Most recent common ancestor
Encyclopedia
In genetics
, the most recent common ancestor (MRCA) of any set of organism
s 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 large set of individuals, but an estimate of the time at which the MRCA lived can often be given. Such time to MRCA (TMRCA) estimates can be given based on DNA test results and established mutation rate
s as practiced in genetic genealogy
, or by reference to a non-genetic, mathematical model or computer simulation.
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. Such genealogies in reality trace ancestry of individual genes, not organisms. As a result, TMRCA estimates for genetic MRCAs are necessarily greater than those for MRCAs of organisms.
of parents, grandparents, great-grandparents and so on. The number of individuals in an ancestor tree grows exponentially
and will eventually become impossibly high. A 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 world population
at the time.
An ancestor tree is not binary, 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 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 could have co-existed with a large human population, most of whom either have no living descendants or 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 gene) down to every person alive. Because of sexual reproduction
at every generation, an ancestor passes half of his or her genes to each descendant in the next generation. Save for inbreeding, the percentage of genes inherited from the MRCA becomes smaller and smaller in individuals at each generation, sometimes decreasing to zero (at which point the Ship of Theseus situation arises), as genes inherited from contemporaries of MRCA are interchanged via sexual reproduction.
It is estimated that the MRCA lived between 5,000 to 2,000 years ago.
Because ancestors of the MRCA are by definition also common ancestors, we can find (less recent) common ancestors by pushing further back in time to ancient common ancestors of all people alive. 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. 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 the same set of ancestors, all the way to the first single-celled organism.
The identical ancestors point for Homo sapiens has been estimated to between 15,000
and 5,000
years ago.
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 human
s 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 evolution
ary 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 (imaginary, in that the journey is going backwards in time), 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 made by the public as well as some authors is to refer to a proposed last common ancestor as an earliest ancestor. Examples include the book The Link: Uncovering Our Earliest Ancestor by Colin Tudge
, and a documentary of the same name screened on the History Channel (US) and BBC One
(UK), both referring to the primate fossil dubbed Ida
.
s (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 sexual, allogamous
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.
(mtDNA) 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.
Similarly Y chromosome
is present as a single sex chromosome in the male individual and is passed on to male descendants without recombination. It can be used to trace patrilineal
inheritance and to find the Y-chromosomal Adam
, the most recent common ancestor of all humans via the Y-DNA pathway.
Mitochondrial Eve and Y-chromosomal Adam have been established by researchers using genealogical DNA test
s. 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.
It is more complicated to infer human ancestry via autosomal chromosomes. Although an autosomal 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 genetic composition of the chromosome.
Since Mitochondrial Eve and Y-chromosomal Adam are traced by single genes via a single ancestral parent lines, the time to these genetic MRCAs will necessarily be greater than that for the genealogical MRCA. This is because single genes will coalesce slower 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.
.
In the case of Y-Chrosomal DNA, TMRCA is arrived at in a different way. Y-DNA haplogroups are defined by single-nucleotide polymorphism in various regions of the Y-DNA. The time to MRCA within a 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 showing 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 the Y-STR
sequence, also called the microsatellite variation, can be regarded as a measure of the time passed since the ancestor founded this particular 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.
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
.
Depending on the survival of isolated lineages without admixture from modern migrations
and taking into account long-isolated peoples, such as historical societies 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. With the advent of mathematical models and computer simulations, researchers now find that the MRCA of all humans lived remarkably recently, between 2,000 and 4,000 years ago.
Rohde, Olson
and Chang (2004) constructed a mathematical model that considered the tendency of individuals to choose mates from the same group, as well as the relative geographical isolation of such groups. The mathematical model with one particular set of parameters showed that the MRCA lived about the year 300 BC and yielded an identical ancestor point (IAP) of 3,000 BC.
The same 2004 Rohde paper also presented results from a computer program based on Monte Carlo simulation
designed to overcome some of the limits of the mathematical model. The program took into consideration realistic population substructure and migration patterns, allowing the researchers to simulate historical human demography. A conservative simulation yielded a mean MRCA date of 1,415 BC and a mean IA date of 5,353 BC. A less conservative simulation gave an MRCA date of AD 55 and an IA date of 2,158 BC.
An explanation of this recent MRCA date 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 remote habitats. The possibility remains that an 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 only by genetically testing every living human individual.
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
".
