| Haplogroup L | |
|---|---|
 | |
| Possible time of origin | c. 100–230 kya |
| Possible place of origin | East Africa |
| Ancestor | n/a |
| Descendants | Mitochondrial macro-haplogroups L0, L1, and L5 |
| Defining mutations | None |
In human genetics, the Mitochondrial Eve (also mt-Eve, mt-MRCA) is the matrilineal most recent common ancestor (MRCA) of all currently living humans, i.e., the most recent woman from whom all living humans descend in an unbroken line purely through their mothers, and through the mothers of those mothers, back until all lines converge on one woman.
In terms of mitochondrial haplogroups, the mt-MRCA is situated at the divergence of macro-haplogroup L into L0 and L1–6. As of 2013, estimates on the age of this split ranged at around 150,000 years ago, consistent with a date later than the speciation of Homo sapiens but earlier than the recent out-of-Africa dispersal.
The male analog to the "Mitochondrial Eve" is the "Y-chromosomal Adam" (or Y-MRCA), the individual from whom all living humans are patrilineally descended. As the identity of both matrilineal and patrilineal MRCAs is dependent on genealogical history (pedigree collapse), they need not have lived at the same time. As of 2013, estimates for the age Y-MRCA are subject to substantial uncertainty, with a wide range of times from 180,000 to 580,000 years ago (with an estimated age of between 120,000 and 156,000 years ago, roughly consistent with the estimate for mt-MRCA.).
The name "Mitochondrial Eve" alludes to biblical Eve. This led to repeated misrepresentations or misconceptions in journalistic accounts on the topic. Popular science presentations of the topic usually point out such possible misconceptions by emphasizing the fact that the position of mt-MRCA is neither fixed in time (as the position of mt-MRCA moves forward in time as mitochondrial DNA (mtDNA) lineages become extinct), nor does it refer to a "first woman", nor the only living female of her time, nor the first member of a "new species".
History
Early research
Early research using molecular clock methods was done during the late 1970s to early 1980s.
Allan Wilson, Mark Stoneking, Rebecca L. Cann and Wesley M. Brown found that mutation
in human mtDNA was unexpectedly fast, at 0.02 substitution per base
(1%) in a million years, which is 5–10 times faster than in nuclear DNA.
Related work allowed for an analysis of the evolutionary relationships among gorillas, chimpanzees (common chimpanzee and bonobo) and humans.
With data from 21 human individuals, Brown published the first estimate on the age of the mt-MRCA at 180,000 years ago in 1980.
A statistical analysis published in 1982 was taken as evidence for recent African origin (a hypothesis which at the time was competing with Asian origin of H. sapiens).
1987 publication
By 1985, data from the mtDNA of 145 women of different populations, and of two cell lines, HeLa and GM 3043, derived from a Black American and a !Kung respectively, was available. After more than 40 revisions of the draft, the manuscript was submitted to Nature in late 1985 or early 1986
and published on 1 January 1987. The published conclusion was that all
current human mtDNA originated from a single population from Africa, at
the time dated to between 140,000 and 200,000 years ago.
The dating for "Eve" was a blow to the multiregional hypothesis, which was being controversially discussed at the time, and a boost to the theory of the recent origin model.
Cann, Stoneking and Wilson did not use the term "Mitochondrial
Eve" or even the name "Eve" in their original paper; it appears to
originate with a 1987 article in Science by Roger Lewin, headlined "The Unmasking of Mitochondrial Eve."
The biblical connotation was very clear from the start. The accompanying research news in Nature had the title "Out of the garden of Eden."
Wilson himself preferred the term "Lucky Mother" and thought the use of the name Eve "regrettable."
But the concept of Eve caught on with the public and was repeated in a Newsweek
cover story (11 January 1988 issue featured a depiction of Adam and Eve
on the cover, with the title "The Search for Adam and Eve"), and a cover story in Time on 26 January 1987.
Criticism and later research
Shortly after the 1987 publication, criticism of its methodology and secondary conclusions was published.
Both the dating of mt-Eve and the relevance of the age of the purely
matrilineal descent for population replacement was controversially
discussed during the 1990s;
Alan Templeton
(1997) asserted that the study did "not support the hypothesis of a
recent African origin for all of humanity following a split between
Africans and non-Africans 100,000 years ago" and also did "not support
the hypothesis of a recent global replacement of humans coming out of
Africa."
