Y-chromosomal Adam
Encyclopedia
In human genetics
, Y-chromosomal Adam ( Y-MRCA
) is the theoretical most recent common ancestor
(MRCA) from whom all living people are descended patrilineally
(tracing back only along the paternal lines of their family tree). 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.
All living humans are also descended matrilineally
from Mitochondrial Eve
who is thought to have lived earlier about 190,000 - 200,000 years ago. Y-chromosomal Adam and Mitochondrial Eve need not have lived at the same time.
to the y-chromosome. Unlike the autosomes, the human y-chromosome does not recombine
with the X-chromosome but is transferred intact from father to son. Mutations periodically occur within the Y-chromosome and these mutations are passed on to males in subsequent generations. These mutations can be used as markers to identify shared patrilineal relationships. Y-chromosomes that share a specific mutation are referred to as Haplogroups. Y-chromosomes within a specific haplogroup share a common patrilineal ancestor who was the first to carry the defining mutation. A family tree of y-chromosomes can be constructed, with the mutations serving as branching points along lineages. Y chromosome Adam is positioned at the root of the family tree as the y-chromosomes of all living males are descended from his y-chromosome.
Researchers can reconstruct ancestral y-chromosome DNA sequences by reversing mutated DNA segments to their original condition. The most likely original or ancestral state of a DNA sequence is determined by comparing human DNA sequences with those of a closely related species, usually non-human primates such as chimpanzees and gorillas. By reversing known mutations in a y-chromosome lineage, a hypothetical ancestral sequence for the MRCA, Y-chromosomal Adam, can be inferred.
Determining Y-chromosomal Adam's DNA sequence, and the time when he lived, involves identifying the human y-chromosome lineages that are most divergent from each other- the lineages that share the least unique mutations with each other when compared to a non-human primate sequence in a phylogenetic tree. The common ancestor of the most divergent lineages is therefore the common ancestor of all lineages.
The existence of Y-chromosomal Adam was confirmed by a worldwide sample of Y-chromosomes that included individuals from all continents. A number of Y-chromosome lineages, or haplogroups, from Africa were found to be the most divergent from each other, and non-African lineages were determined to be subsets of a few lineages found in Africa. This suggested Africa was the most likely home of Y-Chromosomal Adam.
and Haplogroup BT
. Haplogroup A is found at low frequencies in parts of Africa, but is common among certain hunter-gatherer groups. Haplogroup BT lineages represent the majority of African y-chromosome lineages and virtually all non-African lineages. . Y-chromosomal Adam was represented as the root of these two lineages. Haplogroup A and Haplogroup BT represented the lineages of the two sons of Y-chromosomal Adam.
However, a recent paper places this event around 142,000 years ago. Cruciani et al. 2011, determined that the deepest split in the Y-chromosome tree is found between two previously reported subclades of Haplogroup A, rather than between Haplogroup A and Haplogroup BT. Subclades A1b and A1a-T, now descend directly from the root of the tree and now represent the lineages of Y-chromosomal Adam's two sons. The rearrangement of the Y-chromosome family tree implies that lineages classified as Haplogroup A do not necessarily form a monophyletic clade. Haplogroup A therefore refers to a collection of lineages that do not possess the markers that define Haplogroup BT, though haplogroup A includes the most distantly related y-chromosomes.
The M91 and P97 mutations distinguish Haplogroup A from Haplogroup BT
. Within Haplogroup A chromosomes, the M91 marker consists of a stretch of 8 T nucleobase
units. In Haplogroup BT and chimpanzee chromosomes, this marker consists of 9 T nucleobase
units. This pattern suggested that the 9T stretch of Haplogroup BT was the ancestral version and that Haplogroup A was formed by the deletion of one nucleobase
. Haplogroups A1b and A1a were considered subclades of haplogroup A as they both possessed the M91 with 8Ts.
