View source for Common descent

{{Short description|Characteristic of a group of organisms with a common ancestor}}
{{For|use of the term in linguistics and philology|Comparative method|Historical linguistics|Proto-language|Textual criticism}}
{{Redirect|Common ancestor|use of the term in graph theory|Lowest common ancestor}}

{{Evolutionary biology|Key topics}}

'''Common descent''' is a concept in [[evolutionary biology]] applicable when one species is the ancestor of two or more species later in time. According to modern evolutionary biology, all living beings could be descendants of a unique ancestor commonly referred to as the [[last universal common ancestor]] (LUCA) of all [[life]] on [[Earth]].<ref>{{Cite journal|last1=Weiss|first1=Madeline C.|last2=Sousa|first2=Filipa L.|last3=Mrnjavac|first3=Natalia|last4=Neukirchen|first4=Sinje|last5=Roettger|first5=Mayo|last6=Nelson-Sathi|first6=Shijulal|last7=Martin|first7=William F.|date=2016-07-25|title=The physiology and habitat of the last universal common ancestor|url=https://www.nature.com/articles/nmicrobiol2016116|journal=Nature Microbiology|language=en|volume=1|issue=9|page=16116|doi=10.1038/nmicrobiol.2016.116|pmid=27562259|s2cid=2997255|issn=2058-5276}}</ref><ref>{{Cite journal|last1=Forterre|first1=Patrick|last2=Gribaldo|first2=Simonetta|last3=Brochier|first3=Céline|date=October 2005|title=[Luca: the last universal common ancestor]|journal=Médecine/Sciences|volume=21|issue=10|pages=860–865|doi=10.1051/medsci/20052110860|issn=0767-0974|pmid=16197904|doi-access=free}}</ref><ref name="theobald">{{cite journal |last=Theobald |first=Douglas L. |date=13 May 2010 |title=A formal test of the theory of universal common ancestry |journal=[[Nature (journal)|Nature]] |volume=465 |issue=7295 |pages=219–222 |doi=10.1038/nature09014 |pmid=20463738|bibcode=2010Natur.465..219T |s2cid=4422345 }}</ref><ref name="steel">{{cite journal |last1=Steel |first1=Mike |author1-link=Mike Steel (mathematician) |last2=Penny |first2=David |date=13 May 2010 |title=Origins of life: Common ancestry put to the test |journal=Nature |volume=465 |issue=7295 |pages=168–169 |doi=10.1038/465168a |pmid=20463725|bibcode=2010Natur.465..168S |s2cid=205055573 |doi-access=free }}</ref>

