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Genetic genealogy

In genetics, 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 often applied to human genealogy.

The MRCA of a set of individuals can sometimes be determined by referring to an established pedigree. However, 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. This time estimate is referred to as TMRCA in scientific papers.

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. To avoid confusion, last common ancestor (LCA) or the equivalent term concestor is sometimes used in place of MRCA when discussing ancestry between species.

The term MRCA may also be used to identify a common ancestor between a set of organisms via specific gene pathways. Mitochondrial Eve and Y-chromosomal Adam are examples of such MRCAs.

MRCA of all living humans[edit | edit source]

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.[1]

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[2]), as genes inherited from contemporaries of MRCA are interchanged via sexual reproduction.[3]

Ways to find the MRCA[edit | edit source]

There are a number of ways to estimate the MRCA such as genetics, archaeology, mathematical models, computer simulations and History. DNA studies have a problem in telling us about the MRCA. As Chang notes, the MRCA will be much more recent than any MRCA that could ever be found in DNA studies, even if one were to study the ancestry of every single gene. The reason being that we are considering people who are simply ancestors, through any route, whether or not any of their genes actually survived the journey. As the human genome consists of roughly 232 base pairs, the genetic contribution of a single ancestor may be flushed out of an individual's genome completely after 32 generations, or roughly 1,000 years.[4]

Time estimates[edit | edit source]

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.

According to Rohde and his colleagues, if we consider not just our all-female and all-male lines, but our ancestors along all parental lines, it turns out that everyone on earth may share a common ancestor who is remarkably recent. Rohde, Olson, and Chang (2004),[5] 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). The paper suggests, "No matter the languages we speak or the color of our skin, we share ancestors who planted rice on the banks of the Yangtze, who first domesticated horses on the steppes of the Ukraine, who hunted giant sloths in the forests of North and South America, and who labored to build the Great Pyramid of Khufu". Rohde (2005)[6] 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. Besides dating our most recent common ancestor, Rohde's team also calculates that in 5,400 BC everyone alive was either an ancestor of all of humanity, or of nobody alive today. The researchers call this the 'identical ancestors' point: the time before which all the family trees of people today are composed of exactly the same individuals.

However, the assumption that there are no isolated populations anymore is strongly questionable in view of the continuing existence of various uncontacted peoples, such as the Sentinelese, who are suspected to have been isolated completely (not only from the western world, but also from the Asian mainland in the case of the Sentinelese) possibly for many millennia.

Other models reported in Rohde, Olson, and Chang (2004)[5] suggest that the MRCA of Western Europeans and people of Western European ancestry 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.[3][6]

A 2008 study found only six women contributed mitrochondrial DNA for 95% of all surviving Native Americans, and examination of mutation rates has been used to date waves of migration.[7]

MRCA of different species[edit | edit source]


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 'pilgrimage' backwards in time, during which we humans travel back through our own evolutionary history and as we do so are joined at each successive stage by all the other species of organism with which we share each respective common ancestor. Dawkins uses the word "concestor" (coined by Nicky Warren) 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.

A common mistake is to refer to a proposed last common ancestor as an earliest ancestor (as in the book The Link: Uncovering Our Earliest Ancestor by Colin Tudge,[8] and the documentary Uncovering Our Earliest Ancestor: The Link screened on the History Channel (US) and BBC One (UK),[9] both referring to the primate fossil dubbed Ida).

MRCA of a population identified by a single genetic marker[edit | edit source]

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 produced by non-autogamous sexual reproduction 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 becomes. 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.[1]

Patrilineal and matrilineal MRCA[edit | edit source]

It is not possible to trace human ancestry via autosomal chromosomes. Although a chromosome contains genes that are passed down from parents to children via independent assortment from only one of the two parents, genetic recombination (chromosomal crossover) mixes genes from non-sister chromatids from both parents during meiosis, thus changing the nature of the chromosome. In addition, each parent will pass on only one of their autosomal chromosomes to their offspring.

However, the mitochondrial DNA (mtDNA) is nearly immune to sexual mixing. Mitochondrial DNA, therefore, can be used to trace matrilineal inheritance of a population of related individuals. Similarly Y-DNA is present as a single chromosome in the male individual and is passed on to sons and grandsons without recombination.

Time estimates or TMRCA[edit | edit source]

Both Y-DNA and mtDNA are thus used to trace ancestry. Populations are defined by the accumulation of mutations on the gene and special trees are created for the mutations and the order in which they occurred in a population. The tree is formed through the testing of a large number of individuals all over the world for the presence or lack of a certain set of mutations. Once this is done it is possible to determine how many mutations separate one population from another in the case of mtDNA. The number of mutations in turn allow scientists to determine the approximate time passed, or the TMRCA, since the populations separated. This estimate is based on the estimated mutation rate of the mtDNA in the regions tested. The TMRCA would be the time when both populations still shared the same set of mutations or belonged to the same haplogroup.

