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Mitochondrial Eve
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Mitochondrial Eve (mt-mrca) is the name given by researchers to the woman who is defined as the matrilineal most recent common ancestor (MRCA) for all currently living humans. Passed down from mother to offspring, her mitochondrial DNA (mtDNA) is now found in all living humans: every mtDNA in every living person is derived from hers. Mitochondrial Eve is the female counterpart of Y-chromosomal Adam, the patrilineal most recent common ancestor, although they lived at different times.
She is believed to have lived about 140,000 years ago in what is now Ethiopia, Kenya or Tanzania.
Mitochondrial Eve is the MRCA of all humans via the mitochondrial DNA pathway, not the unqualified MRCA of all humanity.
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Encyclopedia
Mitochondrial Eve (mt-mrca) is the name given by researchers to the woman who is defined as the matrilineal most recent common ancestor (MRCA) for all currently living humans. Passed down from mother to offspring, her mitochondrial DNA (mtDNA) is now found in all living humans: every mtDNA in every living person is derived from hers. Mitochondrial Eve is the female counterpart of Y-chromosomal Adam, the patrilineal most recent common ancestor, although they lived at different times.
She is believed to have lived about 140,000 years ago in what is now Ethiopia, Kenya or Tanzania.
Mitochondrial Eve is the MRCA of all humans via the mitochondrial DNA pathway, not the unqualified MRCA of all humanity. All living humans can trace their ancestry back to the MRCA via at least one of their parents, but Mitochondrial Eve is defined via the maternal line. Therefore, she necessarily lived at least as long, though likely much longer, ago than the MRCA of all humanity.
The existence of Mitochondrial Eve and Y-chromosomal Adam does not imply the existence of population bottlenecks or a first couple. They each may have lived within a large human population at a different time.
Matrilineal descent
To find the Mitochondrial Eve of all living humans, one can start by tracing a line from every individual to his/her mother, then continue those lines from each of those mothers to their mothers and so on, effectively tracing a family tree backward in time based purely on mitochondrial lineages. Going back through time these mitochondrial lineages will converge when two or more women have the same mother. The further back in time one goes, the fewer mitochondrial ancestors of living humans there will be. Eventually only one is left, and this one is the most recent common matrilineal ancestor of all humans alive today, i.e. Mitochondrial Eve.
It is possible to draw the same matrilineal tree forward in time by starting with all human female contemporaries of Mitochondrial Eve. Some of these women may have died childless. Others left only male children. For the rest who became mothers with at least one daughter, one can trace a line forward in time connecting them to their daughter(s). As the forward lineages progress in time, more and more lineage lines become extinct, as the last female in a line dies childless or leaves no female children. Eventually, only one single lineage remains, which includes all mothers, and in the next generation, all people, and hence all people alive today.
Misconceptions
Misconception: The Mitochondrial Eve and Most Recent Common Ancestor (MRCA) are the same
Mitochondrial Eve is the most recent common matrilineal ancestor, not the MRCA of all humans. The MRCA's offspring have led to all living humans via sons and daughters, but Mitochondrial Eve must be traced only through female lineages, so she is estimated to have lived much longer ago than the MRCA. According to probabilistic studies, Mitochondrial Eve is thought to have lived around 140,000 years ago. On the arbitrary assumption that people mate with a random individual drawn from the whole of the global population, the theoretical MRCA could have lived as recently as 3,000 years ago. The most recent common ancestor (MRCA) is the most recent person whom all of humanity can count as one of their ancestors. Because each person's number of ancestors double with each generation backwards, the MRCA of all humanity overlaps relatively recently in history. The MRCA answers the question, "Do any of my four grandparents overlap with any of your four grandparents? If not, then do any of my 8 great grandparents overlap with any of your 8 great grandparents?" Between one-hundred and two-hundred generations back, a single person will appear in every living person's family tree. However, each person has only one mitochondrial ancestor with each generation backwards because each person inherits their mitochondria from their mother. The question of "when did Mitochondrial Eve live?" is answering the question of "When does my mother's mother's mother ... overlap with everyone else's mother's mother's mother ... (all the way up the female lineage)?"
Misconception: Mitochrondrial Eve was the only woman alive at that time
Allan Wilson's naming Mitochondrial Eve after Eve of the Genesis creation story has led to some misunderstandings among the general public. A common misconception is that Mitochondrial Eve was the only living human female of her time. Had this been the case, humanity would have long since become extinct due to an extreme example of a population bottleneck.
Indeed, not only were many women alive at the same time as Mitochondrial Eve but many of them have living descendants through their sons. While the mtDNA of these women is gone, their Nuclear genes are present in today's population.
What distinguishes Mitochondrial Eve (and her matrilineal ancestors) from all her female contemporaries is that she has a purely matrilineal line of descent to all humans alive today, whereas all her female contemporaries with descendants alive today have at least one male in every line of descent. Because mitochondrial DNA is only passed through matrilineal descent, all humans alive today have mitochondrial DNA that is traceable back to Mitochondrial Eve.
Furthermore, it can be shown that every female contemporary of Mitochondrial Eve either has no living descendant today or is an ancestor to all living people. Starting with 'the' MRCA at around 3,000 years ago, one can trace all ancestors of the MRCA backward in time. At every ancestral generation, more and more ancestors (via both paternal and maternal lines) of MRCA are found. These ancestors are by definition also common ancestors of all living people. Eventually, there will be a point in 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 today. This point in time is termed the identical ancestors point and is estimated to be between 5,000 and 15,000 years ago. Since Mitochondrial Eve is estimated to have lived more than hundred thousand years before the identical ancestors point, every woman contemporary to her is either not an ancestor of any living people, or a common ancestor of all living people.
