Diz-se que um grupo de organismos tem '''origem comum''' se todos partilham um ancestral. Em biologia, a teoria da ''origem comum universal'' postula que todos os organismos na Terra descendem dum ancestral comum, ou dum pool de genes ancestral.

A primeira teoria da '''origem comum universal''' baseada em princípios evolutivos foi proposta por 3.5 mil milhões de anos (ver: origem da vida).

== História ==

A primeira sugestão de que todos os organismos podem ter tido um ancestral comum e divergido através de variação aleatória e selecção natural foi avançada em 1745 pelo matemático e cientista francês Pierre-Louis Moreau de Maupertuis (1698-1759) na sua obra ''Vénus physique'':

:"Não se poderia dizer, considerando as combinações fortuitas dos produtos da natureza, e a certeza de entre elas haver as que se caracterizam por uma certa condição de forma que lhes permite subsistir, que não é de admirar que esta boa forma esteja presente em todas as espécies que existem no presente? O acaso, poder-se-ia dizer, produziu uma multiplicidade inquantificável de indivíduos; uma pequena porção destes resultou de tal forma que as partes do animal eram capazes de satisfazer as suas necessidades; numa outra porção, infinitamente maior, não havia nem ordem nem boa forma: destes, todos pereceram. Animais sem uma boca não podiam viver; outros sem órgãos reprodutivos não se podiam perpetuar... As espécies que vemos hoje são apenas a mais pequena parte do que o destino cego produziu..." 

Em animais implica um tipo original comum e, portanto, um ancestral comum.

Em 1795, Erasmus Darwin, o avô de Charles Darwin, levantou a hipótese de todos os animais de sangue quente descenderem dum único "filamento vivo":

:"...seria ousadia imaginar que todos os animais de sangue quente tenham surgido a partir dum filamento vivo, que '''a grande causa primeira''' dotou de animalidade...?" (''Zoonomia'', 1795, section 39, "Generation") 

Em 1859 foi publicada ''A Origem das Espécies'' de Charles Darwin. As opiniões aí expressas acerca da origem comum variam entre a sugestão duma única "primeira criatura" à concessão de poder ter havido mais do que uma, como se pode ver a partir dos seguintes extractos da ''Conclusão'':

:"Provavelmente todos os seres orgânicos que alguma vez viveram nesta Terra descenderam duma única forma primordial na qual a vida foi pela primeira vez instilada." 

:"Toda a História do Mundo, tal como a conhecemos no presente, [...] será de ora em diante reconhecida como um mero fragmento de tempo, quando comparada com as eras que passaram desde a criação da primeira criatura, o progenitor das inúmeras espécies descendentes extintas e existentes." 

:"Quando penso neles, não como criações especiais, mas como descendentes directos dum punhado de seres que viveu muito antes do primeiro estrato do sistema silúrico ter sido depositado, todos os seres me parecem mais nobres." 

A famosa frase de fecho descreve a "grandiosidade desta visão da vida, com todo o seu potencial, ter sido originalmente instilada em poucas ou numa única forma." A escolha de palavras não deixa de ser notável pela sua consistência com ideias recentes acerca da existência dum único pool de genes ancestral.

Recentemente, baseando-se na universalidade do código genético, cientistas como Francis Crick postularam que o ancestral comum universal pode ter tido origem extraterrestre, uma teoria conhecida como panspermia.

== Provas a favor da origem comum ==

A group of organisms is said to have '''common descent''' if they have a common lateral gene transfer amongst bacteria. For this and other reasons, the most recent common ancestor may have been a genetic pool rather than an organism.</ref>

A theory of '''universal common descent''' based on The Descent of Man'' (1871).
This theory is now generally accepted by biologists, and the last universal common ancestor (LUCA or LUA), that is, the 3.9 billion years ago. The theory of a common ancestor between all organisms is one of the principles of evolution, although for single cell organisms and viruses, single phylogeny is disputed (see: origin of life).

== History ==

The first suggestion that all organisms may have had a common ancestor and diverged through random variation and natural selection was made in 1745 by the French mathematician and scientist Pierre-Louis Moreau de Maupertuis (1698-1759) in his work ''Vénus physique''. Specifically:
:''"Could one not say that, in the fortuitous combinations of the productions of nature, as there must be some characterized by a certain relation of fitness which are able to subsist, it is not to be wondered at that this fitness is present in all the species that are currently in existence? Chance, one would say, produced an innumerable multitude of individuals; a small number found themselves constructed in such a manner that the parts of the animal were able to satisfy its needs; in another infinitely greater number, there was neither fitness nor order: all of these latter have perished. Animals lacking a mouth could not live; others lacking reproductive organs could not perpetuate themselves... The species we see today are but the smallest part of what blind destiny has produced..."''

In 1790, Immanuel Kant (Königsberg (Kaliningrad) 1724 - 1804), in his ''Kritik der Urtheilskraft'', states that the analogy of animal forms implies a common original type and thus a common parent.

Charles Darwin's grandfather, Erasmus Darwin, hypothesized in 1795 that all warm-blooded animals were descended from a single "living filament":
:''"...would it be too bold to imagine, that all warm-blooded animals have arisen from one living filament, which THE GREAT FIRST CAUSE endued with animality...?"'' (''Zoonomia'', 1795, section 39, "Generation")

In 1859, Charles Darwin's ''The Origin of Species'' was published. The views about common descent expressed therein vary between suggesting that there was a single "first creature" to allowing that there may have been more than one. Here are the relevant quotations from the ''Conclusion'':

:''"[P]robably all of the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."''

:''"The whole history of the world, as at present known, ... will hereafter be recognized as a mere fragment of time, compared with the ages which have elapsed since the first creature, the progenitor of innumerable extinct and living descendants, was created."''

:''"When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Silurian system was deposited, they seem to me to become ennobled."''

The famous closing sentence describes the "grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one." The phrase "one form" here seems to hark back to the phrase "some few beings"; in any case, the choice of words is remarkable for its consistency with recent ideas about there having been a single ancestral "genetic pool".

== Evidence of universal common descent ==

=== Common biochemistry and genetic code ===
All known forms of life are based on the same fundamental biochemical organisation: genetic information encoded in DNA, transcribed into RNA, through the effect of protein- and RNA-enzymes, then translated into proteins by (highly similar) ribosomes, with ATP, NADH and others as energy currencies, etc. Furthermore, the genetic code (the "translation table" according to which DNA information is translated into proteins) is nearly identical for all known lifeforms, from bacteria to humans, with minor local differences. The universality of this code is generally regarded by biologists as definitive evidence in favor of the theory of universal common descent. Analysis of the small differences in the genetic code has also provided support for universal common descent.<ref></ref>

==== Selectively neutral similarities ====
Similarities which have no relevance to evolution and therefore cannot be explained by convergence, tend to be very compelling support for the universal common descent theory.

Such evidence has come from two domains: amino acid sequences and DNA sequences. Proteins with the same 3-d 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 codons (DNA triplets) that code for the same amino acid. Thus, if two 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 recency of a common ancestor.

==== Other similarities ====
The universality of many aspects of cellular life is often pointed to as supportive evidence to the more compelling evidence listed above. These similarities include the energy carrier ATP, and the fact that all amino acids found in proteins are left-handed. It is possible that these similarities resulted because of the laws of physics and chemistry, rather than universal common descent and therefore resulted in convergent evolution.