But according to Cruciani et al. 2011, the region surrounding the M91 marker is a mutational hotspot prone to recurrent mutations. It is therefore possible that the 8T stretch of Haplogroup A may be the ancestral state of M91 and the 9T of Haplogroup BT
may be the derived state that arose by an insertion
of 1T. This would explain why subclades A1b and A1a-T, the deepest branches of Haplogroup A, both possess the same version of M91 with 8Ts. Furthermore Cruciani et al. 2011 determined that the P97 marker, which is also used to identify haplogroup A, possessed the ancestral state in haplogroup A but the derived state in Haplogroup BT
.
achieve their highest frequencies in these regions. But according to Cruciani et al. 2011, the most basal lineages have been detected in West
, Northwest and Central Africa
. In a sample of 2204 African y-chromosomes, 8 chromosomes belonged to either haplogroup A1b or A1a. Haplogroup A1a was identified in two Moroccan
Berbers, one Fulbe and one Tuareg from Niger
. Haplogroup A1b was identified in three Bakola pygmies from Southern Cameroon and one Algerian Berber. Cruciani et al. 2011 suggest a Y-chromosome Adam living somewhere in Central-Northwest Africa, fits well with the data. However they suggest that this interpretation is still very preliminary due to the incomplete sampling of African Y-chromosomes, and incomplete knowledge of past demographic events.
, who lived 150,000-200,000 years ago. This date also meant that Y-chromosome Adam lived at a time very close to, and possibly after, the out of Africa migration which is believed to have taken place 50,000-80,000 years ago.
One explanation given for this discrepancy in the dates of Adam and Eve was that females have a better chance of reproducing than males due to the practice of polygyny
. When a male individual has several wives, he has effectively prevented other males in the community from reproducing and passing on their y-chromosomes to subsequent generations. On the other hand, polygyny doesn't prevent most females in a community from passing on their mitochondrial DNA to subsequent generations. This differential reproductive success of males and females can lead to fewer male lineages relative to female lineages persisting into the future. These fewer male lineages are more sensitive to drift and would most likely coalesce on a more recent common ancestor. This would potentially explain the more recent dates associated with Y-chromosome Adam.
The 2011 study by Cruciani et al. found that Y-chromosome Adam lived about 142,000 years ago, significantly earlier than previous estimates, such as the 59,000 years ago estimate proposed by Thomson et al. 2000. The older TMRCA was due to the discovery of additional mutations and the rearrangement of the backbone of the y-chromosome phylogeny following the resequencing of Haplogroup A
lineages. According to the study, determining the precise date when Y-chromosome Adam lived depends on the accuracy of the mutation rate used. But the repositioning of the MRCA from the root of haplogroups A and BT to the root of Haplogroups A1b and A1a still entails that Y-chromosome Adam is older than previously thought. According to Cruciani et al., the much older date is easier to reconcile with models of human origins.
Human genetics
Human genetics describes the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics,...
, Y-chromosomal Adam ( Y-MRCA
Most recent common ancestor
In genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
) is the theoretical most recent common ancestor
Most recent common ancestor
In genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
(MRCA) from whom all living people are descended patrilineally
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....
(tracing back only along the paternal lines of their family tree). 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.
All living humans are also descended matrilineally
Matrilineality
Matrilineality is a system in which descent is traced through the mother and maternal ancestors. Matrilineality is also a societal system in which one belongs to one's matriline or mother's lineage, which can involve the inheritance of property and/or titles.A matriline is a line of descent from a...
from 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...
who is thought to have lived earlier about 190,000 - 200,000 years ago. Y-chromosomal Adam and Mitochondrial Eve need not have lived at the same time.
Hypothesis
The existence of a Y-chromosomal Adam was determined by applying the theories of molecular evolutionMolecular evolution
Molecular evolution is in part a process of evolution at the scale of DNA, RNA, and proteins. Molecular evolution emerged as a scientific field in the 1960s as researchers from molecular biology, evolutionary biology and population genetics sought to understand recent discoveries on the structure...
to the y-chromosome. Unlike the autosomes, the human y-chromosome does not recombine
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...
with the X-chromosome but is transferred intact from father to son. Mutations periodically occur within the Y-chromosome and these mutations are passed on to males in subsequent generations. These mutations can be used as markers to identify shared patrilineal relationships. Y-chromosomes that share a specific mutation are referred to as Haplogroups. Y-chromosomes within a specific haplogroup share a common patrilineal ancestor who was the first to carry the defining mutation. A family tree of y-chromosomes can be constructed, with the mutations serving as branching points along lineages. Y chromosome Adam is positioned at the root of the family tree as the y-chromosomes of all living males are descended from his y-chromosome.
Researchers can reconstruct ancestral y-chromosome DNA sequences by reversing mutated DNA segments to their original condition. The most likely original or ancestral state of a DNA sequence is determined by comparing human DNA sequences with those of a closely related species, usually non-human primates such as chimpanzees and gorillas. By reversing known mutations in a y-chromosome lineage, a hypothetical ancestral sequence for the MRCA, Y-chromosomal Adam, can be inferred.
