Most recent common ancestor

Inbiologyandgenetic genealogy,themost recent common ancestor(MRCA), also known as thelast common ancestor(LCA), of a set oforganismsis the most recent individual from which all the organisms of the set aredescended.The term is also used in reference to the ancestry of groups ofgenes(haplotypes) rather than organisms.

The MRCA of a set of individuals can sometimes be determined by referring to an establishedpedigree.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. Suchtime to most recent common ancestor(TMRCA) estimates can be given based on DNA test results and establishedmutation ratesas practiced in genetic genealogy, or by reference to a non-genetic,mathematical modelor computer simulation.

In organisms usingsexual reproduction,thematrilineal MRCAandpatrilineal MRCAare the MRCAs of a given population considering onlymatrilinealandpatrilinealdescent, respectively. The MRCA of a population by definition cannot be older than either its matrilineal or its patrilineal MRCA. In the case ofHomo 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 around 200,000 years.

Unlike in pedigrees of individual humans or domesticated lineages where historical parentage is known, in the inference of relationships among species or higher groups oftaxa(systematicsorphylogenetics), ancestors are not directly observable or recognizable. They are inferences based on patterns of relationship among taxa inferred in aphylogenetic analysisof extant organisms and/orfossils.^[1]

Thelast 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 thePaleoarchean).^[2]^[3]^{[note 1]}

MRCA of different species

Evolutionary treeshowing the divergence of modern species from thelast universal ancestorin the center.^[5]The threedomainsare colored, withbacteriablue,archaeagreen, andeukaryotesred.

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 hypothesizedclades;for example, the MRCA of allCarnivora(cats,dogs,etc) is estimated to have diverged some 42 million years ago (Miacidae).^[6]

The concept of the last common ancestor from the perspective ofhuman evolutionis described for a popular audience inThe Ancestor's TalebyRichard Dawkins(2004). Dawkins lists "concestors" of thehuman lineagein order of increasing age, includinghominin(human–chimpanzee),hominine(human–gorilla),hominid(human–orangutan),hominoid(human–gibbon), and so on in 40 stages in total, down to thelast universal common ancestor(human–bacteria).

MRCA of a population identified by a single genetic marker

It is also possible to consider the ancestry of individualgenes(or groups of genes,haplotypes) instead of an organism as a whole.Coalescent theorydescribes a stochastic model of how the ancestry of suchgenetic markersmaps to the history of a population.

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 mutateddescendant genes.While organisms have ancestry graphs and progeny graphs viasexual reproduction,a gene has a single chain of ancestors and a tree of descendants. An organism produced by sexual cross-fertilization (allogamy) has at least two ancestors (its immediate parents), but a gene always has one ancestor per generation.

Patrilineal and matrilineal MRCA

Throughrandom driftor selection, lineage will trace back to a single person. In this example over 5 generations, the colors represent extinct matrilineal lines and black the matrilineal line descended from the mt-MRCA.

Mitochondrial DNA(mtDNA) is nearly immune to sexual mi xing, unlike thenuclear DNAwhose chromosomes are shuffled and recombined inMendelian inheritance.Mitochondrial DNA, therefore, can be used to tracematrilineal inheritanceand to find theMitochondrial Eve(also known as theAfrican Eve), the most recent common ancestor of all humans via the mitochondrial DNA pathway.

Likewise,Y chromosomeis present as a single sex chromosome in the male individual and is passed on to male descendants without recombination. It can be used to tracepatrilinealinheritance and to find theY-chromosomal Adam,the most recent common ancestor of all humans via the Y-DNA pathway.

Approximate dates for Mitochondrial Eve and Y-chromosomal Adam have been established by researchers usinggenealogical DNA tests.Mitochondrial Eve is estimated to have lived about 200,000 years ago. A paper published in March 2013 determined that, with95% confidenceand that provided there are nosystematic errorsin the study's data, Y-chromosomal Adam lived between 237,000 and 581,000 years ago.^[7]^[8]

The MRCA of all humans alive today would, therefore, need to have lived more recently than either.^[9]^{[note 2]}

It is more complicated to infer human ancestry viaautosomal chromosomes.Although an autosomal chromosome contains genes that are passed down from parents to children viaindependent assortmentfrom only one of the two parents,genetic recombination(chromosomal crossover) mixes genes from non-sisterchromatidsfrom both parents duringmeiosis,thus changing the genetic composition of the chromosome.