Cann, Stoneking & Wilson (1987)'s
placement of a relatively small population of humans in sub-Saharan
Africa was consistent with the hypothesis of Cann (1982) and lent
considerable support for the "recent out-of-Africa" scenario.
In 1999 Krings et al. eliminated problems in molecular clocking postulated by Nei (1992)[citation needed]
when it was found that the mtDNA sequence for the same region was
substantially different from the MRCA relative to any human sequence.
Although the original research did have analytical limitations, the estimate on the age of the mt-MRCA has proven robust. More recent age estimates have remained consistent with the 140–200 kya
estimate published in 1987: A 2013 estimate dated Mitochondrial Eve to
about 160 kya (within the reserved estimate of the original research)
and Out of Africa II to about 95 kya.
Another 2013 study (based on genome sequencing of 69 people from 9 different populations) reported the age of Mitochondrial Eve between 99 to 148 kya and that of the Y-MRCA between 120 and 156 kya.
Female and mitochondrial ancestry
Through random drift
or selection the female-lineage will trace back to a single female,
such as Mitochondrial Eve. In this example over five generations colors
represent extinct matrilineal lines and black the matrilineal line
descended from mtDNA MRCA.
Without a DNA sample, it is not possible to reconstruct the complete genetic makeup (genome)
of any individual who died very long ago. By analysing descendants'
DNA, however, parts of ancestral genomes are estimated by scientists.
Mitochondrial DNA (mtDNA) and Y-chromosome DNA are commonly used to
trace ancestry in this manner. mtDNA is generally passed un-mixed from
mothers to children of both sexes, along the maternal line, or matrilineally. Matrilineal descent goes back to our mothers, to their mothers, until all female lineages converge.
Branches are identified by one or more unique markers which give a mitochondrial "DNA signature" or "haplotype" (e.g. the CRS is a haplotype). Each marker is a DNA base-pair that has resulted from an SNP mutation.
Scientists sort mitochondrial DNA results into more or less related
groups, with more or less recent common ancestors. This leads to the
construction of a DNA family tree where the branches are in biological terms clades, and the common ancestors such as Mitochondrial Eve sit at branching points in this tree. Major branches are said to define a haplogroup (e.g. CRS belongs to haplogroup H), and large branches containing several haplogroups are called "macro-haplogroups".
Simplified Human mitochondrial phylogeny
The mitochondrial clade which Mitochondrial Eve defines is the species Homo sapiens sapiens itself, or at least the current population or "chronospecies"
as it exists today. In principle, earlier Eves can also be defined
going beyond the species, for example one who is ancestral to both
modern humanity and Neanderthals, or, further back, an "Eve" ancestral to all members of genus Homo and chimpanzees in genus Pan. According to current nomenclature,
Mitochondrial Eve's haplogroup was within mitochondrial haplogroup L because this macro-haplogroup contains all surviving human mitochondrial lineages today, and she must predate the emergence of L0.
The variation of mitochondrial DNA between different people can
be used to estimate the time back to a common ancestor, such as
Mitochondrial Eve. This works because, along any particular line of
descent, mitochondrial DNA accumulates mutations at the rate of
approximately one every 3,500 years per nucleotide.
A certain number of these new variants will survive into modern times
and be identifiable as distinct lineages. At the same time some
branches, including even very old ones, come to an end, when the last
family in a distinct branch has no daughters.
Mitochondrial Eve is the most recent common matrilineal ancestor
for all modern humans. Whenever one of the two most ancient branch lines
dies out, the MRCA will move to a more recent female ancestor, always
the most recent mother to have more than one daughter with living
maternal line descendants alive today. The number of mutations that can
be found distinguishing modern people is determined by two criteria:
firstly and most obviously, the time back to her, but secondly and less
obviously by the varying rates at which new branches have come into
existence and old branches have become extinct. By looking at the number
of mutations which have been accumulated in different branches of this
family tree, and looking at which geographical regions have the widest
range of least related branches, the region where Eve lived can be
proposed.
Popular reception and misconceptions
Newsweek reported on Mitochondrial Eve based on the Cann et al.
study in January 1988, under a heading of "Scientists Explore a
Controversial Theory About Man's Origins". The edition sold a record
number of copies.
