understand the most fundamental aspe%ts of life. 2

%losel1 related approa%h to 0$B and s1stems biolog1, %alled relational biolog1, is %on%erned

mainl1 with understanding life pro%esses in terms of the most important relations, and %ategories

of su%h relations among the essential fun%tional %omponents of organisms; for multi%ellular

organisms, this has been defined as =%ategori%al biolog1=, or a model representation of organisms

as a %ategor1 theor1 of biologi%al relations, and also an algebrai% topolog1 of the fun%tional

organization of li6ing organisms in terms of their d1nami%, %omple8 networ's of metaboli%,

geneti%, epigeneti% pro%esses and signaling pathwa1s.+?-+A-

&arwinian d1nami%

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7t has also been argued that the e6olution of order in li6ing s1stems and %ertain ph1si%al s1stems

obe1 a %ommon fundamental prin%iple termed the &arwinian d1nami%.+A-+A- 5he &arwinian

d1nami% was formulated b1 first %onsidering how ma%ros%opi% order is generated in a simple

non*biologi%al s1stem far from thermod1nami% eGuilibrium, and then e8tending %onsideration to

short, repli%ating R2 mole%ules. 5he underl1ing order generating pro%ess for both t1pes of

s1stem was %on%luded to be basi%all1 similar.+A-

Cperator theor1

2nother s1stemi% definition, %alled the Cperator theor1, proposes that Ilife is a general term for

the presen%e of the t1pi%al %losures found in organisms; the t1pi%al %losures are a membrane and

an auto%atal1ti% set in the %ellI,+A- and also proposes that an organism is Ian1 s1stem with an

organisation that %omplies with an operator t1pe that is at least as %omple8 as the %ell.+A?-+AA-

+A-+AD- Life %an also be modeled as a networ' of inferior negati6e feedba%'s of regulator1

me%hanisms subordinated to a superior positi6e feedba%' formed b1 the potential of e8pansion

and reprodu%tion.+A>-

9istor1 of stud1

"aterialism

"ain arti%le "aterialism

!lant growth in the 9oh Rainforest

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9erds of zebra and impala gathering on the "aasai "ara plain

2n aerial photo of mi%robial mats around the :rand !rismati% $pring of Kellowstone ational

!ar' 

$ome of the earliest theories of life were materialist, holding that all that e8ists is matter, and that

life is merel1 a %omple8 form or arrangement of matter. Empedo%les (? B0) argued that

e6er1thing in the uni6erse is made up of a %ombination of four eternal =elements= or =roots of

all= earth, water, air, and fire. 2ll %hange is e8plained b1 the arrangement and rearrangement of

these four elements. 5he 6arious forms of life are %aused b1 an appropriate mi8ture of elements.

+A-

&emo%ritus (? B0) thought that the essential %hara%teristi% of life is ha6ing a soul (ps1%he).

Li'e other an%ient writers, he was attempting to e8plain what ma'es something a li6ing thing.

9is e8planation was that fier1 atoms ma'e a soul in e8a%tl1 the same wa1 atoms and 6oid

a%%ount for an1 other thing. 9e elaborates on fire be%ause of the apparent %onne%tion between

life and heat, and be%ause fire mo6es.+-

!latoIs world of eternal and un%hanging Forms, imperfe%tl1 represented in matter b1 a di6ine

2rtisan, %ontrasts sharpl1 with the 6arious me%hanisti% 4eltans%hauungen, of whi%h atomism

was, b1 the fourth %entur1 at least, the most prominent ... 5his debate persisted throughout the

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an%ient world. 2tomisti% me%hanism got a shot in the arm from Epi%urus ... while the $toi%s

adopted a di6ine teleolog1 ... 5he %hoi%e seems simple either show how a stru%tured, regular

world %ould arise out of undire%ted pro%esses, or ine%t intelligen%e into the s1stem.+-

 H R. A) is a

me%hanisti% e8planation for the origin of spe%ies b1 means of natural sele%tion.+-

91lomorphism