An assumption that there are no isolated populations is questionable in view of the existence of various uncontacted peoples
, who are suspected to have been isolated for many millennia, including the Sentinelese who have been isoltated from the western world and also from the Asian mainland.
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
, the most recent common ancestor (MRCA) of any set of organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s is the most recent individual from which all organisms in the group are directly descended
Common descent
In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
. The term is often applied to human
Human
Humans are the only living species in the Homo genus...
genealogy
Genealogy
Genealogy is the study of families and the tracing of their lineages and history. Genealogists use oral traditions, historical records, genetic analysis, and other records to obtain information about a family and to demonstrate kinship and pedigrees of its members...
.
The MRCA of a set of individuals can sometimes be determined by referring to an established pedigree
Pedigree chart
A pedigree chart is a diagram that shows the occurrence and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the next, most commonly humans, show dogs, and race horses....
.
However, in general, it is impossible to identify the specific MRCA of a large set of individuals, but an estimate of the time at which the MRCA lived can often be given. Such time to MRCA (TMRCA) estimates can be given based on DNA test results and established mutation rate
Mutation rate
In genetics, the mutation rate is the chance of a mutation occurring in an organism or gene in each generation...
s as practiced in genetic genealogy
Genetic genealogy
Genetic genealogy is the application of genetics to traditional genealogy. Genetic genealogy involves the use of genealogical DNA testing to determine the level of genetic relationship between individuals.-History:...
, or by reference to a non-genetic, mathematical model or computer simulation.
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
Mitochondrial Eve
In the field of human genetics, Mitochondrial Eve refers to the matrilineal "MRCA" . In other words, she was the woman from whom all living humans today descend, on their mother's side, and through the mothers of those mothers and so on, back until all lines converge on one person...
and Y-chromosomal Adam
Y-chromosomal Adam
In human genetics, Y-chromosomal Adam is the theoretical most recent common ancestor from whom all living people are descended patrilineally . Many studies report that Y-chromosomal Adam lived as early as around 142,000 years ago and possibly as recently as 60,000 years ago...
are examples of such MRCAs. Such genealogies in reality trace ancestry of individual genes, not organisms. As a result, TMRCA estimates for genetic MRCAs are necessarily greater than those for MRCAs of organisms.
MRCA of all living humans
Tracing one person's lineage back in time forms a binary treeBinary tree
In computer science, a binary tree is a tree data structure in which each node has at most two child nodes, usually distinguished as "left" and "right". Nodes with children are parent nodes, and child nodes may contain references to their parents. Outside the tree, there is often a reference to...
of parents, grandparents, great-grandparents and so on. The number of individuals in an ancestor tree grows exponentially
Exponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...
and will eventually become impossibly high. A human alive today would over 30 generations (going back to about the High Middle Ages
High Middle Ages
The High Middle Ages was the period of European history around the 11th, 12th, and 13th centuries . The High Middle Ages were preceded by the Early Middle Ages and followed by the Late Middle Ages, which by convention end around 1500....
), have 230 or about 1.07 billion ancestors, more than the world population
World population
The world population is the total number of living humans on the planet Earth. As of today, it is estimated to be billion by the United States Census Bureau...
at the time.
An ancestor tree is not binary, rather pedigree collapse
Pedigree collapse
In genealogy, pedigree collapse describes how reproduction between two individuals who knowingly or unknowingly share an ancestor causes the number of distinct ancestors in the family tree of their offspring to be smaller than it could otherwise be. Robert C...
changes the binary tree to a directed acyclic graph
Directed acyclic graph
In mathematics and computer science, a directed acyclic graph , is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of...
.
Consider the formation, one generation at a time, of the ancestor graph of all 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 could have co-existed with a large human population, most of whom either have no living descendants or are ancestors of a subset of people alive today. The existence of an MRCA does therefore not imply the existence of a population bottleneck
Population bottleneck
A population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing....
or first couple.
It is incorrect to assume that the MRCA passed all of his or her genes (or indeed any gene) down to every person alive. Because of sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
at every generation, an ancestor passes half of his or her genes to each descendant in the next generation. Save for inbreeding, the percentage of genes inherited from the MRCA becomes smaller and smaller in individuals at each generation, sometimes decreasing to zero (at which point the Ship of Theseus situation arises), as genes inherited from contemporaries of MRCA are interchanged via sexual reproduction.
It is estimated that the MRCA lived between 5,000 to 2,000 years ago.
