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.
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 they are related. The most recent common ancestor of all currently living organisms is the last universal ancestor, which lived about 3.9 billion years ago. The two earliest pieces of evidence for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in western Greenland and microbial mat found in 3.48 billion-year-old sandstone discovered in Western Australia. All currently living organisms on Earth share a common Genetics heritage, though the suggestion of substantial horizontal gene transfer during early evolution has led to questions about the monophyly (single ancestry) of life. 6,331 groups of common to all living animals have been identified; these may have arisen from a single common ancestor that lived Cryogenian in the Precambrian.
Universal common descent through an process was first proposed by the British Natural history Charles Darwin in the concluding sentence of his 1859 book On the Origin of Species:
In 1790, the philosopher Immanuel Kant wrote in Kritik der Urteilskraft ( Critique of Judgment) that the similarity of animal forms implies a common original type, and thus a common parent.: "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."
In 1794, Charles Darwin's grandfather, Erasmus Darwin asked:
Would 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 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?
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:
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.
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,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 he asserts rather,
"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".
Common descent was widely accepted amongst the scientific community after Darwin's publication.Krogh, David. (2005). Biology: A Guide to the Natural World. Pearson/Prentice Hall. p. 323. "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." In 1907, Vernon Kellogg commented that "practically no naturalists of position and recognized attainment doubt the theory of descent."Kellogg, Vernon L. (1907). Darwinism To-Day. Henry Holt and Company. p. 3
In 2008, biologist T. Ryan Gregory noted that:
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.
nonpolar | polar | basic | acidic | Stop codon |
+ Standard genetic code !rowspan=2 | 1st base !colspan=8 | 2nd base | ||||||
!rowspan=4 | } | Phenylalanine | }C} | Serine | }A} | Tyrosine | }G} | Cysteine |
}}C | }CC | }AC | }GC | |||||
}}A | Leucine | }CA | }AA | Stop codon | }GA | Stop codon | ||
}}G | }CG | }AG | Stop codon | }GG | Tryptophan | |||
! rowspan="4" | C | CC} | Proline | CA} | Histidine | CG} | Arginine | |
C}C | CCC | CAC | CGC | |||||
C}A | CCA | CAA | Glutamine | CGA | ||||
C}G | CCG | CAG | CGG | |||||
! rowspan="4" | A | Isoleucine | AC} | Threonine | AA} | Asparagine | AG} | Serine |
A}C | ACC | AAC | AGC | |||||
A}A | ACA | AAA | Lysine | AGA | Arginine | |||
A}G | Methionine | ACG | AAG | AGG | ||||
! rowspan="4" | G | Valine | GC} | Alanine | GA} | Aspartic acid | GG} | Glycine |
G}C | GCC | GAC | GGC | |||||
G}A | GCA | GAA | Glutamic acid | GGA | ||||
G}G | GCG | GAG | GGG | |||||
The genetic code (the "translation table" according to which DNA information is translated into , and hence proteins) is nearly identical for all known lifeforms, from bacteria and archaea to and . The universality of this code is generally regarded by biologists as definitive evidence in favor of universal common descent.
The way that Genetic code (DNA triplets) are mapped to 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 with water-repelling regions able to support the essential electron exchange (redox) reactions for energy transfer.
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.
Selectively neutral similarities
Other similarities
Phylogenetic trees
Objections
Gene exchange clouds phylogenetic analysis
Convergent evolution
RNA world
See also
Bibliography
Notes
External links
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