Common descent is an effect of [[speciation]], in which multiple species derive from a single ancestral population. The more recent the ancestral population two species have in common, the more closely are they related. The most recent common ancestor of all currently living organisms is the last universal ancestor,<ref name="theobald" /> which lived about [[Timeline of the evolutionary history of life|3.9 billion years ago]].<ref>{{cite journal |last=Doolittle |first=W. Ford |author-link=Ford Doolittle |date=February 2000 |title=Uprooting the Tree of Life |url=http://shiva.msu.montana.edu/courses/mb437_537_2004_fall/docs/uprooting.pdf |journal=[[Scientific American]] |volume=282 |issue=2 |pages=90–95 |doi=10.1038/scientificamerican0200-90 |pmid=10710791 |archive-url=https://web.archive.org/web/20060907081933/http://shiva.msu.montana.edu/courses/mb437_537_2004_fall/docs/uprooting.pdf |archive-date=2006-09-07 |access-date=2015-11-22|bibcode=2000SciAm.282b..90D }}</ref><ref>{{cite journal |last1=Glansdorff |first1=Nicolas |author2=Ying Xu |last3=Labedan |first3=Bernard |date=9 July 2008 |title=The Last Universal Common Ancestor: emergence, constitution and genetic legacy of an elusive forerunner |journal=[[Biology Direct]]  |volume=3 |page=29 |doi=10.1186/1745-6150-3-29  |pmc=2478661 |pmid=18613974 |doi-access=free }}</ref> The two earliest pieces of evidence for life on Earth are [[graphite]] found to be [[Biogenic substance|biogenic]] in 3.7 billion-year-old [[metasedimentary rock]]s discovered in western [[Greenland]]<ref name="NG-20131208">{{cite journal |last1=Ohtomo |first1=Yoko |last2=Kakegawa |first2=Takeshi |last3=Ishida |first3=Akizumi |last4=Nagase |first4=Toshiro |last5=Rosing |first5=Minik T. |display-authors=3 |date=January 2014 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=[[Nature Geoscience]] |volume=7 |issue=1 |pages=25–28 |bibcode=2014NatGe...7...25O |doi=10.1038/ngeo2025}}</ref> and [[microbial mat]] [[fossil]]s found in 3.48 billion-year-old [[sandstone]] discovered in [[Western Australia]].<ref name="AP-20131113">{{cite news |last=Borenstein |first=Seth |date=13 November 2013 |title=Oldest fossil found: Meet your microbial mom |url=https://www.yahoo.com/news/oldest-fossil-found-meet-microbial-mom-223338964.html |publisher=[[Yahoo News]] |agency=[[Associated Press]] |access-date=2015-11-22 |archive-date=2019-11-10 |archive-url=https://web.archive.org/web/20191110152138/https://www.yahoo.com/news/oldest-fossil-found-meet-microbial-mom-223338964.html |url-status=dead }}</ref><ref name="AST-20131108">{{cite journal |last1=Noffke |first1=Nora |author1-link=Nora Noffke |last2=Christian |first2=Daniel |last3=Wacey |first3=David |last4=Hazen |first4=Robert M. |author-link4=Robert Hazen |date=16 December 2013 |title=Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ''ca''. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia |journal=[[Astrobiology (journal)|Astrobiology]] |volume=13 |issue=12 |pages=1103–1124 |doi=10.1089/ast.2013.1030 |pmc=3870916 |pmid=24205812|bibcode=2013AsBio..13.1103N }}</ref> All currently living organisms on Earth share a common [[Genetics|genetic]] heritage, though the suggestion of substantial [[horizontal gene transfer]] during early evolution has led to questions about the [[monophyly]] (single ancestry) of life.<ref name="theobald" /> 6,331 groups of [[gene]]s common to all living animals have been identified; these may have arisen from a single [[Animal#Phylogeny|common ancestor]] that lived [[Cryogenian|650 million years ago]] in the [[Precambrian]].<ref name="NYT-20180504" /><ref name="NC-20150430" />

Universal common descent through an [[evolution]]ary process was first proposed by the British [[Natural history|naturalist]] [[Charles Darwin]] in the concluding sentence of his 1859 book ''[[On the Origin of Species]]'':

{{Blockquote|There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.<ref name="origin_490">{{harvnb|Darwin|1859|p=490}}</ref>}}

==History==
{{See also|History of evolutionary thought}}

The idea that all living things (including things considered non-living by science) are related is a recurring theme in many indigenous worldviews across the world.<ref>{{Cite news|last=Staff|first=I. C. T.|title=We Are All Related: Indigenous Knowledge Reaffirmed by Digitized Tree of Life|url=https://indiancountrytoday.com/archive/we-are-all-related-indigenous-knowledge-reaffirmed-by-digitized-tree-of-life|access-date=2021-05-05|newspaper=Ict News|date=13 September 2018 |language=en}}</ref>  Later on, in the 1740s, the French [[mathematician]] [[Pierre Louis Maupertuis]] arrived at the idea that all organisms had a common ancestor, and had diverged through random variation and [[natural selection]].<ref>{{harvnb |Crombie |Hoskin |1970 |pp=[https://books.google.com/books?id=OOgzAAAAIAAJ&dq=Maupertuis+%22for+the+first+time%22&pg=PA62 62–63]}}</ref><ref>{{harvnb |Treasure |1985 |p=142}}</ref>