Time to MRCA estimates

Different types of MRCAs are estimated to have lived at different times in the past. Thesetime to MRCA(TMRCA) estimates are also computed differently depending on the type of MRCA being considered. Patrilineal and matrilineal MRCAs (Mitochondrial Eve and Y-chromosomal Adam) are traced by single gene markers, thus their TMRCA are computed based on DNA test results and established mutation rates as practiced in genetic genealogy. The time to the genealogical MRCA (most recent common ancestor by any line of descent) of all living humans cannot be traced genetically because the DNA of the great majority of ancestors is completely lost after a few hundred years. It is therefore computed based on non-genetic, mathematical models and computer simulations.

Since Mitochondrial Eve and Y-chromosomal Adam are traced by single genes via a single ancestral parent line, the time to these genetic MRCAs will necessarily be greater than that for the genealogical MRCA. This is because single genes will coalesce more slowly than tracing of conventional human genealogy via both parents. The latter considers only individual humans, without taking into account whether any gene from the computed MRCA actually survives in every single person in the current population.^[11]

TMRCA via genetic markers

Mitochondrial DNA can be used to trace the ancestry of a set of populations. In this case, populations are defined by the accumulation of mutations on the mtDNA, and special trees are created for the mutations and the order in which they occurred in each 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. The number of mutations, together with estimated mutation rate of the mtDNA in the regions tested, allows scientists to determine the approximate time to MRCA (TMRCA) which indicates time passed since the populations last shared the same set of mutations or belonged to the samehaplogroup.

In the case of Y-Chromosomal DNA, TMRCA is arrived at in a different way.Y-DNA haplogroupsare defined bysingle-nucleotide polymorphismin various regions of the Y-DNA. The time to MRCA within a haplogroup is defined by the accumulation of mutations inSTRsequences of the Y-Chromosome of that haplogroup only. Y-DNA network analysis ofY-STR haplotypesshowing a non-star cluster indicates Y-STR variability due to multiple founding individuals. Analysis yielding a star cluster can be regarded as representing a population descended from a single ancestor. In this case the variability of theY-STRsequence, also called themicrosatellitevariation, can be regarded as a measure of the time passed since the ancestor founded this particular population. Thedescendants of Genghis Khanor one of his ancestors represents a famous star cluster that can be dated back to the time of Genghis Khan.^[12]

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 thedemic diffusionversuscultural diffusionduring theEuropean Neolithic.^[13]

TMRCA of all living humans

The age of the MRCA ofall living humansis unknown. It is necessarily younger than the age of either the matrilinear or the patrilinear MRCA, both of which have an estimated age of between roughly 100,000 and 200,000 years ago.^[14]

A study by mathematicians Joseph T. Chang, Douglas Rohde and Steve Olson used a theoretical model to calculate that the MRCA may have lived remarkably recently, possibly as recently as 2,000 years ago. It concludes that the MRCA of all humans probably lived in East Asia, which would have given them key access to extremely isolated populations in Australia and the Americas. Possible locations for the MRCA include places such as the Chuckchi and Kamchatka Peninsulas that are close to Alaska, places such as Indonesia and Malaysia that are close to Australia or a place such as Taiwan or Japan that is more intermediate to Australia and the Americas. European colonization of the Americas and Australia was found by Chang to be too recent to have had a substantial impact on the age of the MRCA. In fact, if the Americas and Australia had never been discovered by Europeans, the MRCA would only be about 2.3% further back in the past than it is.^[15]^[16]^[17]

Note that the age of the MRCA of a population does not correspond to apopulation bottleneck,let alone a "first couple". It rather reflects the presence of a single individual with high reproductive success in the past, whose genetic contribution has become pervasive throughout the population over time. It is also incorrect to assume that the MRCA passed all, or indeed any, genetic information to every living person. Throughsexual reproduction,an ancestor passes half of his or her genes to each descendant in the next generation; in the absence ofpedigree collapse,after just 32 generations the contribution of a single ancestor would be on the order of 2⁻³²,a number proportional to less than a single basepair within thehuman genome.^[18]

Identical ancestors point

The MRCA is the most recentcommonancestor shared by all individuals in the population under consideration. This MRCA may well have contemporaries who are also ancestral to some but not all of the extant population. Theidentical ancestors pointis a point in the past more remote than the MRCA at which time there are no longer organisms which are ancestral to some but not all of the modern population. Due topedigree collapse,modern individuals may still exhibit clustering, due to vastly different contributions from each of ancestral population.^[19]

Notes

^The composition of the LUCA is not directly accessible as a fossil, but can be studied bycomparing the genomes of its descendants,organisms living today. By this means, a 2016 study identified a set of 355genesinferred to have been present in the LUCA.^[4]
^Notions such asMitochondrial EveandY-chromosomal Adamyield common ancestors that are more ancient than for all living humans.^[10]

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