Mitochondrial Eve
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. The time she lived is calculated based on the molecular clock technique of correlating elapsed time with observed genetic drift.
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.
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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
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.
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 are not extant, 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. 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. 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 rapidly evolving D-loop in the analysis.
- An outgroup (chimpanzee) mtDNA sequence was used to root the tree (outgroup rooting). Outgroup rooting is much more reliable than midpoint rooting.
The study by Ingman et al. verifies the major conclusions of Cann et al. of an African origin of human mtDNA within the past 172 000 ± 50 000 years.
In popular science
- Bryan Sykes has written a popular science book entitled The Seven Daughters of Eve.
- In River Out of Eden, Richard Dawkins discusses human ancestry in the context of a river of genes and shows that Mitochondrial Eve is one of the many common ancestors we can trace back to via different gene pathways.
- The Discovery Channel produced a documentary entitled The Real Eve, based on the book Out of Eden by Stephen Oppenheimer.
External links
- Krishna Kunchithapadam, "What if anything is a Mitochondrial Eve?": a simple explanation
- Mitochondrial Eve and Y-chromosomal Adam Diagrams
License
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Mitochondrial Eve".
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