In 1790, the philosopher [[Immanuel Kant]] wrote in ''Kritik der Urteilskraft'' (''[[Critique of Judgment]]'') that the similarity{{efn|Now called [[Homology (biology)|homology]].}} of animal forms implies a common original type, and thus a common parent.<ref>{{harvnb |Kant |1987 |p=[https://books.google.com/books?id=y5wQD-ioaUUC&dq=Critique+of+Judgment+%22original+mother%22&pg=PA304 304]}}: "Despite all the variety among these forms, they seem to have been produced according to a common archetype, and this analogy among them reinforces our suspicion that they are actually akin, produced by a common original mother."</ref>

In 1794, Charles Darwin's grandfather, [[Erasmus Darwin]] asked:
<blockquote>[W]ould it be too bold to imagine, that in the great length of time, since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind, would it be too bold to imagine, that all warm-blooded animals have arisen from one living filament, which {{Smallcaps |the great First Cause}} endued with animality, with the power of acquiring new parts attended with new propensities, directed by irritations, sensations, volitions, and associations; and thus possessing the faculty of continuing to improve by its own inherent activity, and of delivering down those improvements by generation to its posterity, world without end?<ref>{{harvnb |Darwin |1818 |p=397 [§&nbsp;39.4.8]}}</ref></blockquote>

[[Charles Darwin]]'s views about common descent, as expressed in ''[[On the Origin of Species]]'', were that it was probable that there was only one progenitor for all life forms:

<blockquote>Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.<ref>{{harvnb |Darwin |1859 |p=484}}</ref></blockquote>

But he precedes that remark by, "Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype. But analogy may be a deceitful guide." And in the subsequent edition,<ref>Darwin, C. R. 1860. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. 2nd edition, second issue, page 466</ref> he asserts rather, <blockquote>"We do not know all the possible transitional gradations between the simplest and the most perfect organs; it cannot be pretended that we know all the varied means of Distribution during the long lapse of years, or that we know how imperfect the Geological Record is. Grave as these several difficulties are, in my judgment they do not overthrow the theory of descent from a few created forms with subsequent modification". </blockquote>

Common descent was widely accepted amongst the [[scientific community]] after Darwin's publication.<ref>Krogh, David. (2005). ''Biology: A Guide to the Natural World''. Pearson/Prentice Hall. p. 323. {{ISBN|978-0321946768}} "Descent with modification was accepted by most scientists not long after publication of Darwin's ''On the Origin of Species by Means of Natural Selection'' in 1859. Scientists accepted it because it explained so many facets of the living world."</ref> In 1907, [[Vernon Lyman Kellogg|Vernon Kellogg]] commented that "practically no naturalists of position and recognized attainment doubt the theory of descent."<ref>[[Vernon Lyman Kellogg|Kellogg, Vernon L]]. (1907). [https://archive.org/stream/darwinismtodaydi00kell#page/3/mode/2up ''Darwinism To-Day'']. Henry Holt and Company. p. 3</ref>

In 2008, biologist [[T. Ryan Gregory]] noted that:

<blockquote>No reliable observation has ever been found to contradict the general notion of common descent. It should come as no surprise, then, that the scientific community at large has accepted evolutionary descent as a historical reality since Darwin's time and considers it among the most reliably established and fundamentally important facts in all of science.<ref>{{Cite journal | doi=10.1007/s12052-007-0001-z|title = Evolution as Fact, Theory, and Path| journal=Evolution: Education and Outreach| volume=1| pages=46–52|year = 2008|last1 = Gregory|first1 = T. Ryan|doi-access=free}}</ref></blockquote>

