Most recent common ancestor

Overview
The most recent common ancestor (MRCA) of any set of organisms is the most recent individual from which all organisms in the group are directly descended. 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.

The term MRCA is usually used to describe a common ancestor of individuals within a species. It can also be used to describe a common ancestor between species (i.e. the species founder). To avoid confusion, last common ancestor (LCA) or the equivalent term concestor, coined by Richard Dawkins, is sometimes used in place of MRCA when discussing ancestry between species.

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 estimated 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 existence of a 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 a subset of people alive today. This seemingly paradoxical phenomenon can be easily explained, if the nature of lineage is taken into account.

When tracing human lineage back in time, most people look at parents, grandparents, great-grandparents and so on. The same approach is often taken when tracing descendants via children and grandchildren. This approach is misguided, as the numbers of ancestors and descendants grows exponentially as generations are added to the lineage tree. In 80 generations, the number of ancestors is 280, more than a trillion trillion.

This simple calculation does not take into account the fact that every fertilisation is really a fertilisation between distant cousins. The ancestry tree is therefore not a tree, but a directed, acyclic graph. People with no descendants are placed at the bottom of the graph and ancestors are placed above their descendants. Starting with the people with no descendants one can connect their parents above. (Half-)siblings thus become connected via one or two common ancestors, their parent(s). As each generation of ancestors is added at the top of the graph, more and more people become related to one another (first cousins, second cousin, third cousins and so on). Eventually a generation is reached where one or more of the many top-level ancestors is an MRCA from whom it is possible to trace a path of direct descendants all the way down to every living person at the bottom generations of the graph.

It is incorrect to assume that the MRCA passed all (or indeed any) of his or her genes down to every person alive today. Because of sexual reproduction, at every generation, an ancestor only passes half of his or her genes to the next generation. The percentage of genes inherited from the MRCA becomes smaller and smaller at every successive generation, as genes inherited from contemporaries of MRCA are interchanged via sexual reproduction.

Identical Ancestors Point
The MRCA had many contemporary companions of both sexes. Many of these contemporaries left direct descendants, but not all of them left an unbroken link of descendants all the way down to today's population. That is, some contemporaries of the MRCA are ancestors of no one in the current population. The rest of the contemporaries of the MRCA may claim ancestry over a subset of current population, but not the entirety of current population.

Because ancestors of the MRCA are by definition also common ancestors, the farther we push back in time, the more common ancestors we find, from this single MRCA. Eventually we will reach a point in the past where all humans can be divided into two groups: those who left no descendants today and those who are common ancestors of all living humans today. This point in time is termed the identical ancestors point. Even though each living person receives genes (in original or mutated forms) in dramatically different proportions from these ancestors from the identical ancestors point, all living people share exactly the same set of ancestors from this point back, all the way to the very first single-celled organism.

Gene ancestry
It is also possible to consider the ancestry of individual genes (coalescent theory), instead of a person (an organism) as a whole. Unlike organisms, a gene is passed down from a generation of organisms to the next generation either as perfect replicas of itself or as slightly mutated descendant genes. While organisms have ancestry graphs and progeny graphs via sexual reproduction, a gene has a single chain of ancestors and a tree of descendants. An organism has at least two ancestors (immediate parents), but a gene always has one single ancestor.

Given any gene in the body of a person, we can trace a single chain of human ancestors back in time, following the lineage of this one gene. Because a typical organism is built from tens of thousands of genes, there are numerous ways to trace the ancestry of organisms using this mechanism. But all these inheritance pathways share one common feature. If we start with all humans alive in 1995 and trace their ancestry by one particular gene (actually a locus), we find that the farther we move back in time, the smaller the number of ancestors become. The pool of ancestors continues to shrink until we find the most recent common ancestor (MRCA) of all humans who were alive in 1995, via this particular gene pathway.

Patrilineal and matrilineal ancestry
In theory, one can also trace human ancestry via a single chromosome, as a chromosome contains a set of genes and is passed down from parents to children via independent assortment from only one of the two parents. But genetic recombination (chromosomal crossover) mixes genes from non-sister chromatids from both parents during meiosis, thus muddling the ancestry path.

However, the mitochondrial DNA (mtDNA) is immune to sexual mixing, unlike the nuclear DNA whose chromosomes are shuffled and recombined in Mendelian inheritance. Mitochondrial DNA, therefore, can be used to trace matrilineal inheritance and to find the Mitochondrial Eve (also known as the African Eve), the most recent common ancestor of all humans via the mitochondrial DNA pathway.

Mitochondrial Eve and the most recent common patrilineal ancestor of all living male humans, known as Y-chromosomal Adam, have been established by researchers using tests of the same kinds of DNA as for two individuals. Mitochondrial Eve is estimated to have lived about 140,000 years ago. Y-chromosomal Adam is estimated to have lived around 60,000 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 was 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). Rohde (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 be resolved authoritatively only 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. In the past, the term MRCA was used interchangeably with last common ancestor (LCA) to denote both the common ancestor within a species and that between species. But MRCA is now more frequently used to describe common ancestors within a species. On the other hand, LCA now describes the common ancestor between two species.

The concept of the last common ancestor 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 LCA.

In The Ancestor's Tale, 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, until we finally meet the concestor of all living organisms, known as the last universal ancestor.