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Mitochondrial Eve

2007 Schools Wikipedia Selection. Related subjects: Evolution and
reproduction

   Mitochondrial Eve (mt-mrca) is the name given by researchers to the
   woman who is the matrilineal most recent common ancestor for all living
   humans; the mitochondrial DNA (mtDNA) in all living humans is derived
   from hers. Mitochondrial Eve is the female counterpart of the
   Y-chromosomal Adam, the patrilineal most recent common ancestor.

   The holder of this title is believed by some to have lived about
   150,000 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.

Matrilineal descent

   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 — she was not. Had she been the only living
   female of her time, humanity would most likely have become extinct due
   to population bottleneck. Many women alive at the same time as
   Mitochondrial Eve have descendants alive today. Some of those women may
   even be ancestors to all humans alive today while others may be
   ancestors to only some of the humans alive today. However, only
   Mitochondrial Eve, and her matrilineal ancestors, have a pure
   matrilineal line of descent to all humans alive today. Because
   mitochondrial DNA is passed through matrilineal descent, all humans
   alive today have mitochondrial DNA that is traceable back to
   Mitochondrial Eve.

   To find the Mitochondrial Eve of all humans living today, one can start
   by listing all individuals alive today. For every individual (males and
   females), trace a line from the 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, until only one is left. This 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 by starting with all
   contemporary human females of Mitochondrial Eve. Some of these women
   may have died childless. Others left only male children. For the rest
   who became mothers with a least one daughter, one can trace a line
   forward in time connecting them to their daughters. As the forward
   lineages progress in time, more and more lineage lines become extinct,
   because the last female in the line dies childless or left no female
   children. Eventually, only one single lineage remains which includes
   all mothers alive today and their male and female children.

   Mitochondrial Eve is the most recent common matrilineal
   (female-lineage) ancestor for mtDNA, not the most recent common
   ancestor (MRCA) of all humans. The MRCA's offspring have led to all
   living humans, but Mitochondrial Eve must be traced only through female
   lineages, so she is estimated to have lived much longer ago than the
   MRCA. While Mitochondrial Eve is thought to have been living around
   150,000 years ago, the MRCA is estimated to have been living only
   10,000 plus years ago.

Mitochondrial DNA

   We know about Eve because of mitochondrial organelles that are passed
   only from mother to offspring. Each mitochondrion contains
   Mitochondrial DNA (mtDNA). A comparison of DNA sequences from mtDNA in
   a population reveals a molecular phylogeny. Unlike mtDNA, which is
   outside the nucleus, genes in nuclear DNA become mixed because of
   genetic recombination, and therefore we can be statistically less
   certain about their origins. Diversity is magnified in mtDNA and
   population bottlenecks are particularly magnified (Wilson et al 1985).

   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.

Academic investigation

   The original paper by Cann et al. (1987) troubled some people. Some
   criticism was indeed legitimate:
     * Of the 147 persons sampled, only two were from sub-Saharan Africa.
       The other 18 'Africans' in the study were Afro-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 to estimate
       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 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
   at. that is: a recent (172 ± 50 kyr) African origin of human mtDNA.

Eve and the Out-of-Africa theory

   Mitochondrial Eve 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 are indigenous
   Africans, whereas the lineages of indigenous peoples on other
   continents all branch off from African lines. Researchers therefore
   reason that all living humans descend from Africans, some of whom
   migrated out of Africa to populate 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.

   Since phylogeny has theoretical as well as practical (computational)
   limitations, it is hard if not impossible to find the best tree to
   match experimental data, and therefore room is left for discussion.
   Critics of the "African genesis" model argue that the mitochondrial
   evidence can be explained by natural selection acting on a single gene.
   This explanation is compelling because nuclear genes do not show the
   same "evidence" that the mitochondrial gene does. Moreover, some trees
   associate Eve most closely to the indigenous peoples of Asia. As of
   2003 the current debate focuses more on questions of dating an event
   that is generally considered proven. This will continue to prove futile
   because of natural selection.

   The strongest support that mitochondrial DNA offers for the
   African-origin hypothesis may not depend on family trees. One finding
   not subject to interpretation is that the greatest diversity of
   mitochondrial DNA sequences exists among Africans. This diversity would
   not have accumulated, researchers argue, if humans had not been living
   longer in Africa than anywhere else. Yet this too proves to be a mixed
   blessing of "support" for the Eve theory since the same relative
   diversity is explained if more people lived in Africa than in other
   regions - an interpretation of the past that all evolutionary models
   accept, even those that contradict the Eve theory, such as
   Multiregional evolution.

In popular culture

     * 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 the Mitochondrial
       Eve is one of the many common ancestors we can trace back to via
       different gene pathways.
     * The Discovery Channel has produced a documentary entitled The Real
       Eve.
     * The Japanese novel, horror film and video game series Parasite Eve
       uses the Mitochondrial Eve theory as the basis for a fantasy about
       a scientist resurrecting his wife by regenerating her liver cells,
       with disastrous effects.
     * Greg Egan has written a short story called Mitochondrial Eve.

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