==Evidence==
{{further|Evidence of common descent}}

===Common biochemistry===

All known forms of life are based on the same fundamental biochemical organization: genetic information encoded in [[DNA]], transcribed into [[RNA]], through the effect of [[protein]]- and RNA-[[enzyme]]s, then translated into proteins by (highly similar) [[ribosome]]s, with [[Adenosine triphosphate|ATP]], [[Nicotinamide adenine dinucleotide phosphate|NADPH]] and others as energy sources. Analysis of small sequence differences in widely shared substances such as [[cytochrome c]] further supports universal common descent.<ref name=Knight>{{cite journal |last1=Knight |first1=Robin |last2=Freeland |first2=Stephen J. |last3=Landweber |first3=Laura F. |date=January 2001 |title=Rewiring the keyboard: evolvability of the genetic code |journal=[[Nature Reviews Genetics]] |volume=2 |issue=1 |pages=49–58 |doi=10.1038/35047500 |pmid=11253070 |s2cid=12267003 }}</ref> Some 23 proteins are found in all organisms, serving as [[enzyme]]s carrying out core functions like DNA replication. The fact that only one such set of enzymes exists is convincing evidence of a single ancestry.<ref name="theobald" /><ref name="Than2010">{{cite magazine |last1=Than|first1=Ker |title=All Species Evolved From Single Cell, Study Finds |url=https://news.nationalgeographic.com/news/2010/05/100513-science-evolution-darwin-single-ancestor/ |archive-url=https://web.archive.org/web/20100515123853/http://news.nationalgeographic.com/news/2010/05/100513-science-evolution-darwin-single-ancestor |url-status=dead |archive-date=May 15, 2010 |magazine=National Geographic |access-date=22 November 2017 |date=14 May 2010}}</ref> 6,331 [[gene]]s common to all living animals have been identified; these may have arisen from a single [[Animal#Phylogeny|common ancestor]] that lived [[Cryogenian|650 million years ago]] in the [[Precambrian]].<ref name="NYT-20180504">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=The Very First Animal Appeared Amid an Explosion of DNA |url=https://www.nytimes.com/2018/05/04/science/first-animal-genes-evolution.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2018/05/04/science/first-animal-genes-evolution.html |archive-date=2022-01-01 |url-access=limited |date=4 May 2018 |work=[[The New York Times]] |access-date=4 May 2018 }}{{cbignore}}</ref><ref name="NC-20150430">{{cite journal |last1=Paps |first1=Jordi |last2=Holland |first2=Peter W. H. |title=Reconstruction of the ancestral metazoan genome reveals an increase in genomic novelty |date=30 April 2018 |journal=[[Nature Communications]] |volume=9 |pages=1730 |number=1730 (2018) |doi=10.1038/s41467-018-04136-5 |pmid=29712911 |pmc=5928047 |bibcode=2018NatCo...9.1730P }}</ref>

===Common genetic code===
{{further|Genetic code}}
{| class="wikitable floatright" style="border: none; background: none; text-align: center;"
| '''[[Amino acid]]s'''
| style="background-color:#ffe75f; width: 50px;" | nonpolar
| style="background-color:#b3dec0; width: 50px;" | polar
| style="background-color:#bbbfe0; width: 50px;" | basic
| style="background-color:#f8b7d3; width: 50px;" | acidic
| style="border: none; background: none; width: 75px;" |
| style="background-color:#B0B0B0;" | Stop codon
|}

{| class="wikitable floatright"
|+ Standard genetic code
!rowspan=2| 1st<br />base
!colspan=8| 2nd base
|-
!colspan=2| {{{T|T}}}
!colspan=2| C
!colspan=2| A
!colspan=2| G
|-