In the case of Y-DNA TMRCA is arrived at in a different way. Haplogroups are defined by single-nucleotide polymorphism in various regions of the Y-DNA. The TMRCA within the haplogroup is defined by the accumulation of mutations in STR sequences of the Y-Chromosome of that haplogroup only. Y-DNA network analysis of Y-STR haplotypes yielding a star cluster can be regarded as representing a population descended from a single ancestor. In this case the variability of the DYS sequence, also called the microsatellite variation, can be regarded as a measure of the time passed since the ancestor founded this particular population. Variability due to multiple founding individuals will not display as a star cluster and overall variability in the Y-DNA is thus due in part to variability in the original founding population. The descendants of Genghis Khan or one of his ancestors represents a famous star cluster than can be dated back to the time of Genghis Khan.[10]

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.[11] 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 would therefore need to have lived more recently than either.[3]

TMRCA calculations are considered critical evidence when attempting to determine migration dates of various populations as they spread around the world. For example, if a mutation is deemed to have occurred 30,000 years ago, then this mutation should be found amongst all populations that diverged after this date. If archeological evidence indicates cultural spread and formation of regionally isolated populations then this must be reflected in the isolation of subsequent genetic mutations in this region. If genetic divergence and regional divergence coincide it can be concluded that the observed divergence is due to migration as evidenced by the archaeological record. However, if the date of genetic divergence occurs at a different time than the archaeological record, then scientists will have to look at alternate archaeological evidence to explain the genetic divergence. The issue is best illustrated in the debate surrounding the demic diffusion versus cultural diffusion during the European Neolithic.[12]

Identical ancestors point[edit | edit source]

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.

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,[5] from this point back, all living people share exactly the same set of ancestors, all the way to the very first single-celled organism.[1]

See also[edit | edit source]

References[edit | edit source]

  1. ^ a b c See the chapter All Africa and her progenies in Dawkins, Richard (1995). River Out of Eden. New York: Basic Books. ISBN 0-465-06990-8. 
  2. ^ Zhaxybayeva, Olga; Lapierre, Pascal; Gogarten, J. Peter (May 2004). "Genome mosaicism and organismal lineages" (PDF). Trends in Genetics 20 (5): 254–260. DOI:10.1016/j.tig.2004.03.009. PMID 15109780. Retrieved on 2009-02-19. “The Ship of Theseus paradox […] is frequently invoked to illustrate this point […]. Even moderate levels of gene transfer will make it impossible to reconstruct the genomes of early ancestors; …” 
  3. ^ a b c Dawkins, Richard (2004). The Ancestor's Tale, A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company. ISBN 0-618-00583-8. 
  4. ^ Chang 1999
  5. ^ a b c Rohde DL, Olson S, Chang JT (September 2004). "Modelling the recent common ancestry of all living humans". Nature 431 (7008): 562–6. DOI:10.1038/nature02842. PMID 15457259. 
  6. ^ a b Rohde, DLT , On the common ancestors of all living humans. Submitted to American Journal of Physical Anthropology. (2005)
  7. ^ Achilli A, Perego UA, Bravi CM, Coble MD, Kong Q-P, et al. (2008). "The Phylogeny of the Four Pan-American MtDNA Haplogroups: Implications for Evolutionary and Disease Studies". PLoS ONE 3 (3): e1764. DOI:10.1371/journal.pone.0001764. PMID 18335039. 
  8. ^ Tudge, Colin. (2009). The Link: Uncovering Our Earliest Ancestors. Little Brown.
  9. ^ Elizabeth Cline (May 22, 2009). "Ida-lized! The Branding of a Fossil". Seed Magazine. 
  10. ^ Tatiana Zerjal (2003), The Genetic Legacy of the Mongols,
  11. ^ Notions such as Mitochondrial Eve and Y-chromosomal Adam yield common ancestors that are more ancient than for all living humans (Hartwell 2004:539).
  12. ^ Morelli L, Contu D, Santoni F, Whalen MB, Francalacci P, et al. (2010). "A Comparison of Y-Chromosome Variation in Sardinia and Anatolia Is More Consistent with Cultural Rather than Demic Diffusion of Agriculture". PLoS ONE 5: e10419. DOI:10.1371/journal.pone.0010419. PMID 20454687. 

Further reading[edit | edit source]

External links[edit | edit source]

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