檢視生命的原始碼

'''生命'''(''' life''' )，泛指一类具有稳定的[[物质]]和[[能量]][[代谢]]现象并且能回应[[刺激]]、能进行自我复制（[[繁殖]]）的半开放物质系统。简单来说，也就是由有生命机制的物体<ref name=Koshland>{{Cite journal |title=The Seven Pillars of Life |journal=Science |date=March 22, 2002 |first=Daniel E. |last=Koshland, Jr. |volume=295 |issue=5563 |pages=2215–2216 |doi= 10.1126/science.1068489 |url=http://www.sciencemag.org/cgi/content/full/295/5563/2215 |accessdate=2009-05-25 |pmid=11910092}}</ref><ref name=AHDLife>The ''American Heritage Dictionary'' of the English Language, 4th edition, published by Houghton Mifflin Company, via [http://www.answers.com/topic/life Answers.com]:
* "The property or quality that distinguishes living organisms from dead organisms and inanimate matter, manifested in functions such as metabolism, growth, reproduction, and response to stimuli or adaptation to the environment originating from within the organism."
* "The characteristic state or condition of a living organism."</ref>。生命个体一定会经历[[出生]]、[[成长]]、[[衰老]]和[[死亡]]。生命种群则在一代代个体的更替中经过[[自然选择]]发生[[进化]]以适应环境。[[生物学]]则是以研究生命为中心的[[科学]]。

生命的最小单位是[[生物]]，生物是由一个或多个[[细胞]]组成，能够[[新陈代谢]]，维持[[稳态|恒定性]]，可以[[发育生物学|成长]]，回应[[刺激]]，可以[[繁殖]]甚至[[演化]]，以适应外界[[环境]]，继续繁殖并产生后代<ref name=Koshland/>。在地球的[[生物圈]]内可以找到许多不同的生物，在这些生物中（包括[[植物]]、[[动物]]、[[真菌]]、[[原生生物]]、[[古菌]]及[[细菌]]），都有共同的[[特徵]]，都是由以{{link-en|碳生命|carbon-based life|碳}}和[[水#对生物|水]]为基础的[[细胞]]构成，有其[[组织 (生物学)|组织]]以及可以[[遗传]]的[[基因]]资讯。

[[地球]]约在[[地球的年龄|45.4亿年前]]形成。[[生命起源|最早的生命]]至少约在35亿年产生。<ref name="Origin1">Schopf, JW, Kudryavtsev, AB, Czaja, AD, and Tripathi, AB. (2007). ''Evidence of Archean life: Stromatolites and microfossils.'' Precambrian Research 158:141-155.</ref><ref name="Origin2">Schopf, JW (2006). ''Fossil evidence of Archaean life.'' Philos Trans R Soc Lond B Biol Sci 29;361(1470) 869-85.</ref><ref name="RavenJohnson2002">{{cite book | first=Peter | last=Hamilton Raven | first2=George | last2=Brooks Johnson | title=Biology | url=http://books.google.com/books?id=GtlqPwAACAAJ | year=2002 | publisher=McGraw-Hill Education | isbn=978-0-07-112261-0 | page=68 | accessdate=2013-07-07 }}</ref>当时是[[始太古代]]，[[冥古宙]]熔化的地壳已经固化。地球上最早的生物证据是在西[[格陵兰]]发现的37亿年前[[变质岩]]中的{{link-en|生物物质|Biogenic substance}}[[石墨]]<ref name="NG-20131208">{{cite web | url=http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2025.html | first=Yoko | last=Ohtomo | first2=Takeshi | last2=Kakegawa | first3=Akizumi | last3=Ishida | first4=Toshiro | last4=Nagase | first5=Minik T. | last5=Rosing | title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks | publisher=Nature Geoscience | doi=10.1038/ngeo2025 | date=8 December 2013 }}</ref>，及西澳洲34.8亿年前[[砂岩]]中的远古生物[[化石]]<ref name="AP-20131113">{{cite news |last=Borenstein |first=Seth |title=Oldest fossil found: Meet your microbial mom |url=http://apnews.excite.com/article/20131113/DAA1VSC01.html |date=13 November 2013 |work=AP News }}</ref><ref name="AST-20131108">{{cite journal | last1=Noffke | first1=Nora | last2=Christian | first2=Daniel | last3=Wacey | first3=David | last4=Hazen | first4=Robert M. | title=Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia | url=http://online.liebertpub.com/doi/abs/10.1089/ast.2013.1030 | date=8 November 2013 | journal=[[Astrobiology (journal)]] | volume=13 | issue=12 | pages=1103–24 | doi=10.1089/ast.2013.1030 | bibcode=2013AsBio..13.1103N | pmid=24205812 | pmc=3870916}}</ref>。不过[[后期重轰炸期|很多研究]]推测地球在更早之前就已有生命<ref name="AB-20021014">{{cite web |last=Tenenbaum |first=David |title=When Did Life on Earth Begin? Ask a Rock |url=http://www.astrobio.net/exclusive/293/when-did-life-on-earth-begin-ask-a-rock |date=October 14, 2002 |work=Astrobiology Magazine |accessdate=April 13, 2014 }}</ref>。根据其中一种研究，可能在42.5亿年前就已有生命<ref name="NS-20080702">{{cite web |last=Courtland |first=Rachel |title=Did newborn Earth harbour life? |url=http://www.newscientist.com/article/dn14245-did-newborn-earth-harbour-life.html |date=July 2, 2008 |work=[[New Scientist]] |accessdate=April 13, 2014 }}</ref>，另一个研究则认为是44亿年前<ref name="RN-20090520">{{cite web |last=Steenhuysen |first=Julie |title=Study turns back clock on origins of life on Earth |url=http://www.reuters.com/article/2009/05/20/us-asteroids-idUSTRE54J5PX20090520 |date=May 20, 2009 |work=Reuters |accessdate=April 13, 2014 }}</ref>。目前仍不确定地球上产生生命的机制，不过已有许多的[[生命起源]]假说。生命形成后，变成许多不同的形式，生物学家则将其[[生物分类法|分类]]成许多[[分类单元]]的体系。生命可以在许多不同的条件下生存。