In human genetics, Y-chromosomal most recent common ancestor refers to the
most recent common ancestor from whom all currently living people are
descended patrilineally. The term Y-MRCA reflects the fact that the Y chromosomes
of all currently living 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.
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. 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. Current estimates of the Y-MRCA range
around 200,000 to 300,000 years ago, consistent with the emergence of
anatomically modern humans and overlapping with age estimates for the
mt-MRCA. By definition, it is not necessary that
the Y-MRCA and the mt-MRCA should have lived at the same time, even though
current estimates suggest the possibility that the two individuals may
well have been roughly contemporaneous. Definition
The existence of a Y-MRCA for any given human population is a consequence of the
fact that every human being has a biological father, and that every human
male inherits his Y chromosome from his biological father. We assume quite
reasonably that every man inherited his Y chromosome from some common ancestor.
So there had to be a male who was the last one from whom all men inherit their
Y chromosome. 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 also have
descendants alive today, but not, by definition, through solely patrilineal
descent. 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. Age estimate
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. = Method =
The time when Y-MRCA lived is determined by applying a molecular clock to human
Y-chromosomes. In contrast to mitochondrial DNA, which has a short
sequence of 16,000 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; as a consequence, they have reduced the accuracy of
Y-chromosome mutation rate estimates. 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. Y chromosomes within a
specific haplogroup are assumed to 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. = Estimates =
Current estimates for the age for the Y-MRCA are roughly compatible with the
estimate for the emergence of anatomically modern humans some 200,000
years ago, although there are substantial uncertainties.
Early estimates published during the 1990s ranged between roughly 200 and 300
kya, Such estimates were later substantially corrected 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
discovery of additional mutations and 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 men,
which indicated an estimate of 180,000 to 200,000 years for the common origin
of all humans through paternal lineage. or again as high as 180 to 200 kya. 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 of yet another
discovery of a previously unknown lineage, haplogroup A00, in 2013,
resulted in another shift in the estimate for the age of Y-chromosomal.
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 50kya, and an additional bottleneck for non-African
populations at about 10kya, interpreted as reflecting cultural changes
increasing the variance in male reproductive success in the Neolithic.
Family tree Initial sequencing of the human Y
chromosome suggested that two most basal Y-chromosome lineages were Haplogroup A
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 male descendants of Y-chromosomal Adam.
Cruciani et al. 2011, determined that the deepest split in the Y-chromosome
tree was found between two previously reported subclades of Haplogroup A,
rather than between Haplogroup A and Haplogroup BT. Subclades A1b and A1a-T
are now believed to 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.
Likely geographic origin As current estimates on TMRCA converge
with estimates for the age of anatomically modern humans and well
predate the Out of Africa migration, geographical origin hypotheses continue
to be limited to the African continent. According to Cruciani et al. 2011, the
most basal lineages have been detected in West, Northwest and Central Africa,
suggesting plausibility for the Y-MRCA living in the general region of
"Central-Northwest Africa". Scozzari et al. agreed with a plausible
placement in "the north-western quadrant of the African continent" for the
emergence of the A1b haplogroup. The 2013 report of haplogroup A00 found
among the Mbo people of western present-day Cameroon is also compatible
with this picture. The revision of Y-chromosomal phylogeny
since 2011 has affected estimates for the likely geographical origin of Y-MRCA
as well as estimates on time depth. By the same reasoning, future discovery of
presently-unknown archaic haplogroups in living people would again lead to such
revisions. In particular, the possible presence of between 1% and 4%
Neanderthal-derived DNA in Eurasian genomes implies that the event of a
discovery of a single living Eurasian male exhibiting a Neanderthal
patrilineal line would immediately push back TMRCA to at least twice its current
estimate. Questions of geographical origin would become part of the debate
on Neanderthal evolution from Homo erectus.
See also References
Further reading External links
Documentary Redraws Humans' Family Tree DNA Mysteries – The Search for Adam
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