Y-chromosomal Adam
In human genetics, the Y-chromosomal Adam, is the patrilineal most recent common ancestor from whom all currently living humans are descended. He is the most recent male from whom all living humans are descended through an unbroken line of their male ancestors. The term Y-MRCA reflects the fact that the Y chromosomes of all currently living human males are directly derived from the Y chromosome of this remote ancestor.
The analogous concept of the matrilineal most recent common ancestor is known as "Mitochondrial Eve", the most recent woman from whom all living humans are descended matrilineally. As with "Mitochondrial Eve", the title of "Y-chromosomal Adam" is not permanently fixed to a single individual, but can advance over the course of human history as paternal lineages become extinct.
Estimates of the time when Y-MRCA lived have also shifted as modern knowledge of human ancestry changes. For example, in 2013, the discovery of a previously unknown Y-chromosomal haplogroup was announced,
which resulted in a slight adjustment of the estimated age of the human Y-MRCA.
By definition, it is not necessary that the Y-MRCA and the mt-MRCA should have lived at the same time.
While estimates as of 2014 suggested the possibility that the two individuals may well have been roughly contemporaneous, the discovery of the archaic Y-haplogroup has pushed back the estimated age of the Y-MRCA beyond the most likely age of the mt-MRCA. As of 2015, estimates of the age of the Y-MRCA range around 200,000 to 300,000 years ago, roughly consistent with the emergence of anatomically modern humans.
Y-chromosomal data taken from a Neanderthal from El Sidrón, Spain, produced a Y-T-MRCA of 588,000 years ago for Neanderthal and Homo sapiens patrilineages, dubbed ante Adam, and 275,000 years ago for Y-MRCA.
Definition
The Y-chromosomal most recent common ancestor is the most recent common ancestor of the Y-chromosomes found in currently living human males.Due to the definition via the "currently living" population, the identity of a MRCA, and by extension of the human Y-MRCA, is time-dependent.
The MRCA of a population may move forward in time as archaic lineages within the population go extinct:
once a lineage has died out, it is irretrievably lost. This mechanism can thus only shift the title of Y-MRCA forward in time. Such an event could be due to the total extinction of several basal haplogroups.
The same holds for the concepts of matrilineal and patrilineal MRCAs: it follows from the definition of Y-MRCA that he had at least two sons who both have unbroken lineages that have survived to the present day. If the lineages of all but one of those sons die out, then the title of Y-MRCA shifts forward from the remaining son through his patrilineal descendants, until the first descendant is reached who had at least two sons who both have living, patrilineal descendants. The title of Y-MRCA is not permanently fixed to a single individual, and the Y-MRCA for any given population would himself have been part of a population which had its own, more remote, Y-MRCA.
Although the informal name "Y-chromosomal Adam" is a reference to the biblical Adam, this should not be misconstrued as implying that the bearer of the chromosome was the only human male alive during his time.
His other male contemporaries may also have descendants alive today, but not, by definition, through solely patrilineal descent; in other words, none of them have an unbroken male line of descendants connecting them to currently living people.
By the nature of the concept of most recent common ancestors, these estimates can only represent a terminus ante quem, until the genome of the entire population has been examined.
Age estimate
Estimates on the age of the Y-MRCA crucially depend on the most archaic known haplogroup extant in contemporary populations., this is haplogroup A00. Age estimates based on this published during 2014-2015 range between 160,000 and 300,000 years, compatible with the time of emergence and early dispersal of Homo sapiens.Method
In addition to the tendency of the title of Y-MRCA to shift forward in time, the estimate of the Y-MRCA's DNA sequence, his position in the family tree, the time when he lived, and his place of origin, are all subject to future revisions.The following events would change the estimate of who the individual designated as Y-MRCA was:
- 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.
- Revision of the Y-chromosome mutation rate can change the estimate of the time when he lived.
Methods of estimating the age of the Y-MRCA for a population of human males whose Y-chromosomes have been sequenced are based on applying the theories of molecular evolution to the Y chromosome. Unlike the autosomes, the human Y-chromosome does not recombine often with the X chromosome during meiosis, but is usually transferred intact from father to son; however, it can recombine with the X chromosome in the pseudoautosomal regions at the ends of the Y chromosome. Mutations occur periodically 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. Men belonging to a specific Y-DNA 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. The Y-MRCA 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 the Y-MRCA'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 fewest 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.
History of estimates
Early estimates of the age for the Y-MRCA published during the 1990s ranged between roughly 200 and 300 thousand years ago.Such estimates were later substantially revised downward, as in Thomson et al. 2000, which proposed an age of about 59,000.
This date suggested that the Y-MRCA lived about 84,000 years after his female counterpart mt-MRCA, who lived 150,000–200,000 years ago.
This date also meant that Y-chromosomal Adam lived at a time very close to, and possibly after, the migration from Africa which is believed to have taken place 50,000–80,000 years ago.
One explanation given for this discrepancy in the time depths of patrilineal vs. matrilineal lineages 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 does not 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 the Y-MRCA.
The "hyper-recent" estimate of significantly below 100 kya was again corrected upward in studies of the early 2010s, which ranged at about 120 kya to 160 kya.
This revision was due to the rearrangement of the backbone of the Y-chromosome phylogeny following the resequencing of Haplogroup A lineages.
In 2013, Francalacci et al. reported the sequencing of male-specific single-nucleotide Y-chromosome polymorphisms from 1204 Sardinian males, which indicated an estimate of 180,000 to 200,000 years for the common origin of all humans through paternal lineage.
Also in 2013, Poznik et al. reported the Y-MRCA to have lived between 120,000 and 156,000 years ago, based on genome sequencing of 69 men from 9 different populations.
In addition, the same study estimated the age of Mitochondrial Eve to about 99,000 and 148,000 years. As these ranges overlap for a time-range of 28,000 years, the results of this study have been cast in terms of the possibility that "Genetic Adam and Eve may have walked on Earth at the same time" in the popular press.
The announcement by Mendez et al. of the discovery of a previously unknown lineage, haplogroup A00, in 2013, resulted in another shift in the estimate for the age of Y-chromosomal Adam. The authors estimated the split from the other haplogroups at 338,000 years ago, but later Elhaik et al. dated it to between 163,900 and 260,200 years ago, and Karmin et al. dated it to between 192,000 and 307,000 years ago. The same study reports that non-African populations converge to a cluster of Y-MRCAs in a window close to 50 kya, and an additional bottleneck for non-African populations at about 10 kya, interpreted as reflecting cultural changes increasing the variance in male reproductive success in the Neolithic.