'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 living
humans. Passed down from mothers to offspring for over a hundred thousand years, 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 most recent common ancestor 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 can only be reached via the maternal line. Therefore, she necessarily lived 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 co-existed within a large human population. Some of these contemporaries have no living descendants today, and others are ancestors of all people alive today. No contemporary of Mitochondrial Eve is an ancestor of only a subset of people alive today, because she lived much longer ago than the
identical ancestors point.
Matrilineal descent
Mitochondrial Eve is the
most recent common ancestor (MRCA) of all humans ''via the mitochondrial DNA pathway''. In other words, she is the MRCA found when ancestry of all living humans is traced back in time, following only the maternal lineage. The mitochondrial DNA pathway is equivalent to maternal lineage, because
Mitochondrial DNA is only passed down from mother to child, never father to child.
[1]
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 forward in time 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 at 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, 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
most recent common ancestor (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. While Mitochondrial Eve is thought to have been living around 140,000 years ago, according to probabilistic studies,
[2] the MRCA could have been living as recently as 3,000 years ago.
[3]
Allan Wilson's naming Mitochondrial Eve
[4] after
Eve of the
Genesis creation story may be considered unfortunate in that it 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 an extreme
population bottleneck.
Indeed not only were many women alive at the same time as Mitochondrial Eve but many of them have descendants alive today. They may have left descendants via either son or daughters (and grandsons or granddaughters, and so on).
Nuclear genes from these contemporary women of Mitochondrial Eve may be present in today's population, but mitochondrial DNA from them is not.
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 contemporaries 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 contemporary woman 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 contemporary woman of hers is either not an ancestor of any living people, or a common ancestor of all living people.
Mitochondrial DNA
Main articles: 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 outside the nucleus, genes in
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.
[6][Bryan Sykes ''The Seven Daughters of Eve: The Science That Reveals Our Genetic Ancestry'', W.W. Norton, 2001, hardcover, 306 pages, ISBN 0-393-02018-5]
Just as mitochondria are inherited matrilineally,
Y-chromosomes are inherited patrilineally.
[7] 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.
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 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 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.
[8]
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 widely 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
The original paper by Cann, Stoneking and Wilson
troubled some people. Some criticisms include:
★ Of the 147 persons sampled, only two were from sub-Saharan Africa. The other 18 'Africans' in the study were African-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)
[9] 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 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 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'', based on the book ''Out of Eden'' by
Stephen Oppenheimer.
★ 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".
★ The anime/manga series
K.R.I.E.G made several references to mitochondria and the existence of the Mitochondrial Eve theory, among many other theories of evolution and human creation.
★
Arthur C. Clarke's novel ''
The Light of Other Days'' describes a sequence where mitochondrial DNA is visually traced back to a Mitochondrial Eve.
See also
References
1. See the chapter ''All Africa and her progenies'' in River Out of Eden, , Richard, Dawkins, Basic Books, 1995, ISBN 0-465-06990-8
2. The Ancestor's Tale, A Pilgrimage to the Dawn of Life, , Richard, Dawkins, Houghton Mifflin Company, 2004, ISBN 0-618-00583-8
3. Rohde DLT, Olson S, Chang JT (2004) "Modelling the recent common ancestry of all living humans". ''Nature'' 431: 562-566.
4. Cann, R.L., Stoneking, M., and Wilson, A.C., 1987, "Mitochondrial DNA and human evolution", ''Nature'' 325; pp 31–36. Online: Cann ''et al.'' (1987)
5. Rohde, DLT , ''On the common ancestors of all living humans''. Submitted to American Journal of Physical Anthropology. (2005)
6. A. C. Wilson, R. L. Cann, S. M. Carr, M. George Jr., U. B. Gyllensten, K. Helm- Bychowski, R. G. Higuchi, S. R. Palumbi, E. M. Prager, R. D. Sage, and M. Stoneking (1985) "Mitochondrial DNA and two perspectives on evolutionary genetics". ''Biological Journal of the Linnean Society'' 26:375-400
7. Bryan Sykes (2004). ''Adam's Curse. A Future without Men.'' New York: W. W. Norton.
8. Out of Africa Again and Again by Templeton in Nature
9. Nature 408, 708-713 (7 December 2000) | doi:10.1038/35047064. Online:Ingman ''et al.'' (2000)
★ Excoffier, L., and Yang, Z., "Substitution Rate Variation Among Sites in Mitochondrial Hypervariable Region I of Humans and Chimpanzees", 1999, ''Mol. Biol. Evol.'' 16; pp 1357–1368
★ Kaessmann, H., and Pääbo, S.: "The genetical history of humans and the great apes". ''Journal of Internal Medicine'' 251: 1–18 (2002).
pubmed
★ Laurence Loewe and Siegfried Scherer, “Mitochondrial Eve: The Plot Thickens,” Trends in Ecology & Evolution, Vol. 12,
11 November 1997, p. 422.
★ Nicole Maca-Meyer , Ana M González , José M Larruga, Carlos Flores and Vicente M Cabrera (2001) "Major genomic mitochondrial lineages delineate early human expansions". BMC Genetics
Biomed central
★ Oppenheimer, Stephen. The Real Eve, Carroll & Graf; (September 9, 2004) ISBN 0-7867-1334-8
★ Vigilant, L., Pennington, R., Harpending, H., Kocher, T.D., Wilson, A.C., 1989, "Mitochondrial DNA Sequences in Single Hairs from a Southern African Population", ''Proc. Natl. Acad. Sci. USA'' 86; pp 9350-9354
★ Thomas J. Parsons et al., “A High Observed Substitution Rate in the Human Mitochondrial DNA Control Region,” Nature Genetics, Vol.
15 April 1997, p. 365.
★ Vigilant, L., Stoneking, M., Harpending, H., Hawkes, K., Wilson, A.C., 1991, "African Populations and the Evolution of the Human Mitochondrial DNA", ''Science'' 253; pp 1503–1507
Pubmed
★ Watson E., Forster P., Richards M., Bandelt H.-J. (1997). "Mitochondrial Footprints of Human Expansions in Africa." ''American Journal of Human Genetics''. 61: 691-704
pubmed
★ Spencer Wells
''The Journey of Man: A Genetic Odyssey'', Princeton University Press, January 2003, hardcover, 246 pages, ISBN 0-691-11532-X
External links
★
Krishna Kunchithapadam, "What if anything is a Mitochondrial Eve?": a simple explanation
★
Mitochondrial Eve and Y-chromosomal Adam Diagrams
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