Determining Y-chromosomal Adam's DNA sequence, and the time when he lived, involves identifying the human y-chromosome lineages that are most divergent from each other- the lineages that share the least unique mutations with each other when compared to a non-human primate sequence in a phylogenetic tree. The common ancestor of the most divergent lineages is therefore the common ancestor of all lineages.
The existence of Y-chromosomal Adam was confirmed by a worldwide sample of Y-chromosomes that included individuals from all continents. A number of Y-chromosome lineages, or haplogroups, from Africa were found to be the most divergent from each other, and non-African lineages were determined to be subsets of a few lineages found in Africa. This suggested Africa was the most likely home of Y-Chromosomal Adam.
Many Adams
The title "Y-chromosome Adam" is not permanently fixed on a single individual. Because knowledge of human Y-chromosomes is still incomplete, Y-chromosome Adam's DNA sequence, his position in the family tree, the time when lived, and his place of origin, are all subject to future revisions. In addition, demographic changes during the course of human evolution would have frequently caused the title of Y-chromosome Adam to change hands. The following events would change the individual designated Y chromosome Adam:- Further sampling of y-chromosomes could uncover previously unknown divergent lineages. If this happens, y-chromosome lineages would converge on an individual who lived further back in time.
- The discovery of additional deep rooting mutations in known lineages could lead to a rearrangement of the family tree.
- When deep rooting haplogroups are permanently lost from the world's population, living human y-chromosomes converge on a more recent common ancestor. A Y-chromosome lineage is halted when a male dies without reproducing or when a male individual only has daughters. Phenomena such as bottlenecksPopulation bottleneckA population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing....
and genetic driftGenetic driftGenetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
during human evolutionHuman evolutionHuman 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...
would have caused the total extinction of several basal haplogroups. Because of these factors, the title "Y-chromosome Adam" has changed hands numerous times.
Family tree
Y-chromosomal Adam had at least two sons and two of his sons have unbroken lineages that have survived to the present day. Initial sequencing of the human y-chromosome suggested that two most basal y-chromosome lineages were Haplogroup AHaplogroup A (Y-DNA)
In human genetics, Haplogroup A refers to a group of y-chromosome lineages that were among the first to branch off from the root of the human y-chromosome phylogeny...
and Haplogroup BT
Haplogroup BT (Y-DNA)
In human genetics, Haplogroup BT is a Y-chromosome haplogroup. The notation BT refers to the derived set of haplogroups between B and T which forms its own cladistic set....
. Haplogroup A is found at low frequencies in parts of Africa, but is common among certain hunter-gatherer groups. Haplogroup BT lineages represent the majority of African y-chromosome lineages and virtually all non-African lineages. . Y-chromosomal Adam was represented as the root of these two lineages. Haplogroup A and Haplogroup BT represented the lineages of the two sons of Y-chromosomal Adam.
However, a recent paper places this event around 142,000 years ago. Cruciani et al. 2011, determined that the deepest split in the Y-chromosome tree is found between two previously reported subclades of Haplogroup A, rather than between Haplogroup A and Haplogroup BT. Subclades A1b and A1a-T, now descend directly from the root of the tree and now represent the lineages of Y-chromosomal Adam's two sons. The rearrangement of the Y-chromosome family tree implies that lineages classified as Haplogroup A do not necessarily form a monophyletic clade. Haplogroup A therefore refers to a collection of lineages that do not possess the markers that define Haplogroup BT, though haplogroup A includes the most distantly related y-chromosomes.
The M91 and P97 mutations distinguish Haplogroup A from Haplogroup BT
Haplogroup BT (Y-DNA)
In human genetics, Haplogroup BT is a Y-chromosome haplogroup. The notation BT refers to the derived set of haplogroups between B and T which forms its own cladistic set....
. Within Haplogroup A chromosomes, the M91 marker consists of a stretch of 8 T nucleobase
Thymine
Thymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be derived by methylation of uracil at...
units. In Haplogroup BT and chimpanzee chromosomes, this marker consists of 9 T nucleobase
Thymine
Thymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be derived by methylation of uracil at...
units. This pattern suggested that the 9T stretch of Haplogroup BT was the ancestral version and that Haplogroup A was formed by the deletion of one nucleobase
Nucleobase
Nucleobases are a group of nitrogen-based molecules that are required to form nucleotides, the basic building blocks of DNA and RNA. Nucleobases provide the molecular structure necessary for the hydrogen bonding of complementary DNA and RNA strands, and are key components in the formation of stable...