!rowspan=4| {{{T|T}}}
| {{{T|T}}}{{{T|T}}}{{{T|T}}}
|rowspan=2 style="background-color:#ffe75f" | [[Phenylalanine|Phenyl-<br>alanine]]
| {{{T|T}}}C{{{T|T}}}
|rowspan=4 style="background-color:#b3dec0" | [[Serine]]
| {{{T|T}}}A{{{T|T}}}
|rowspan=2 style="background-color:#b3dec0" | [[Tyrosine]]
| {{{T|T}}}G{{{T|T}}}
|rowspan=2 style="background-color:#b3dec0" | [[Cysteine]]

|-
| {{{T|T}}}{{{T|T}}}C
| {{{T|T}}}CC
| {{{T|T}}}AC
| {{{T|T}}}GC

|-
| {{{T|T}}}{{{T|T}}}A
|rowspan=6 style="background-color:#ffe75f" | [[Leucine]]
| {{{T|T}}}CA
| {{{T|T}}}AA
| style="background-color:#B0B0B0;" | [[Stop codon|Stop]] <!--(''Ochre'')-->
| {{{T|T}}}GA
| style="background-color:#B0B0B0;" | [[Stop codon|Stop]] <!--(''Opal'')-->

|-
| {{{T|T}}}{{{T|T}}}G
| {{{T|T}}}CG
| {{{T|T}}}AG
| style="background-color:#B0B0B0;" | [[Stop codon|Stop]] <!--(''Amber'')-->
| {{{T|T}}}GG
| style="background-color:#ffe75f;" | [[Tryptophan]]&nbsp;<!-- to make the columns roughly the same width -->

|-

! rowspan="4" | C
| C{{{T|T}}}{{{T|T}}}
| CC{{{T|T}}}
|rowspan=4 style="background-color:#ffe75f" | [[Proline]]
| CA{{{T|T}}}
|rowspan=2 style="background-color:#bbbfe0" | [[Histidine]]
|CG{{{T|T}}}
|rowspan=4 style="background-color:#bbbfe0" | [[Arginine]]

|-
|C{{{T|T}}}C
|CCC
|CAC
|CGC

|-
|C{{{T|T}}}A
|CCA
|CAA
|rowspan=2 style="background-color:#b3dec0" | [[Glutamine]]
|CGA

|-
|C{{{T|T}}}G
|CCG
|CAG
|CGG

|-

! rowspan="4" | A
|A{{{T|T}}}{{{T|T}}}
|rowspan=3 style="background-color:#ffe75f" | [[Isoleucine]]
|AC{{{T|T}}}
|rowspan=4 style="background-color:#b3dec0" | [[Threonine]]&nbsp;<!-- to make the columns roughly the same width -->
|AA{{{T|T}}}
|rowspan=2 style="background-color:#b3dec0" | [[Asparagine]]
|AG{{{T|T}}}
|rowspan=2 style="background-color:#b3dec0" | [[Serine]]

|-
|A{{{T|T}}}C
|ACC
|AAC
|AGC

|-
|A{{{T|T}}}A
|ACA
|AAA
|rowspan=2 style="background-color:#bbbfe0" | [[Lysine]]
|AGA
|rowspan=2 style="background-color:#bbbfe0" | [[Arginine]]

|-
|A{{{T|T}}}G
| style="background-color:#ffe75f;" | [[Methionine]]
|ACG
|AAG
|AGG

|-

! rowspan="4" | G
|G{{{T|T}}}{{{T|T}}}
|rowspan=4 style="background-color:#ffe75f" | [[Valine]]
|GC{{{T|T}}}
|rowspan=4 style="background-color:#ffe75f" | [[Alanine]]
|GA{{{T|T}}}
|rowspan=2 style="background-color:#f8b7d3" | [[Aspartic acid|Aspartic<br>acid]]
|GG{{{T|T}}}
|rowspan=4 style="background-color:#ffe75f" | [[Glycine]]

|-
|G{{{T|T}}}C
|GCC
|GAC
|GGC

|-
|G{{{T|T}}}A
|GCA
|GAA
|rowspan=2 style="background-color:#f8b7d3" | [[Glutamic acid|Glutamic<br>acid]]
|GGA

|-
|G{{{T|T}}}G
|GCG
|GAG
|GGG

|-
|}

The [[genetic code]] (the "translation table" according to which DNA information is translated into [[amino acid]]s, and hence proteins) is nearly identical for all known lifeforms, from [[bacteria]] and [[archaea]] to [[animal]]s and [[plant]]s. The universality of this code is generally regarded by biologists as definitive evidence in favor of universal common descent.<ref name=Knight/>

