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.^{[1] (http://en.wikipedia.org/wiki/Common_descent#endnote_transfer)} An ancestor is a parent or (recursively) the parent of an ancestor. ... Biology is the science of life (from the Greek words bios = life and logos = reasoned account). ... In biology and ecology, an organism (in Greek organon = instrument) is a living being. ... Earth, also known as the Earth or Terra, is the third planet outward from the Sun. ...

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 3.5 billion years ago (see: origin of life). Charles Darwin, the father of evolutionary theory Although generally, evolution is taken to mean any process of change over time, in the context of life science, evolution is a change in the traits of living organisms over generations, including the emergence of new species. ... Charles Darwin, about the same time as the publication of The Origin of Species. ... The 1859 edition of On the Origin of Species First published in 1859, The Origin of Species (full title On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life) by British naturalist Charles Darwin is one of the pivotal... 1859 is a common year starting on Saturday. ... The Descent of Man and Selection in Relation to Sex by British naturalist Charles Darwin was first published in 1871. ... 1871 was a common year starting on Sunday (see link for calendar). ... Last universal ancestor (LUA), the hypothetical latest living organism from which all currently living organisms descend. ... The most recent common ancestor (MRCA) of any set of organisms is the most recent individual which is an ancestor of all of them. ... This timeline outlines the major events in the development of life on planet Earth. ... Pre-Cambrian stromatolites in the Siyeh Formation, Glacier National Park. ...

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: Events May 11 - War of Austrian Succession: Battle of Fontenoy - At Fontenoy, French forces defeat an Anglo-Dutch-Hanoverian army including the Black Watch June 4 – Frederick the Great destroys Austrian army at Hohenfriedberg August 19 - Beginning of the 45 Jacobite Rising at Glenfinnan September 12 - Francis I is elected... Pierre Louis Maupertuis, here wearing lapmudes or a fur coat from his Lapland expedition. ...

"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. 1790 was a common year starting on Friday (see link for calendar). ... A painting of Immanuel Kant in his middle age Immanuel Kant (April 22, 1724 in Königsberg – February 12, 1804) was a German philosopher from Prussia, generally regarded as one of Europes most influential thinkers and the last major philosopher of the Enlightenment. ...

In 1795, Charles Darwin's grandfather, Erasmus Darwin, hypothesized that all warm-blooded animals were descended from a single "living filament": 1795 was a common year starting on Thursday (see link for calendar). ... Erasmus Darwin Erasmus Darwin (December 12, 1731 – April 18, 1802) trained as a physician and wrote extensively on medicine and botany, as well as poetry. ...

"...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: 1859 is a common year starting on Saturday. ...

"[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 recognised 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".

More recently, scientists such as Francis Crick have postulated that the universal common ancestor could have come from space (panspermia). He was led to this conclusion by the universality of the genetic code (see below). Photomontage of Francis Crick lecturing Francis Harry Compton Crick, OM (June 8, 1916 – July 28, 2004) was one of the discoverers of the structure of the DNA molecule. ... Panspermia is a theory (more directly described as a hypothesis, as there is no compelling evidence yet available to support or contradict it) that suggests that the seeds of life are prevalent throughout the Universe and life on Earth began by such seeds landing on Earth and propagating. ... RNA codons. ...

Evidence for common descent

Universality and similarity

The universality of the genetic code is generally regarded by biologists as definitive evidence in favor of the theory of universal common descent (UCD) for all bacteria, archaea, and eukaryotes (see Three domain system). Analysis of the small differences in the genetic code has also provided support for UCD.^{[2] (http://en.wikipedia.org/wiki/Common_descent#endnote_UCD)} RNA codons. ... Phyla/Divisions Actinobacteria Aquificae Bacteroidetes/Chlorobi Chlamydiae/Verrucomicrobia Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus_Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Nitrospirae Omnibacteria Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Bacteria (singular, bacterium) are a major group of living organisms. ... Phyla / Classes Phylum Crenarchaeota Phylum Euryarchaeota     Halobacteria     Methanobacteria     Methanococci     Methanopyri     Archaeoglobi     Thermoplasmata     Thermococci Phylum Korarchaeota Phylum Nanoarchaeota The Archaea are a major group of prokaryotes. ... Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... The three-domain system is a biological classification introduced by Carl Woese in 1990 that emphasizes his separation of prokaryotes into two groups, originally called Eubacteria and Archaebacteria. ...

Another important piece of evidence is the fact that it is possible to construct a detailed phylogenetic tree for all three domains based on similarity. One such tree showing the paths of descent from a common ancestor is depicted in the article on phylogenetic trees. Exactly how viruses fit into the picture is still uncertain, especially since some are based on RNA rather than DNA. However, viruses are not usually regarded as organisms. A phylogenetic tree is a tree showing the evolutionary interrelationships among various species or other entities that are believed to have a common ancestor. ... Three types of viruses: a bacterial virus, otherwise called a bacteriophage (left center); an animal virus (top right); and a retrovirus (bottom right). ... Ribonucleic acid (RNA) is a nucleic acid consisting of a string of covalently-bound nucleotides. ... Space-filling model of a section of DNA molecule Deoxyribose nucleic acid (DNA) is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life (and many viruses). ...

The universality of ATP, and the fact that all amino acids found in proteins are left-handed, are also important pieces of evidence. Adenosine triphosphate (ATP) Adenosine triphosphate (ATP) is the nucleotide known in biochemistry as the molecular currency of intracellular energy transfer; that is, ATP is able to store and transport chemical energy within cells. ... In chemistry, an amino acid is any molecule that contains both amino and carboxylic acid functional groups. ...

The argument from irrelevant differences

There are very strong pieces of evidence for UCD based on universality and similarity, but such arguments become complicated because they run into a potential difficulty:

• universality might be the result of the laws of physics and chemistry, rather than universal common descent;
• similarity might be the result of convergent evolution.

The simplest way to circumvent such difficulties would be to produce evidence based on "irrelevant differences", that is, differences which have no relevance to evolution and therefore cannot be explained by convergence.

Such evidence has come from two domains — amino acid sequences and DNA sequences:

1. 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.
2. 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.

Relevance to creationism

Some creationists do not accept the theory of universal common descent, arguing that humanity was created by God in a distinct act of creation, whereas the rest of life evolved. Other creationists believe that God created many different forms of life and that evolution and speciation has subsequently occurred within some of these forms (as described in creation biology). Still others reject all forms of evolution, arguing that similar structures merely point to a Common Creator, who (purposely) designed the DNA code to be used (almost) universally. Similar to how we use the circle and sphere for so many different purposes (i.e. Wheels, ball bearings, door knobs, stereo knobs, mouse balls, etc.). These last Creationists also believe that each species was individually created. This is an article on wide range of beliefs in creation ex nihilo. ... Baraminology is a method of biological taxonomy proposed by some creationists. ... Speciation refers to the appearance of a new species of life on earth, particularly as seen in the fossil record. ... Creation biology is an attempt to study biology from a creationary perspective. ...

Footnotes

1. ^  The earliest life-like forms probably exchanged genetic material laterally in a manner that is analogous to 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.
2. ^  Robin Knight et. al., (2001) "Rewiring The Keyboard: Evolvability Of The Genetic Code," Nature Reviews - Genetics. 2: 49-58.

Horizontal gene transfer is any process in which an organism transfers genetic material (i. ...

External links

• The Tree of Life Web Project (http://tolweb.org/tree/phylogeny.html)