Identical ancestors point
The MRCA of living humans had many 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. 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 only a subset of current population.Because ancestors of the MRCA are by definition also common ancestors, we can find (less recent) common ancestors by pushing further back in time to ancient common ancestors of all people alive. 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. 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 the same set of ancestors, all the way to the first single-celled organism.
The identical ancestors point for Homo sapiens has been estimated to between 15,000
Mesolithic
The Mesolithic is an archaeological concept used to refer to certain groups of archaeological cultures defined as falling between the Paleolithic and the Neolithic....
and 5,000
4th millennium BC
The 4th millennium BC saw major changes in human culture. It marked the beginning of the Bronze Age and of writing.The city states of Sumer and the kingdom of Egypt were established and grew to prominence. Agriculture spread widely across Eurasia...
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
Richard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
' book, The Ancestor's Tale
The Ancestor's Tale
The Ancestor's Tale is a 2004 popular science book by Richard Dawkins, with contributions from Dawkins' research assistant Yan Wong. It follows the path of humans backwards through evolutionary history, meeting humanity's cousins as they converge on common ancestors...
, in which he imagines a 'pilgrimage' backwards in time, during which we human
Human
Humans are the only living species in the Homo genus...
s 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
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
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
The Ancestor's Tale
The Ancestor's Tale is a 2004 popular science book by Richard Dawkins, with contributions from Dawkins' research assistant Yan Wong. It follows the path of humans backwards through evolutionary history, meeting humanity's cousins as they converge on common ancestors...
, following the human evolution
Human evolution
Human evolution refers to the evolutionary history of the genus Homo, including the emergence of Homo sapiens as a distinct species and as a unique category of hominids and mammals...
ary tree backwards, we first meet the concestor which we share with the species that are our closest relatives, the chimpanzee
Chimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
and bonobo
Bonobo
The bonobo , Pan paniscus, previously called the pygmy chimpanzee and less often, the dwarf or gracile chimpanzee, is a great ape and one of the two species making up the genus Pan. The other species in genus Pan is Pan troglodytes, or the common chimpanzee...
. 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 (imaginary, in that the journey is going backwards in time), we meet the concestor we share with the Gorilla
Gorilla
Gorillas are the largest extant species of primates. They are ground-dwelling, predominantly herbivorous apes that inhabit the forests of central Africa. Gorillas are divided into two species and either four or five subspecies...
, our next nearest relative, then the Orangutan
Orangutan
Orangutans are the only exclusively Asian genus of extant great ape. The largest living arboreal animals, they have proportionally longer arms than the other, more terrestrial, great apes. They are among the most intelligent primates and use a variety of sophisticated tools, also making sleeping...
, and so on, until we finally meet the concestor of all living organisms, known as the last universal ancestor
Last universal ancestor
The last universal ancestor , also called the last universal common ancestor , or the cenancestor, is the most recent organism from which all organisms now living on Earth descend. Thus it is the most recent common ancestor of all current life on Earth...
.
A common mistake made by the public as well as some authors is to refer to a proposed last common ancestor as an earliest ancestor. Examples include the book The Link: Uncovering Our Earliest Ancestor by Colin Tudge
Colin Tudge
Colin Tudge is a British science writer and broadcaster. A biologist by training, he is the author of numerous works on food, agriculture, genetics, and species diversity....
, and a documentary of the same name screened on the History Channel (US) and BBC One
BBC One
BBC One is the flagship television channel of the British Broadcasting Corporation in the United Kingdom. It was launched on 2 November 1936 as the BBC Television Service, and was the world's first regular television service with a high level of image resolution...
(UK), both referring to the primate fossil dubbed Ida
Darwinius
Darwinius is a genus of Adapiformes, a group of basal primates from the Eocene epoch. Its only known species is Darwinius masillae, dated to 47 million years ago based on dating of the fossil site....
.
MRCA of a population identified by a single genetic marker
It is also possible to consider the ancestry of individual geneGene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s (coalescent theory
Coalescent theory
In genetics, coalescent theory is a retrospective model of population genetics. It attempts to trace all alleles of a gene shared by all members of a population to a single ancestral copy, known as the most recent common ancestor...
