Mitochondrial Eve

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, from whom all mitochondrial DNA (mtDNA) in living humans is derived. Mitochondrial Eve is the female counterpart of the Y-chromosomal Adam, the patrilineal most recent common ancestor.

The current holder of the 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 female of her time — she was not (indeed, had she been, humanity would have probably become extinct). Many women alive at the same time as Mitochondrial Eve have descendants alive today. However, only Mitochondrial Eve produced an unbroken line of female descendents carrying her mitochondrial DNA that persists today.

Imagine a family tree of all humans living today. Now imagine a line from each individual to their mother, and continue those lines from each of those mothers to their mothers, and so on. Going back through time their mitochondrial lineages will converge. 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 latest common matrilineal ancestor of all humans alive today, i.e. Mitochondrial Eve.

Now, going in the opposite direction of the family tree (from ancient times to today), imagine the same line, which now connects mothers to their daughters. Starting with the entire human population alive at some time in the past, lineages will become extinct as women die childless or only have male children. Eventually, only a single lineage remains, which is the same as before.

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 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.

References

Cann, R.L., Stoneking, M., and Wilson, A.C., 1987, "Mitochondrial DNA and human evolution", Nature 325; pp 31–36
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
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
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
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