Ортақ Тек және Эволюциялық Туыстастық

Characteristic of a group of organisms with a common ancestor
use of the term in linguistics and philology

Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. According to modern evolutionary biology, all living beings could be descendants of a unique ancestor commonly referred to as the last universal common ancestor (LUCA) of all life on Earth. Common descent is an effect of speciation, in which multiple species derive from a single ancestral population. The more recent the ancestral population two species have in common, the more closely are they related. The most recent common ancestor of all currently living organisms is the last universal ancestor, The two earliest pieces of evidence for life on Earth are graphite found to be biogenic in 3.7 billion year old metasedimentary rocks discovered in western Greenland and microbial mat fossils found in 3.48 billion year old sandstone discovered in Western Australia. All currently living organisms on Earth share a common genetic heritage, though the suggestion of substantial horizontal gene transfer during early evolution has led to questions about the monophyly (single ancestry) of life.

All known forms of life are based on the same fundamental biochemical organization: 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. Analysis of small sequence differences in widely shared substances such as cytochrome c further supports universal common descent. Some 23 proteins are found in all organisms, serving as enzymes carrying out core functions like DNA replication. The fact that only one such set of enzymes exists is convincing evidence of a single ancestry. 6,331 genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian.

Similarities which have no adaptive relevance cannot be explained by convergent evolution, and therefore they provide compelling support for 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 redundantly for the same amino acid. Since many 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 their sharing a recent common ancestor. Had the amino acid sequences come from different ancestors, they would have been coded for by any of the redundant codons, and since the correct amino acids would already have been in place, natural selection would not have driven any change in the codons, however much time was available. Genetic drift could change the codons, but it would be extremely unlikely to make all the redundant codons in a whole sequence match exactly across multiple lineages. Similarly, shared nucleotide sequences, especially where these are apparently neutral such as the positioning of introns and pseudogenes, provide strong evidence of common ancestry.

Biologists often point to the universality of many aspects of cellular life 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 through universal common descent and therefore resulted in convergent evolution. In contrast, there is evidence for homology of the central subunits of transmembrane ATPases throughout all living organisms, especially how the rotating elements are bound to the membrane. This supports the assumption of a LUCA as a cellular organism, although primordial membranes may have been semipermeable and evolved later to the membranes of modern bacteria, and on a second path to those of modern archaea also.

Another important piece of evidence is from detailed phylogenetic trees (i. e., "genealogic trees" of species) mapping out the proposed divisions and common ancestors of all living species. In 2010, Douglas L. Theobald published a statistical analysis of available genetic data,

Theobald noted that substantial horizontal gene transfer could have occurred during early evolution. Bacteria today remain capable of gene exchange between distantly related lineages. This weakens the basic assumption of phylogenetic analysis, that similarity of genomes implies common ancestry, because sufficient gene exchange would allow lineages to share much of their genome whether or not they shared an ancestor (monophyly). This has led to questions about the single ancestry of life. for not including consideration of convergence. They argued that Theobald's test was insufficient to distinguish between the competing hypotheses. Theobald has defended his method against this claim, arguing that his tests distinguish between phylogenetic structure and mere sequence similarity. Therefore, Theobald argued, his results show that "real universally conserved proteins are homologous."

The possibility is mentioned, above, that all living organisms may be descended from an original single celled organism with a DNA genome, and that this implies a single origin for life. Although such a universal common ancestor may have existed, such a complex entity is unlikely to have arisen spontaneously from non life and thus a cell with a DNA genome cannot reasonably be regarded as the origin of life. To understand the origin of life, it has been proposed that DNA based cellular life descended from relatively simple pre cellular self replicating RNA molecules able to undergo natural selection. During the course of evolution, this RNA world was replaced by the evolutionary emergence of the DNA world. A world of independently self replicating RNA genomes apparently no longer exists (RNA viruses are dependent on host cells with DNA genomes). Because the RNA world is apparently gone, it is not clear how scientific evidence could be brought to bear on the question of whether there was a single origin of life event from which all life descended.