. Haplogroups A1b and A1a were considered subclades of haplogroup A as they both possessed the M91 with 8Ts.
But according to Cruciani et al. 2011, the region surrounding the M91 marker is a mutational hotspot prone to recurrent mutations. It is therefore possible that the 8T stretch of Haplogroup A may be the ancestral state of M91 and the 9T of Haplogroup BT
Haplogroup BT (Y-DNA)
In human genetics, Haplogroup BT is a Y-chromosome haplogroup. The notation BT refers to the derived set of haplogroups between B and T which forms its own cladistic set....
may be the derived state that arose by an insertion
Insertion (genetics)
In genetics, an insertion is the addition of one or more nucleotide base pairs into a DNA sequence. This can often happen in microsatellite regions due to the DNA polymerase slipping...
of 1T. This would explain why subclades A1b and A1a-T, the deepest branches of Haplogroup A, both possess the same version of M91 with 8Ts. Furthermore Cruciani et al. 2011 determined that the P97 marker, which is also used to identify haplogroup A, possessed the ancestral state in haplogroup A but the derived state in Haplogroup BT
Haplogroup BT (Y-DNA)
In human genetics, Haplogroup BT is a Y-chromosome haplogroup. The notation BT refers to the derived set of haplogroups between B and T which forms its own cladistic set....
.
Origin
Initial studies implicated East Africa and Southern Africa as the likely sources of human y-chromosome diversity. This was because the basal lineages, Haplogroup A and Haplogroup BHaplogroup B (Y-DNA)
In human genetics, Haplogroup B is a Y-chromosome haplogroup.-Distribution:Haplogroup B is localized to sub-Saharan Africa, especially to tropical forests of West-Central Africa. After Y-haplogroup A, it is the second oldest and one of the most diverse human Y-haplogroups...
achieve their highest frequencies in these regions. But according to Cruciani et al. 2011, the most basal lineages have been detected in West
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...
, Northwest and Central Africa
Central Africa
Central Africa is a core region of the African continent which includes Burundi, the Central African Republic, Chad, the Democratic Republic of the Congo, and Rwanda....
. In a sample of 2204 African y-chromosomes, 8 chromosomes belonged to either haplogroup A1b or A1a. Haplogroup A1a was identified in two Moroccan
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
Berbers, one Fulbe and one Tuareg from Niger
Niger
Niger , officially named the Republic of Niger, is a landlocked country in Western Africa, named after the Niger River. It borders Nigeria and Benin to the south, Burkina Faso and Mali to the west, Algeria and Libya to the north and Chad to the east...
. Haplogroup A1b was identified in three Bakola pygmies from Southern Cameroon and one Algerian Berber. Cruciani et al. 2011 suggest a Y-chromosome Adam living somewhere in Central-Northwest Africa, fits well with the data. However they suggest that this interpretation is still very preliminary due to the incomplete sampling of African Y-chromosomes, and incomplete knowledge of past demographic events.
Nomenclature
Y-chromosomal Adam is named after the Biblical Adam. This may lead to a misconception that he was the only living male of his times, even though he co-existed with plenty of men around, including his own father who was not the "most recent". However, all his other male contemporaries failed to produce a direct unbroken male line to the present day.Time frame
The time when Y-chromosome Adam lived is determined by applying a molecular clock to human y-chromosomes. In contrast to Mitochondrial DNA, which has a short sequence of 16000 base pairs, and mutates frequently, the y-chromosome is significantly longer at 60 million base pairs, and has a lower mutation rate. These features of the y-chromosome have slowed down the identification of its polymorphisms and as a consequence, reduced the accuracy of y-chromosome mutation rate estimates. Initial studies, such as Thomson et al. 2000 proposed that Y-chromosome Adam lived about 59,000 years ago. This date suggested that Y chromosome Adam lived tens of thousands of years after his female counterpart Mitochondrial EveMitochondrial 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...
, who lived 150,000-200,000 years ago. This date also meant that Y-chromosome Adam lived at a time very close to, and possibly after, the out of Africa migration which is believed to have taken place 50,000-80,000 years ago.
One explanation given for this discrepancy in the dates of Adam and Eve was that females have a better chance of reproducing than males due to the practice of polygyny
Polygyny
Polygyny is a form of marriage in which a man has two or more wives at the same time. In countries where the practice is illegal, the man is referred to as a bigamist or a polygamist...
