Most recent common ancestor
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In genetic genealogy, the most recent common ancestor (MRCA) of any set of individuals is the most recent individual from which all the people in the group are directly descended.
The MRCA of a set of individuals can sometimes be determined by referring to an established pedigree. In general, 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 all living humans
Tracing one person's lineage back in time forms a binary tree of parents, grandparents, great-grandparents and so on. However, the number of individuals in such an ancestor tree grows exponentially and will eventually become impossibly high. For example, an individual human alive today would, over 30 generations, going back to about the High Middle Ages, have 230 or about 1.07 billion ancestors, more than the total world population at the time.
In reality, an ancestor tree is not a binary tree. Rather, pedigree collapse changes the binary tree to a directed acyclic graph.
Consider the formation, one generation at a time, of the ancestor graph of all currently living humans with no descendants. Start with living people with no descendants at the bottom of the graph. Adding the parents of all those individuals at the top of the graph will connect (half-) siblings via one or two common ancestors, their parent(s). Adding the next generation will connect all first cousins. As each of the following generations of ancestors is added to the top of the graph, the relationship between more and more people is mapped (second cousins, 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.
The MRCA of everyone alive today could thus 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. The existence of an MRCA does therefore not imply the existence of a population bottleneck or "first couple".
It is incorrect to assume that the MRCA passed all of his or her genes (or indeed any single gene) down to every person alive today. Because of sexual reproduction, at every generation, an ancestor only passes half of his or her genes to each particular descendant in the next generation. Save for inbreeding, the percentage of genes inherited from the MRCA becomes smaller and smaller in individuals at every successive generation, sometimes even decreasing to zero (at which point the Ship of Theseus situation arises. Even moderate levels of gene transfer will make it impossible to reconstruct the genomes of early ancestors, as genes inherited from contemporaries of MRCA are interchanged via sexual reproduction.
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 Upper Paleolithic period.
However, Rohde, Olson, and Chang (2003), 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). 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 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.
In theory, one can 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 is nearly 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 200,000 years ago.
Y-chromosomal Adam is estimated to have lived between 237,000 and 581,000 years ago.
The MRCA of humans alive today would therefore need to have lived more recently than either mitochondrial Eve or Y chromosomal Adam.
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 MRCA are by definition also common ancestors, we can continue to find (less recent) common ancestors by pushing further back in time to the MRCA's ancestors. Eventually we 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, from this point back all living people share exactly the same set of ancestors, all the way to the very first single-celled organism.
- DNA Heritage - Understanding MRCA for genetic genealogy (Internet Archive)
- Sandy Paterson's TMRCA website (retrieved from Internet Archive, version dated 20 December 2012)
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- TMRCA A guide written by Doug McDonald for the Clan McDonald surname project
- TMRCA methods and tools Taylor Family Genes website
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- Y and mtDNA are not Adam and Eve: Part 3 - Resolving a discrepancy by Melissa Wilson Sayres, Mathbionerd, 23 August 2013.
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- Richard Dawkins. The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Houghton Mifflin Company, Boston, 2004.
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