), 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
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
, a gene has a single chain of ancestors and a tree of descendants. An organism produced by sexual, allogamous
Allogamy
Allogamy is a term used in the field of biological reproduction describing the fertilization of an ovum from one individual with the spermatozoa of another. By contrast, autogamy is the term used for self-fertilization. In humans, the fertilization event is an instance of allogamy...
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
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
), 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.
Patrilineal and matrilineal MRCA
Mitochondrial DNAMitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
(mtDNA) is nearly immune to sexual mixing, unlike the nuclear DNA
Nuclear DNA
Nuclear DNA, nuclear deoxyribonucleic acid , is DNA contained within a nucleus of eukaryotic organisms. In mammals and vertebrates, nuclear DNA encodes more of the genome than the mitochondrial DNA and is composed of information inherited from two parents, one male, and one female, rather than...
whose chromosomes are shuffled and recombined in Mendelian inheritance
Mendelian inheritance
Mendelian inheritance is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics...
. Mitochondrial DNA, therefore, can be used to trace matrilineal inheritance and to find the Mitochondrial Eve
Mitochondrial Eve
In the field of human genetics, Mitochondrial Eve refers to the matrilineal "MRCA" . In other words, she was the woman from whom all living humans today descend, on their mother's side, and through the mothers of those mothers and so on, back until all lines converge on one person...
(also known as the African Eve), the most recent common ancestor of all humans via the mitochondrial DNA pathway.
Similarly Y chromosome
Y chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
is present as a single sex chromosome in the male individual and is passed on to male descendants without recombination. It can be used to trace patrilineal
Patrilineality
Patrilineality is a system in which one belongs to one's father's lineage. It generally involves the inheritance of property, names or titles through the male line as well....
inheritance and to find the Y-chromosomal Adam
Y-chromosomal Adam
In human genetics, Y-chromosomal Adam is the theoretical most recent common ancestor from whom all living people are descended patrilineally . Many studies report that Y-chromosomal Adam lived as early as around 142,000 years ago and possibly as recently as 60,000 years ago...
, the most recent common ancestor of all humans via the Y-DNA pathway.
Mitochondrial Eve and Y-chromosomal Adam have been established by researchers using genealogical DNA test
Genealogical DNA test
A genealogical DNA test examines the nucleotides at specific locations on a person's DNA for genetic genealogy purposes. The test results are not meant to have any informative medical value and do not determine specific genetic diseases or disorders ; they are intended only to give genealogical...
s. 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.
It is more complicated to infer human ancestry via autosomal chromosomes. Although an autosomal chromosome contains genes that are passed down from parents to children via independent assortment from only one of the two parents, genetic recombination
Genetic recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
(chromosomal crossover
Chromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...
) mixes genes from non-sister chromatids from both parents during meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
, 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. These time 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. Time to genealogical MRCA of all living humans is computed based on non-genetic, methematical models and computer simulations.Since Mitochondrial Eve and Y-chromosomal Adam are traced by single genes via a single ancestral parent lines, the time to these genetic MRCAs will necessarily be greater than that for the genealogical MRCA. This is because single genes will coalesce slower 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.
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 same haplogroupHaplogroup
In the study of molecular evolution, a haplogroup is a group of similar haplotypes that share a common ancestor having the same single nucleotide polymorphism mutation in both haplotypes. Because a haplogroup consists of similar haplotypes, this is what makes it possible to predict a haplogroup...
.
In the case of Y-Chrosomal DNA, TMRCA is arrived at in a different way. Y-DNA haplogroups are defined by single-nucleotide polymorphism in various regions of the Y-DNA. The time to MRCA within a haplogroup is defined by the accumulation of mutations in STR
Short tandem repeat
A short tandem repeat in DNA occurs when a pattern of two or more nucleotides are repeated and the repeated sequences are directly adjacent to each other. The pattern can range in length from 2 to 5 base pairs and is typically in the non-coding intron region...
sequences of the Y-Chromosome of that haplogroup only. Y-DNA network analysis of Y-STR
Y-STR
A Y-STR is a short tandem repeat on the Y-chromosome. Y-STRs are often used in forensics, paternity, and genealogical DNA testing.-Nomenclature:Y-STRs are assigned names by the HUGO gene nomenclature committee....
haplotypes showing 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 the Y-STR
Y-STR
A Y-STR is a short tandem repeat on the Y-chromosome. Y-STRs are often used in forensics, paternity, and genealogical DNA testing.-Nomenclature:Y-STRs are assigned names by the HUGO gene nomenclature committee....
sequence, also called the microsatellite variation, can be regarded as a measure of the time passed since the ancestor founded this particular population. The descendants of Genghis Khan
Descent from Genghis Khan
Descent from Genghis Khan is traceable primarily in Central Asia. His four sons and other immediate descendants are famous by names and by deeds. Later Asian potentates attempted to claim descent from the House of Borjigin even on flimsy grounds. In the 14th century, valid sources all but dried...
or one of his ancestors represents a famous star cluster than can be dated back to the time of Genghis Khan.
