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Most recent common ancestor

The 'most recent common ancestor' ('MRCA') of any set of organisms is the most recent individual which is an ancestor of all of them, except itself. The term is most frequently used of humans.

The MRCA of a set of individuals can sometimes be determined by referring to an established pedigree. In general, however, it is impossible to identify the specific MRCA of a set of individuals, but an estimate of the time at which the MRCA lived can often be given; such estimates can be given based on DNA test results and established mutation rates, or by reference to a non-genetic genealogical model.

MRCA of two individuals

The most recent common patrilineal ancestor of any two males, and the most recent common matrilineal ancestor of any two individuals can be determined by genealogical DNA tests. The tests use mitochondrial DNA for matrilineal inheritance or Y-chromosome-DNA for patrilineal inheritance.

MRCA of all living humans

The existence of an MRCA does not imply any sort of population bottleneck or first couple. The MRCA of everyone alive today could have co-existed with a large human population, most of whom either have no living descendants today or else are ancestors of almost everyone alive today.

Patrilineal and matrilineal ancestry

The most recent common patrilineal ancestor of all living male humans, and the most recent common matrilineal ancestor of all living female humans have been established by researchers using tests of the same kinds of DNA as for two individuals. Notions such as Mitochondrial Eve and Y-chromosomal Adam yield common ancestors that are more ancient than for all living humans (Hartwell 2004:539). Mitochondrial Eve is estimated to have lived about 150,000 years ago. Y-chromosomal Adam is estimated to have lived between 60,000 and 90,000 years ago. The MRCA of humans alive today necessarily lived more recently than either.

Time estimates

Depending on the survival of isolated lineages without admixture from Modern migrations and taking into account long-isolated peoples, such as historical tribes in central Africa, Australia and remote islands in the South Pacific, the human MRCA is generally assumed to have lived in the Paleolithic period.

However, Rohde, Olson, and Chang (2004), using a non-genetic model, estimated that the MRCA of all living humans may have lived within historical times (3rd millennium BC to 1st millennium AD). Rhode (2005) refined the simulation with parameters from estimated historical human migrations and of population densities. For conservative parameters, he pushes back the date for the MRCA to the 6th millennium BC (p. 20), but still concludes with a "surprisingly recent" estimate of a MRCA living in the second or first millennium BC (p. 27). An explanation of this result is that, while humanity's MRCA was indeed a Paleolithic individual up to Early Modern times, the European explorers of the 16th and 17th centuries would have fathered enough offspring so that some "mainland" ancestry by today pervades even remote habitats. The possibility remains, however, that a single isolated population with no recent "mainland" admixture persists somewhere, which would immediately push back the date of humanity's MRCA by many millennia. While simulations help estimate probabilities, the question can only be resolved authoritatively by genetically testing every living human individual.

Other models reported in Rohde, Olson, and Chang (2004) suggest that the MRCA of Western Europeans lived as recently as AD 1000. The same article provides surprisingly recent estimates for the 'identical ancestors point', the most recent time when each person then living was either an ancestor of all the persons alive today or an ancestor of none of them. The estimates for this are similarly uncertain, but date to considerably earlier than the MRCA, according to Rohde (2005) roughly to between 15,000 and 5,000 years ago.

MRCA of different species

It is also possible to use the term MRCA to describe the common ancestor of two or more different species. This concept is described in Richard Dawkins' book, The Ancestor's Tale, in which he imagines a backwards 'pilgrimage' in time, during which we humans 'meet' all the other species of organism with which we share a common ancestor. Dawkins coined the word, 'concestor', as an alternative to MRCA.

Following the human evolutionary tree backwards, we first meet the concestor which we share with the species that are our closest relatives, the chimpanzee and bonobo. Dawkins estimates this to have occurred between 5 and 7 million years ago. Another way of looking at this is to say that our (approximately) 250,000-greats-grandparent was a creature from which all humans, chimpanzees and bonobos are directly descended.

Further on in Dawkins' imaginary journey, we meet the concestor we share with the Gorilla, our next nearest relative, then the Orangutan, and so on.