Mitochondrial DNA
Mitochondrial organelles, which contain mitochondrial DNA (mtDNA), are passed only from mother to offspring. A comparison of DNA sequences from mtDNA in a population reveals a molecular phylogeny. Unlike mtDNA, which is outside the nucleus, genes containing nuclear DNA become recombined after being inherited from both parents, and therefore we can be statistically less certain about nuclear DNA origins than we can for mtDNA, which is only inherited from the mother. mtDNA also mutates at a higher rate compared to nuclear DNA, so it gives researchers a more useful, magnified view of the diversity present in a population.
Just as mitochondria are inherited matrilineally, Y-chromosomes are inherited patrilineally. Thus it is possible to apply the same principles outlined above to men. The common patrilineal ancestor of all humans alive today has been dubbed Y-chromosomal Adam. Importantly, the genetic evidence suggests that the most recent patriarch of all humanity is much more recent than the most recent matriarch, suggesting that 'Adam' and 'Eve' were not alive at the same time. While 'Eve' is believed to be alive 140,000 years ago, 'Adam' lived only 60,000 years ago.
Mitochondrial Clock
In order to determine the timing of an evolutionary event such as the origins of the common ancestor of human beings living today for which the fossil record is incomplete, Zuckerkandl and Linus Pauling devised the hypothesis of the molecular clock, which dates events based on analysis of change of molecular traits.
An initial assumption for the rate of mutation was made based on the fossil record of human-chimpanzee divergence of 5 million years ago. This rate was then applied to date the mitochondrial eve sequence, placing it between 120,000 to 220,000 years ago. In the 1990s, scientists made a startling discovery. Mitochondrial DNA appeared to mutate faster than expected, which raised troubling questions concerning the dating of evolutionary events. The discovery was made upon comparing the noncoding mitochondrial DNA sequence of the remains of the Russian tsar Nicholas II to that of his relatives. Using this new calibrated clock, puts mitochondrial eve at a mere 6,000 years ago. . A subsequent study that used a far greater area of the mitochondrial sequence, does not confirm the high rate of mutation, and dates mitochondrial eve at 170,000 (+/-50,000) years
Some have suggested a varying mutation rate or an inconsistent molecular clock.
Eve and the Out-of-Africa theory
Since Mitochondrial Eve is believed to have lived in Africa she is sometimes referred to as African Eve, an ancestor who has been hypothesized on the grounds of fossil as well as DNA evidence. According to the most common interpretation of the mitochondrial DNA data, the titles belong to the same hypothetical woman. Family trees (or "phylogenies") constructed on the basis of mitochondrial DNA comparisons show that the living humans whose mitochondrial lineages branched earliest from the tree (L1) are prevalent among the San and the Mbuti people. The subsequent branches of L2 and L3 are also largely confined to Africa, while only the macrogroups M and N, descended from L3, participated in the migration out of Africa.
Researchers therefore reason that all living humans descend from Africans, some of whom migrated out of Africa and populated the rest of the world. If the mitochondrial analysis is correct, then because mitochondrial Eve represents the root of the mitochondrial family tree, she must have predated the exodus and lived in Africa. Therefore many researchers take the mitochondrial evidence as support for the "single-origin" or Out-of-Africa model.
Some people use the mtDNA family tree as evidence of a population bottleneck (e.g. Toba catastrophe theory) giving rise to the human species. There are, however, many ways such family trees can be constructed. A tree can be constructed based on any gene, not just the mitochondrial DNA. When different such trees including the mtDNA tree are compared, no population bottleneck is found because different trees show different coalescent points. The inconsistencies between coalescent points indicate that there had been numerous gene interchanges between population groups around the world, even after the first exodus out of Africa. This idea forms the basis of Alan Templeton's 'Out of Africa Again and Again' theory.
The Mitochondrial DNA provides another support for the Out of Africa hypothesis in the form of gene diversity. One finding not subject to interpretation is that the greatest diversity of mitochondrial DNA sequences exists among Africans. This diversity is believed to have accumulated because humans have been living longer in Africa than anywhere, although the same relative diversity can also be explained if just more people lived in Africa than in other regions - an interpretation of the past that all evolutionary models also accept, even those that contradict the African-origin theory, such as Multiregional evolution.
Academic investigation
Criticisms of the original paper by Cann, Stoneking and Wilson include:
- Of the 147 persons sampled, only two were from sub-Saharan Africa. The other 18 'Africans' in the study were black Americans.
- The method to generate the tree was not guaranteed to find the most parsimonious tree.
- The method used to root the tree placed it at the midpoint of the longest branch (midpoint rooting). This could lead to a wrong position of the root if for example the rate of evolution is higher in Africa.
- Restriction-fragment length polymorphism is ill-suited for estimation of mutation rates which is essential in timing evolutionary events.
- Weak statistical analysis.
Ingman et al. (2000) repeated the study while avoiding its major pitfalls:
- They sampled 53 persons, 32 of whom were Africans from different regions of sub-Saharan Africa.
- They sequenced the complete mtDNA but excluded the evolutionarily unpredictable D-loop in the analysis.
The study by Ingman et al. verifies the major conclusions of Cann et al. of an African origin of human mtDNA within the past 171,500 ± 50,000 years.
In popular science
See also
External links
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