The way that [[Genetic code|codon]]s (DNA triplets) are mapped to [[amino acid]]s seems to be strongly optimised. Richard Egel argues that in particular the [[hydrophobic]] (non-polar) side-chains are well organised, suggesting that these enabled the earliest organisms to create [[peptide]]s with water-repelling regions able to support the essential electron exchange ([[redox]]) reactions for energy transfer.<ref name=Egel>{{cite journal |last1=Egel |first1=Richard |title=Primal Eukaryogenesis: On the Communal Nature of Precellular States, Ancestral to Modern Life |journal=Life |date=March 2012 |volume=2 |issue=1 |pages=170–212 |doi=10.3390/life2010170 |pmc=4187143 |pmid=25382122|bibcode=2012Life....2..170E |doi-access=free }}</ref>

===Selectively neutral similarities===

Similarities which have no adaptive relevance cannot be explained by [[convergent evolution]], and therefore they provide compelling support for universal common descent. Such evidence has come from two areas: [[amino acid]] sequences and DNA sequences. Proteins with the same three-dimensional structure need not have identical amino acid sequences; any irrelevant similarity between the sequences is evidence for common descent. In certain cases, there are several [[Genetic code|codon]]s (DNA triplets) that code redundantly for the same amino acid. Since many species use the same codon at the same place to specify an amino acid that can be represented by more than one codon, that is evidence for their sharing a recent common ancestor. Had the amino acid sequences come from different ancestors, they would have been coded for by any of the redundant codons, and since the correct amino acids would already have been in place, [[natural selection]] would not have driven any change in the codons, however much time was available. [[Genetic drift]] could change the codons, but it would be extremely unlikely to make all the redundant codons in a whole sequence match exactly across multiple lineages. Similarly, shared nucleotide sequences, especially where these are apparently neutral such as the positioning of [[intron]]s and [[pseudogene]]s, provide strong evidence of common ancestry.<ref name="Sharma2005">{{cite book |last=Sharma |first=N. S. |title=Continuity And Evolution Of Animals |url=https://books.google.com/books?id=x7Q47fvCfLIC&pg=PA32 |year=2005 |publisher=Mittal Publications |isbn=978-81-8293-018-6 |pages=32–}}</ref>

===Other similarities===

Biologists often{{quantify|date=March 2018}} point to the universality of many aspects of cellular life as supportive evidence to the more compelling evidence listed above. These similarities include the energy carrier [[adenosine triphosphate]] (ATP), and the fact that all amino acids found in proteins are [[Chirality (chemistry)#In biochemistry|left-handed]]. It is, however, possible that these similarities resulted because of the [[Laws of science|laws of physics and chemistry]] - rather than through universal common descent - and therefore resulted in convergent evolution. In contrast, there is evidence for homology of the central subunits of [[ATPase#Transmembrane ATP synthases|transmembrane ATPases]] throughout all living organisms, especially how the rotating elements are bound to the membrane. This supports the assumption of a LUCA as a cellular organism, although primordial membranes may have been semipermeable and evolved later to the membranes of modern bacteria, and on a second path to those of modern archaea also.<ref>
{{cite book
 |last= Lane |first= Nick |author-link= Nick Lane
 |year= 2015
 |title= The Vital Question: Why Is Life The Way It Is?
 |publisher= Profile Books |isbn= 978-1781250365
 |title-link= The Vital Question
 |page= <!--part 77 of online edition-->
}}
</ref>

===Phylogenetic trees===
{{PhylomapB|size=300px|caption=A [[phylogenetic tree]] based on [[ribosomal RNA]] genes implies a single origin for all life.}}
{{Main |Phylogenetic tree}}
{{See also |Tree of life (biology)}}

