Common ancestry between organisms of different species arises during speciation, in which new species are established from a single ancestral population. Organisms which share a more recent common ancestor are more closely related. The most recent common ancestor of all currently living organisms is the last universal ancestor,[1] which lived about 3.9 billion years ago.[3][4] The earliest evidences for life on Earth are graphite found to be biogenic in 3.7 billion-year-old met sedimentary rocks discovered in Western Greenland[5] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[6][7] All currently living organisms on Earth share a common genetic heritage (universal common descent), with each being the descendant from a single original species, though the suggestion of substantial horizontal gene transfer during early evolution has led to questions about monophyly of life.[1]
Universal common descent through an evolutionary process, that there was
only one progenitor for all life forms, was first proposed by Charles Darwin in On the Origin of
Species, which ended with "There is a grandeur in this view
of life, with its several powers, having been originally breathed into a few
forms or into one".[8] The theory has been
recently popularized by Richard Dawkins, in The Ancestor's Tale,
and others.
History
In the 1740s, Pierre-Louis
Moreau de Maupertuis made the first known suggestion in a series
of essays that all organisms may have had a common ancestor, and that they had
diverged through random variation and natural selection.[9][10] In Essai
de Cosmologie,
Maupertuis noted:
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 ...[11]
In 1790, Immanuel Kant wrote in Kritik
der Urtheilskraft (Critique of Judgement)
that the analogy of animal forms implies a common original type, and thus a
common parent.[12]
[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 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?[13]
In 1859, Charles Darwin's The
Origin of Species was published. The views about common descent
expressed therein were that it was possible 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." [8] (p 484)
Darwin's 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."[8] (p 490)
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,
NADPH and others as energy sources, etc. Furthermore, the genetic code (the "translation
table" according to which DNA information is translated into proteins) is
nearly identical for all known life forms,
from bacteria and archaea to animals and plants. 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. An
example would be Cytochrome c which most organisms
actually share. [14] A statistical comparison of
various alternative hypotheses has shown that universal common ancestry is
significantly more probable than models involving multiple origins.[1][15]
Selectively neutral similarities
Similarities which have no adaptive relevance
cannot be explained by convergent evolution, and therefore they provide
compelling support for the theory of 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 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 a recent 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 adenosine
triphosphate (ATP), and the fact that all amino acids
found in proteins are left-handed. It is, however, possible that these
similarities resulted because of the laws of physics and chemistry, rather than
universal common descent and therefore resulted in convergent evolution.
Phylogenetic trees
A phylogenetic tree based
on rRNA genes.
Another important piece of evidence is that
it is possible to construct detailed phylogenetic trees (that is, "genealogic
trees" of species) mapping out the proposed divisions and common ancestors
of all living species. In 2010 an analysis of available genetic data, mapping
them to phylogenetic trees, gave "firm quantitative support for the unity
of life. ...there is now strong quantitative support, by a formal test,[1] for the unity of life.[2] It should be noted,
however, the 'formal' test is criticised[16] for not including consideration
of convergent evolution,
and Theobald has defended the method against this claim.[17][18]
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.[19]
Illustrations of common descent
Artificial selection
Artificial selection demonstrates
the diversity that can exist among organisms that share a relatively recent
common ancestor. In artificial selection, one species is bred selectively at
each generation, allowing only those organisms that exhibit desired
characteristics to reproduce. These characteristics become increasingly
well-developed in successive generations. Artificial selection was successful
long before science discovered the genetic basis.
The Chihuahua mix and Great Dane both share a common ancestor, the wolf. but show the
power of artificial selection to create diversity of form in a relatively short
period of time.
Dog breeding
The diversity of domesticated dogs is
an example of the power of artificial selection. All breeds share common
ancestry, having descended from wolves. Humans selectively
bred them to enhance specific characteristics, such as color and length or body
size. This created a range of breeds that include the Chihuahua, Great Dane, Basset Hound, Pug,
and Poodle. Wild wolves, which did not undergo artificial
selection, are relatively uniform in comparison.
Wild cabbage
Wild Cabbage plant
Early farmers cultivated many popular
vegetables from the Brassica oleracea (wild cabbage) by
artificially selecting for certain attributes. Common vegetables such as cabbage, kale, broccoli, cauliflower, kohlrabi and Brussels sprouts are all descendants of the
wild cabbage plant.[20] Brussels sprouts were
created by artificially selecting for large bud size.
Broccoli was bred by selecting for large flower stalks. Cabbage was created
by selecting for short petioles. Kale was bred by selecting for large leaves.
Natural selection
Natural selection is the evolutionary process
by which heritable traits that increase an
individual's fitness become more common, and heritable traits that decrease an
individual's fitness become less common.
Darwin's finches
During Charles Darwin's studies on the Galápagos Islands,
Darwin observed 13 species of finches that are closely related
and differ most markedly in the shape of their beaks.
The beak of each species is suited to the food available in its
particular environment, suggesting that beak shapes evolved by natural selection. Large beaks were found on
the islands where the primary source of food for the finches are nuts and therefore the large beaks allowed the
birds to be better equipped for opening the nuts and staying well nourished.
Slender beaks were found on the finches which found insects to be the best source of food on the island
they inhabited; their slender beaks allowed the birds to be better equipped for
pulling out the insects from their tiny hiding places. The finch is also found
on the main land and it is thought that they migrated to the islands and began adapting to their environment through natural selection.
(From Wikipedia and http://www.darwinwasright.org/common_descent.html)