. When a male individual has several wives, he has effectively prevented other males in the community from reproducing and passing on their y-chromosomes to subsequent generations. On the other hand, polygyny doesn't prevent most females in a community from passing on their mitochondrial DNA to subsequent generations. This differential reproductive success of males and females can lead to fewer male lineages relative to female lineages persisting into the future. These fewer male lineages are more sensitive to drift and would most likely coalesce on a more recent common ancestor. This would potentially explain the more recent dates associated with Y-chromosome Adam.
The 2011 study by Cruciani et al. found that Y-chromosome Adam lived about 142,000 years ago, significantly earlier than previous estimates, such as the 59,000 years ago estimate proposed by Thomson et al. 2000. The older TMRCA was due to the discovery of additional mutations and the rearrangement of the backbone of the y-chromosome phylogeny following the resequencing of Haplogroup A
Haplogroup A (Y-DNA)
In human genetics, Haplogroup A refers to a group of y-chromosome lineages that were among the first to branch off from the root of the human y-chromosome phylogeny...
lineages. According to the study, determining the precise date when Y-chromosome Adam lived depends on the accuracy of the mutation rate used. But the repositioning of the MRCA from the root of haplogroups A and BT to the root of Haplogroups A1b and A1a still entails that Y-chromosome Adam is older than previously thought. According to Cruciani et al., the much older date is easier to reconcile with models of human origins.
See also
- ArchaeogeneticsArchaeogeneticsArchaeogenetics, a term coined by Colin Renfrew, refers to the application of the techniques of molecular population genetics to the study of the human past. This can involve:*the analysis of DNA recovered from archaeological remains, i.e...
- Adam's CurseAdam's CurseAdam's Curse: A Future Without Men is a 2003 book by Oxford University human genetics professor Bryan Sykes expounding his hypothesis that with the declining sperm count in men and the continual atrophy of the Y chromosome, within 5,000 generations the male of the...
(book by Bryan SykesBryan SykesBryan Sykes is a former Professor of Human Genetics at the University of Oxford and a current Fellow of Wolfson College.Sykes published the first report on retrieving DNA from ancient bone...
) - Eurasian AdamEurasian AdamEurasian Adam is a name given to the man who was the common male-line ancestor of all men with the single nucleotide polymorphism mutation on the Y chromosome known as "M168"...
- 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...
- 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:...
- Genographic project
- Human Y-chromosome DNA haplogroupsHuman Y-chromosome DNA haplogroupsIn human genetics, a Human Y-chromosome DNA haplogroup is a haplogroup defined by differences in the non-recombining portions of DNA from the Y chromosome ....
- Identical ancestors pointIdentical ancestors pointIn genetic genealogy, the identical ancestors point is that point in a given population's past where each individual then alive turned out to be either the ancestor of every individual alive now, or to have no living descendants at all...
- Most recent common ancestorMost recent common ancestorIn genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
- Paternal mtDNA transmissionPaternal mtDNA transmissionIn genetics, paternal mtDNA transmission and paternal mtDNA inheritance refer to the incidence of mitochondrial DNA being passed from a father to his offspring. Paternal mtDNA inheritance is observed in a small proportion of species; in general, mtDNA is passed unchanged from a mother to her...
- San peopleBushmenThe indigenous people of Southern Africa, whose territory spans most areas of South Africa, Zimbabwe, Lesotho, Mozambique, Swaziland, Botswana, Namibia, and Angola, are variously referred to as Bushmen, San, Sho, Barwa, Kung, or Khwe...
- Single-origin hypothesis
- Y-chromosomal AaronY-chromosomal AaronY-chromosomal Aaron is the name given to the hypothesised most recent common ancestor of many of the patrilineal Jewish priestly caste known as Kohanim . In the Torah, this ancestor is identified as Aaron, the brother of Moses...
Further reading
- "Modern Men Trace Ancestry to African Migrants", A Gibbons, Volume 292, Number 5519, Issue of 11 May 2001, pp. 1051–1052.
- "African Origin of Modern Humans in East Asia: A Tale of 12,000 Y Chromosomes", Yuehai Ke et al., Science 2001 292: 1151–1153
- Bateman, A. J. 1948 Intra-sexual selection in Drosophila. Heredity2, 349–368.
External links
- Documentary Redraws Humans' Family Tree (from National Geographic)
- DNA Mysteries – The Search for Adam (from National Geographic)
- Mitochondrial Eve and Y-chromosomal Adam Diagrams
- Y-Chromosome Biallelic Haplogroups
- Most European males 'descended from farmers'
- Why study the Y: Chromosome reveals path of ancestral humans