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.{{ref|transfer}}

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.{{ref|UCD}}

Related Topics:
Genetic code - Bacteria - Archaea - Eukaryote - Three domain system

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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.

Related Topics:
Phylogenetic tree - Virus - RNA - DNA

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

Related Topics:
ATP - Amino acid

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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:

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• 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.

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

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• 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.