=== Moderne systematik ===
[[Fil:Phylogenetic Tree of Life.png|thumb|310px|Fylogenetisk opstilling af alle levende organismer.]]
Selv omkring år [[2000]] var der ikke enighed om, hvor mange [[rige (biologi)|biologiske riger]], liv skal indplaceres i.
<ref name="AnselmSurvey"/>
<ref>[http://sn2000.taxonomy.nl/ Brands, S.J. (comp.) 1989-2002. Systema Naturae 2000. Amsterdam, The Netherlands.] Citat: "...The classification of life is changing rapidly. As new techniques reveal more about the phylogenetic relationships between taxa, the tree changes shape almost every day....", [http://www.google.com/custom?sa=Google+Search&domains=taxonomy.nl&sitesearch=taxonomy.nl Google søgning i domænet: "taxonomy.nl"].</ref>
<ref>[http://www.mhhe.com/biosci/pae/zoology/animalphylogenetics/section03.mhtml III. Molecular Phylogenetic Trees as Alternative Hypotheses]</ref>
<ref>[http://www.sciencedaily.com/releases/2003/09/030918094208.htm ScienceDaily, 2003-09-19, Bacterial Relationships Revealed] Citat: "...constructing a bacterial family tree is indeed possible..."</ref> 
Grunden er, at man selv i dag opdager nye [[art]]er og nye egenskaber i arter.
<ref>[http://news.bbc.co.uk/1/hi/sci/tech/3003946.stm BBCNews, 19 June, 2003, Ancient organism challenges cell evolution] Citat: "..."It appears that this organelle has been conserved in evolution from prokaryotes to eukaryotes, since it is present in both,"...''Agrobacterium tumefaciens''..."</ref>
<ref>{{Cite web |url=http://www.spaceref.com/news/viewpr.html?pid=4742 |title=SpaceRef.com, July 29, 1997: Scientists Discover Methane Ice Worms On Gulf Of Mexico Sea Floor |access-date= 2. marts 2016 |archive-date= 3. april 2020 |archive-url=https://web.archive.org/web/20200403040654/http://www.spaceref.com/news/viewpr.html?pid=4742 |url-status=dead }}</ref>
<ref>[http://www.science.psu.edu/iceworms/iceworms.html The Eberly College of Science: Methane Ice Worms discovered on Gulf of Mexico Sea Floor] {{Webarchive|url=https://web.archive.org/web/20090601140743/http://science.psu.edu/iceworms/iceworms.html |date= 1. juni 2009 }} download Publication quality photos</ref>
<ref>[http://www.spaceref.com/news/viewnews.html?id=339 SpaceRef.com, May 04, 2001: Redefining "Life as We Know it"] {{Webarchive|url=https://web.archive.org/web/20200403040654/http://www.spaceref.com/news/viewnews.html?id=339 |date= 3. april 2020 }} ''Hesiocaeca methanicola'' In 1997, Charles Fisher, professor of biology at Penn State, discovered this remarkable creature living on mounds of methane ice under half a mile of ocean on the floor of the Gulf of Mexico.</ref>
<ref>[http://news.bbc.co.uk/1/hi/sci/tech/2585235.stm BBCNews, 18 December, 2002, 'Space bugs' grown in lab] Citat: "...''Bacillus simplex'' and ''Staphylococcus pasteuri''...''Engyodontium album''...The strains cultured by Dr Wainwright seemed to be resistant to the effects of UV – one quality required for survival in space...."</ref> 
Siden opdagelsen af [[DNA]] og brugen af [[kladogram]]mer, har man anvendt denne nye måde til at kunne sammenligne udvalgte dele af livs DNA ([[kladistisk systematik]]) og ren DNA systematik, der kaldes [[fylogenetisk systematik]]. Men på grund af DNA`s store informationsmængde er man nødt til at plukke udvalgte dele af DNA ud og benytte det til systematiseringen og sammenholde det med den gamle [[fænetisk systematik|fænetiske systematik]]. Denne systematik kaldes [[evolutionær systematik]]. Hvilke dele af DNA, der skal anvendes, overlades til den enkelte forskers eller forskergruppes vurdering, og da forskellige forskere kan have hver deres syn på, hvad der skal vælges, er det en kilde til [[debat]] og [[diskussion]].
<ref>[http://www.sciencedaily.com/releases/2009/03/090318153803.htm University of Calgary (2009, March 19). Is The Hippopotamus The Closest Living Relative To The Whale?. ScienceDaily. Retrieved March 20, 2009] Citat: "..."And the reason their tree is so different is simple: by excluding all the DNA information they left out all the data that shows a strong relationship between [[Hvaler|whales]] and [[Flodheste|hippos]]."..."
</ref> Det viser sig desuden, at især småt liv forholdsvis tit udveksler gener (nogle DNA-stumper f.eks. i form af [[plasmid]]er), hvilket gør klassificeringen endnu sværere og mere interessant.
<ref>[http://www.sciencedaily.com/releases/2011/04/110411163918.htm University of Gothenburg (2011, April 13). Antibiotic resistance spreads rapidly between bacteria. ScienceDaily]</ref>
<ref>[http://www.sciencedaily.com/releases/2005/07/050703232326.htm European Molecular Biology Laboratory (2005, July 5). Trees, Vines And Nets -- Microbial Evolution Changes Its Face. ScienceDaily] Citat: "...EBI researchers have changed our view of 4 billion years of [[mikrobe|microbial]] evolution...In all, more than 600,000 vertical transfers are observed, coupled with 90,000 [[gen]]e loss events and approximately 40,000 horizontal gene transfers...A few species, including beneficial nitrogen-fixing soil bacteria, appear to be 'champions'of horizontal gene transfer; "it's entirely possible that apparently harmless organisms are quietly spreading [[antibiotika|antibiotic]] resistance under our feet," concludes Christos Ouzounis..."</ref>
<ref>[http://www.sciencedaily.com/releases/2003/08/030808081854.htm 2003-08-08, ScienceDaily: Cross-species Mating May Be Evolutionarily Important And Lead To Rapid Change, Say Indiana University Researchers] Citat: "...the sudden mixing of closely related species may occasionally provide the energy to impel rapid evolutionary change..."</ref>
<ref name="LateralGene">[http://www.sciencedaily.com/releases/2005/11/051111103519.htm University of Queensland (2005, November 11). Lateral Thinking Produces First Map Of Gene Transmission. ScienceDaily] Citat: "...Their results clearly show genetic modification of organisms by lateral transfer is a widespread natural phenomenon, and it can occur even between distantly related organisms... it was assumed that transfer of genes could only be vertical, i.e. from parents to offspring..."</ref> 
Indtil for få årtier siden blev livs "spontane" [[Horisontal genoverførsel|genoverførsel]] anset for stort set umuligt.
<ref name="LateralGene" />
Filosofi Fysik Naturvidenskab