Another important piece of evidence is from detailed phylogenetic trees (i.e., "genealogic trees" of species) mapping out the proposed divisions and common ancestors of all living species. In 2010, Douglas L. Theobald published a statistical analysis of available genetic data,<ref name="theobald" /> mapping them to phylogenetic trees, that gave "strong quantitative support, by a formal test, for the unity of life."<ref name="steel" />

Traditionally, these trees have been built using morphological methods, such as appearance, [[embryology]], etc. Recently, it has been possible to construct these trees using molecular data, based on similarities and differences between genetic and protein sequences. All these methods produce essentially similar results, even though most [[genetic variation]] has no influence over external morphology. That phylogenetic trees based on different types of information agree with each other is strong evidence of a real underlying common descent.<ref>{{cite web |url=http://www.talkorigins.org/faqs/comdesc/section1.html#independent_convergence |title=Prediction 1.3: Consilience of independent phylogenies |last=Theobald |first=Douglas L. |work=29+ Evidences for Macroevolution: The Scientific Case for Common Descent |version=Version 2.89 |publisher=[[TalkOrigins Archive|The TalkOrigins Foundation]] |access-date=2009-11-20}}</ref>

==Objections==

[[File:Tree Of Life (with horizontal gene transfer).svg|thumb|2005 [[tree of life (biology)|tree of life]] shows many [[horizontal gene transfer]]s, implying multiple possible origins.]]

===Gene exchange clouds phylogenetic analysis===
{{further|Horizontal gene transfer}}

Theobald noted that substantial [[horizontal gene transfer]] could have occurred during early evolution. Bacteria today remain capable of gene exchange between distantly-related lineages. This weakens the basic assumption of phylogenetic analysis, that similarity of genomes implies common ancestry, because sufficient gene exchange would allow lineages to share much of their genome whether or not they [[monophyly|shared an ancestor (monophyly)]]. This has led to questions about the single ancestry of life.<ref name="theobald" /> However, biologists consider it very unlikely that completely unrelated proto-organisms could have exchanged genes, as their different coding mechanisms would have resulted only in garble rather than functioning systems. Later, however, many organisms all derived from a single ancestor could readily have shared genes that all worked in the same way, and it appears that they have.<ref name="theobald" />

===Convergent evolution===
{{further|Convergent evolution}}

If early organisms had been driven by the same environmental conditions to [[Convergent evolution|evolve similar biochemistry convergently]], they might independently have acquired similar genetic sequences. Theobald's "formal test" was accordingly criticised by Takahiro Yonezawa and colleagues<ref>{{cite journal |last1=Yonezawa |first1=Takahiro |last2=Hasegawa |first2=Masami |title=Was the universal common ancestry proved? |date=16 December 2010 |journal=Nature |volume=468 |issue=7326 |page=E9 |doi=10.1038/nature09482 |pmid=21164432|bibcode=2010Natur.468E...9Y |s2cid=4318346 |doi-access=free }}</ref> for not including consideration of convergence. They argued that Theobald's test was insufficient to distinguish between the competing hypotheses. Theobald has defended his method against this claim, arguing that his tests distinguish between phylogenetic structure and mere sequence similarity. Therefore, Theobald argued, his results show that "real universally [[Conserved sequence|conserved]] proteins are [[sequence homology|homologous]]."<ref>{{cite journal |last=Theobald |first=Douglas L. |date=16 December 2010 |title=Theobald reply |journal=Nature |volume=468 |issue=7326 |page=E10 |doi=10.1038/nature09483|bibcode=2010Natur.468E..10T |s2cid=4317014 |doi-access=free }}</ref><ref>{{cite journal |last=Theobald |first=Douglas L. |date=24 November 2011 |title=On universal common ancestry, sequence similarity, and phylogenetic structure: The sins of P-values and the virtues of Bayesian evidence |journal=Biology Direct |volume=6 |issue=1 |page=60 |doi=10.1186/1745-6150-6-60 |pmc=3314578 |pmid=22114984 |doi-access=free }}</ref>