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
Demic diffusion
Demic diffusion is a demographic term referring to a migratory model developed by Cavalli-Sforza, that consists of population diffusion into and across an area previously uninhabited by that group, possibly, but not necessarily, displacing, replacing, or intermixing with a pre-existing population...
versus cultural diffusion
Cultural diffusion
In cultural anthropology and cultural geography, cultural diffusion, as first conceptualized by Alfred L. Kroeber in his influential 1940 paper Stimulus Diffusion, or trans-cultural diffusion in later reformulations, is the spread of cultural items—such as ideas, styles, religions, technologies,...
during the European Neolithic
Neolithic Europe
Neolithic Europe refers to a prehistoric period in which Neolithic technology was present in Europe. This corresponds roughly to a time between 7000 BC and c. 1700 BC...
.
TMRCA of all living humans
Estimating time to MRCA of all humans based on the common genealogical usage of the term 'ancestor' is much harder and less accurate compared to estimates of Patrilineal and matrilineal MRCAs. Researches must trace ancestry along both female and male parental lines, and rely on historical and archaeological records.Depending on the survival of isolated lineages without admixture from modern migrations
Human migration
Human migration is physical movement by humans from one area to another, sometimes over long distances or in large groups. Historically this movement was nomadic, often causing significant conflict with the indigenous population and their displacement or cultural assimilation. Only a few nomadic...
and taking into account long-isolated peoples, such as historical societies in central Africa, Australia and remote islands in the South Pacific, the human MRCA was generally assumed to have lived in the Upper Paleolithic
Upper Paleolithic
The Upper Paleolithic is the third and last subdivision of the Paleolithic or Old Stone Age as it is understood in Europe, Africa and Asia. Very broadly it dates to between 40,000 and 10,000 years ago, roughly coinciding with the appearance of behavioral modernity and before the advent of...
period. With the advent of mathematical models and computer simulations, researchers now find that the MRCA of all humans lived remarkably recently, between 2,000 and 4,000 years ago.
Rohde, Olson
Steve Olson (writer)
Steve Olson is a US writer who specializes in science, mathematics, and public policy. He is the author of two nonfiction trade books: Mapping Human History: Genes, Race, and Our Common Origins, which was nominated for the National Book Award in 2002, and Count Down: Six Kids Vie for Glory at...
and Chang (2004) constructed a mathematical model that considered the tendency of individuals to choose mates from the same group, as well as the relative geographical isolation of such groups. The mathematical model with one particular set of parameters showed that the MRCA lived about the year 300 BC and yielded an identical ancestor point (IAP) of 3,000 BC.
The same 2004 Rohde paper also presented results from a computer program based on Monte Carlo simulation
Monte Carlo method
Monte Carlo methods are a class of computational algorithms that rely on repeated random sampling to compute their results. Monte Carlo methods are often used in computer simulations of physical and mathematical systems...
designed to overcome some of the limits of the mathematical model. The program took into consideration realistic population substructure and migration patterns, allowing the researchers to simulate historical human demography. A conservative simulation yielded a mean MRCA date of 1,415 BC and a mean IA date of 5,353 BC. A less conservative simulation gave an MRCA date of AD 55 and an IA date of 2,158 BC.
An explanation of this recent MRCA date is that, while humanity's MRCA was indeed a Paleolithic individual up to early modern times, the European explorers
Age of Discovery
The Age of Discovery, also known as the Age of Exploration and the Great Navigations , was a period in history starting in the early 15th century and continuing into the early 17th century during which Europeans engaged in intensive exploration of the world, establishing direct contacts with...
of the 16th and 17th centuries would have fathered enough offspring so that some "mainland" ancestry by today pervades remote habitats. The possibility remains that an 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 only by genetically testing every living human individual.
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
Ukraine
Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...
