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var newEl = new Element('div', {'class': 'element'}).setHTML('<p>A group of organisms is said to have common descent if they have a common ancestor. In biology, the theory of universal common descent proposes that all organisms on Earth are descended from a common ancestor or ancestral gene pool.</p><p>A theory of universal common descent based on evolutionary principles was proposed by Charles Darwin in his book The Origin of Species (1859), and later in 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 most recent common ancestor of all currently living organisms, is believed to have appeared about 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</p>');
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<h3>
accordeon
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History
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<p>
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
Venus physique. Specifically:
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"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..."
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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".
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Evidence of universal common descent
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Common biochemistry and genetic code
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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.[2]
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Irrelevant differences
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Differences 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.
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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.
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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.
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Phylogenetic trees
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Another important piece of evidence is that it is possible to construct detailed
phylogenetic trees mapping out the proposed divisions and common ancestors of
species, and no matter what method is used, morphological (based on appearance,
embryology, etc) or molecular (based on mutation rates and relative similarities of
important, conserved genes), still get extremely similar results. If there were no
common ancestor, these different methods should give wildly different results, thus
the phylogenetic tree is strong evidence of common descent.
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Examples of common descent
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Artificial selection
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<p>
Artificial selection offers remarkable examples of the amount of diversity that can
exist between individuals sharing a late common ancestor. To perform artificial
selection, one begins with a particular species (following examples include wolves
and wild cabbage) and then, at every generation, only allow certain individuals to
reproduce, based on the degree to which they exhibit certain desirable
characteristics. In time, it is expected that these characteristics become
increasingly well-developed in successive generations. Many examples of artificial
selection, like the ones below, occurred without the guidance of modern scientific
insight.
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Dog breeding
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An obvious example of the power of artificial selection is the diversity found in
various breed in domesticated dogs. The various breeds of dogs all share common
ancestry (being all ultimately descended from wolves) but were domesticated by humans
and then selectively bred in order to enhance various features such as coat color and
length or body size. To see the wide range of difference between the many breeds of
dogs compare the Chihuahua, Great Dane, Basset Hound, Pug, and Poodle. Also compare
this enormous diversity with the relative uniformity of wild wolves.
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