===RNA world===
{{main|RNA world}}
The possibility is mentioned, above, that all living organisms may be descended from an original single-celled organism with a [[DNA]] [[genome]], and that this implies a single origin for life.  Although such a universal common ancestor may have existed, such a complex entity is unlikely to have arisen spontaneously from non-life and thus a cell with a DNA genome cannot reasonably be regarded as the origin of life.  To understand the origin of life, it has been proposed that DNA based cellular life descended from relatively simple pre-cellular self-replicating [[RNA]] molecules able to undergo [[natural selection]].   During the course of evolution, this RNA world was replaced by the evolutionary emergence of the DNA world. A world of independently self-replicating RNA genomes apparently no longer exists (RNA viruses are dependent on host cells with DNA genomes).  Because the RNA world is apparently gone, it is not clear how scientific evidence could be brought to bear on the question of whether there was a single origin of life event from which all life descended.

==See also==

* ''[[The Ancestor's Tale]]''
* [[Urmetazoan]]

==Bibliography==
{{Refbegin}}
* {{cite book |last1=Crombie |first1=A. C. |author-link1=Alistair Cameron Crombie |last2=Hoskin |first2=Michael |author-link2=:sk:Michael Hoskin |year=1970 |chapter=The Scientific Movement and the Diffusion of Scientific Ideas, 1688–1751 |editor-last=Bromley |editor-first=J. S. |title=The Rise of Great Britain and Russia, 1688–1715/25 |series=[[The Cambridge Modern History#The New Cambridge Modern History, 1957–1979|The New Cambridge Modern History]] |volume=6 |location=London |publisher=[[Cambridge University Press]] |isbn=978-0-521-07524-4 |lccn=57014935 |oclc=7588392 }}
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* {{cite book |last=Darwin |first=Erasmus |author-link=Erasmus Darwin |year=1818 |orig-year=Originally published 1794 |title=Zoonomia; or the Laws of Organic Life |volume=1 |edition=4th American |location=Philadelphia, PA |publisher=Edward Earle |title-link=Zoonomia }} {{Internet Archive |id=2551016RX1.nlm.nih.gov |name=Zoonomia; or The laws of organic life: in three parts (Volume 1) (1818)}} Retrieved 2015-11-23.
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* {{cite book |last=Kant |first=Immanuel |author-link=Immanuel Kant |year=1987 |orig-year=Originally published 1790 in Prussia as ''Kritik der Urteilskraft'' |title=Critique of Judgment |others=Translated, with an introduction, by Werner S. Pluhar; foreword by [[Mary J. Gregor]] |location=Indianapolis, IN |publisher=[[Hackett Publishing Company]] |isbn=978-0-87220-025-8 |lccn=86014852 |oclc=13796153 |title-link=Critique of Judgment }}
* {{cite book |last=Treasure |first=Geoffrey |year=1985 |title=The Making of Modern Europe, 1648-1780 |url=https://archive.org/details/makingofmoderneu00grrt |url-access=registration |location=New York |publisher=[[Methuen Publishing|Methuen]] |isbn=978-0-416-72370-0 |lccn=85000255 |oclc=11623262 }}
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{{Refend}}

==Notes==
{{notelist}}

==References==
{{reflist}}

==External links==
* [http://www.talkorigins.org/faqs/comdesc/ 29+ Evidences for Macroevolution: The Scientific Case for Common Descent] from the [[TalkOrigins Archive]].
* [http://tolweb.org/tree/phylogeny.html The Tree of Life Web Project]

{{Origin of life}}
{{Evolution}}
{{portal bar |Evolutionary biology}}

{{DEFAULTSORT:Common Descent}}
[[Category:Evolutionary biology]]
[[Category:Descent]]
[[Category:Most recent common ancestors]]