The popular name "mitochondrial Eve", of 1980s coinage,
has contributed to a number of popular misconceptions. At first, the
announcement of a "mitochondrial Eve" was even greeted with endorsement
from young earth creationists, who viewed the theory as a validation of the biblical creation story.
Due to such misunderstandings, authors of popular science
publications since the 1990s have been emphatic in pointing out that
the name is merely a popular convention, and that the mt-MRCA was not in
any way the "first woman".
Her position is purely the result of genealogical history of human
populations later, and as matrilineal lineages die out, the position of
mt-MRCA keeps moving forward to younger individuals over time.
In River Out of Eden (1995), Richard Dawkins discussed human ancestry in the context of a "river of genes", including an explanation of the concept of Mitochondrial Eve.
The Seven Daughters of Eve (2002) presented the topic of human mitochondrial genetics to a general audience.
The Real Eve: Modern Man's Journey Out of Africa" by Stephen Oppenheimer (2003) was adapted into a Discovery Channel documentary.
Not the only woman
One common misconception surrounding mitochondrial Eve is that since all women alive today descended in a direct unbroken female line from her, she must have been the only woman alive at the time. However, nuclear DNA
studies indicate that the size of the ancient human population never
dropped below tens of thousands. Other women living during Eve's time
may have descendants alive today but not in a direct female line.
Not a fixed individual over time
The
definition of mitochondrial Eve is fixed, but the woman in prehistory
who fits this definition can change. That is, not only can our knowledge
of when and where Mitochondrial Eve lived change due to new
discoveries, but the actual mitochondrial Eve can change. The
mitochondrial Eve can change, when a mother-daughter line comes to an
end. It follows from the definition of Mitochondrial Eve that she had at
least two daughters who both have unbroken female lineages that
have survived to the present day. In every generation mitochondrial
lineages end – when a woman with unique mtDNA dies with no daughters.
When the mitochondrial lineages of daughters of mitochondrial Eve die
out, then the title of "Mitochondrial Eve" shifts forward from the
remaining daughter through her matrilineal descendants, until the first
descendant is reached who had two or more daughters who together have
all living humans as their matrilineal descendants. Once a lineage has
died out it is irretrievably lost and this mechanism can thus only shift
the title of "Mitochondrial Eve" forward in time.
Because mtDNA mapping of humans is very incomplete, the discovery
of living mtDNA lines which predate our current concept of
"Mitochondrial Eve" could result in the title moving to an earlier
woman. This happened to her male counterpart, "Y-chromosomal Adam,"
when older Y lines from Africa were discovered.
Not necessarily a contemporary of "Y-chromosomal Adam"
Sometimes mitochondrial Eve is assumed to have lived at the same time as Y-chromosomal Adam
(from whom all living people are descended patrilineally), and perhaps
even met and mated with him. Even if this were true, which is currently
regarded as highly unlikely, this would only be a coincidence. Like
mitochondrial "Eve", Y-chromosomal "Adam" probably lived in Africa. A
recent study (March 2013) concluded however that "Eve" lived much later
than "Adam" – some 140,000 years later. (Earlier studies considered, conversely, that "Eve" lived earlier than "Adam".) More recent studies indicate that mitochondrial Eve and Y-chromosomal Adam may indeed have lived around the same time.
Mitochondrial Eve is the most recent common matrilineal ancestor, not the most recent common ancestor.
Since the mtDNA is inherited maternally and recombination is either
rare or absent, it is relatively easy to track the ancestry of the
lineages back to a MRCA; however, this MRCA is valid only when
discussing mitochondrial DNA. An approximate sequence from newest to
oldest can list various important points in the ancestry of modern human
populations:
- The human MRCA. Monte Carlo simulations suggest the MRCA was born surprisingly recently, perhaps even within the last 5,000 years, even for people born on different continents.
- The identical ancestors point. Just a few thousand years before the most recent single ancestor shared by all living humans was the time at which all humans who were then alive either left no descendants alive today or were common ancestors of all humans alive today. In other words, "each present-day human has exactly the same set of genealogical ancestors" alive at the "identical ancestors point" in time. This is far more recent than when Mitochondrial Eve lived.
- Mitochondrial Eve, the most recent female-line common ancestor of all living people.
- "Y-chromosomal Adam", the most recent male-line common ancestor of all living people.
