Most recent common ancestor

A most recent common ancestor (MRCA), also known as a last common ancestor (LCA) or concestor (a term coined by Nicky Warren), is the most recent individual from which all organisms of a set are inferred to have descended. The most recent common ancestor of a higher taxon is generally assumed to have been a species. The term is also used in reference to the ancestry of groups of genes (haplotypes) rather than organisms.

The ancestry of a set of individuals can sometimes be determined by referring to an established pedigree, although this may refer only to patrilineal or matrilineal lines for sexually-reproducing organisms with two parents, four grandparents, etc. However, in general, it is impossible to identify the exact MRCA of a large set of individuals, but an estimate of the time at which the MRCA lived can often be given. Such time to most recent common ancestor (TMRCA) estimates can be given based on DNA test results and established mutation rates as practiced in genetic genealogy, or by reference to a non-genetic, mathematical model or computer simulation.

In organisms using sexual reproduction, the matrilineal MRCA and patrilineal MRCA are the MRCAs of a given population considering only matrilineal and patrilineal descent, respectively. The MRCA of a population by definition cannot be older than either its matrilineal or its patrilineal MRCA. In the case of Homo sapiens, the matrilineal and patrilineal MRCA are also known as "Mitochondrial Eve" (mt-MRCA) and "Y-chromosomal Adam" (Y-MRCA) respectively. The age of the human MRCA is unknown. It is no greater than the age of either the Y-MRCA or the mt-MRCA, estimated at 200,000 years.

Unlike in pedigrees of individual humans or domesticated lineages where historical parentage is known for some number of generations into the past, ancestors are not directly observable or recognizable in the inference of relationships among species or higher groups of taxa (systematics or phylogenetics). Ancestors are inferences based on patterns of relationship among taxa inferred in a phylogenetic analysis of extant organisms and/or fossils.

The last universal common ancestor (LUCA) is the most recent common ancestor of all current life on Earth, estimated to have lived some 3.5 to 3.8 billion years ago (in the Paleoarchean).

The project of a complete description of the phylogenetic relationships among all biological species is dubbed the "tree of life". This involves inference of ages of divergence for all hypothesized clades; for example, the MRCA of all Carnivora (cats, dogs, etc.) is estimated to have diverged some 42 million years ago (Miacidae).

The concept of the last common ancestor from the perspective of human evolution is described for a popular audience in The Ancestor's Tale by Richard Dawkins. Dawkins lists "concestors" of the human lineage in order of increasing age, including hominin (human–chimpanzee), hominine (human–gorilla), hominid (human–orangutan), hominoid (human–gibbon), and so on in 40 stages in total, down to the last universal common ancestor (human–bacteria).

It is also possible to consider the ancestry of individual genes (or groups of genes, haplotypes) instead of an organism as a whole. Coalescent theory describes a stochastic model of how the ancestry of such genetic markers maps to the history of a population.