Common descent

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.

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).

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.

In 1795, Charles Darwin's grandfather, Erasmus Darwin, hypothesized 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 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).

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.

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.

The universality of ATP, and the fact that all amino acids found in proteins are left-handed, are also important pieces of evidence.

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. Namely that:
 * 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 &mdash; 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.