, who hunted giant sloths in the forests of North and South America, and who labored to build the Great Pyramid of Khufu
Khufu
Khufu , also known as Cheops or, in Manetho, Suphis , was a Pharaoh of Ancient Egypt's Old Kingdom. He reigned from around 2589 to 2566 BC. Khufu was the second pharaoh of the Fourth Dynasty. He is generally accepted as being the builder of the Great Pyramid of Giza, one of the Seven Wonders of...
".
An assumption that there are no isolated populations is questionable in view of the existence of various uncontacted peoples
Uncontacted peoples
Uncontacted people, also referred to as isolated people or lost tribes, are communities who live, or have lived, either by choice or by circumstance, without significant contact with globalized civilisation....
, who are suspected to have been isolated for many millennia, including the Sentinelese who have been isoltated from the western world and also from the Asian mainland.
See also
- CladeCladeA clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
- a group consisting of an organism and all its descendants - Coalescent theoryCoalescent theoryIn genetics, coalescent theory is a retrospective model of population genetics. It attempts to trace all alleles of a gene shared by all members of a population to a single ancestral copy, known as the most recent common ancestor...
- a retrospective model of population genetics - GenealogyGenealogyGenealogy is the study of families and the tracing of their lineages and history. Genealogists use oral traditions, historical records, genetic analysis, and other records to obtain information about a family and to demonstrate kinship and pedigrees of its members...
- the study of families and the tracing of their lineages and history - Genetic distanceGenetic distanceGenetic distance refers to the genetic divergence between species or between populations within a species. It is measured by a variety of parameters. Smaller genetic distances indicate a close genetic relationship whereas large genetic distances indicate a more distant genetic relationship...
- the genetic divergence between species or between populations within a species - Genetic genealogyGenetic genealogyGenetic genealogy is the application of genetics to traditional genealogy. Genetic genealogy involves the use of genealogical DNA testing to determine the level of genetic relationship between individuals.-History:...
- the application of genetics to traditional genealogyGenealogyGenealogy is the study of families and the tracing of their lineages and history. Genealogists use oral traditions, historical records, genetic analysis, and other records to obtain information about a family and to demonstrate kinship and pedigrees of its members... - Genealogical DNA testGenealogical DNA testA genealogical DNA test examines the nucleotides at specific locations on a person's DNA for genetic genealogy purposes. The test results are not meant to have any informative medical value and do not determine specific genetic diseases or disorders ; they are intended only to give genealogical...
s - examination of nucleotides at specific locations on a person's DNA for genetic genealogy purposes - Lowest common ancestorLowest common ancestorThe lowest common ancestor is a concept in graph theory and computer science. Let T be a rooted tree with n nodes. The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants .The LCA of v and w in T is the shared ancestor of v...
- an analogous concept in graph theory and computer science - Phylogenetic treePhylogenetic treeA phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...
- a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species - Timeline of evolutionTimeline of evolutionThis timeline of evolution of life outlines the major events in the development of life on planet Earth since it first originated until the present day. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations...
- outlines the major events in the development of life on the planet Earth - Timeline of human evolutionTimeline of human evolutionThe timeline of human evolution outlines the major events in the development of human species, and the evolution of humans' ancestors. It includes a brief explanation of some animals, species or genera, which are possible ancestors of Homo sapiens...
- outlines the major events in the development of human species
External links
Documentaries- DNA Mysteries – The Search for Adam – by Spencer Wells – National Geographic, 2008
- The Real Eve: Modern Man's Journey Out of Africa – by Stephen OppenheimerStephen OppenheimerStephen Oppenheimer is a British paediatrician, geneticist, and writer. He is a member of Green Templeton College, Oxford and an honorary fellow of Liverpool School of Tropical Medicine, and carries out and publishes research in the fields of genetics and human prehistory.-Career:Oppenheimer...
– Discovery ChannelDiscovery ChannelDiscovery Channel is an American satellite and cable specialty channel , founded by John Hendricks and distributed by Discovery Communications. It is a publicly traded company run by CEO David Zaslav...
, 2002 - Journey of Man: A Genetic Odyssey (movie) by Spencer Wells – PBS and National Geographic ChannelNational Geographic ChannelNational Geographic Channel, also commercially abbreviated and trademarked as Nat Geo, is a subscription television channel that airs non-fiction television programs produced by the National Geographic Society. Like History and the Discovery Channel, the channel features documentaries with factual...
, 2003