Online Encyclopedia
Mitochondrial Eve
A comparison of the mitochondrial DNA of humans from many races and regions suggests that all of these DNA sequences have evolved molecularly from a common ancestor sequence. Under the assumption that an individual inherits mitochondria only from one's mother, this finding implies that all living humans have a female line of descent from a woman whom researchers have dubbed Mitochondrial Eve. Based on the molecular clock technique of correlating elapsed time with observed genetic drift, Eve is believed to have lived about 150,000 years ago. Phylogenies suggest she lived in Africa.
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Importance of Eve
Although she was named after the Biblical Eve, mitochondrial Eve was not the sole living female of her day. As many as 20,000 individuals of Eve's species may have lived at the same time as her. But only Eve produced an unbroken line of daughters that persists today. As a result, only Eve's mitochondria have descendants in the cells of living humans. Eve was the only female of her generation from whom all living people descend along their maternal lines.
Eve is important because there is a direct female line to her from all living humans. She is not our only common ancestor, nor is she our most recent. However, more recent ancestors are hard to date because of the mixing of the genes in sex.
Chain of events
The surprising fact that no other all-female lines have survived from Eve's day is assumed to be an effect of chance rather than selection. Essentially, the hypothesized process by which all lineages but one disappear is the same as the genetic drift of alleles. As with genetic drift, the process is much slower and much less likely to reach completion in a large population than in a small one. If Eve had lived among a million or a billion other females, it is very unlikely that the matrilineal ancestries of all humans alive today would converge on Eve (or any one contemporary of Eve's). Such assumption of purely neutral drift requires that one likewise believe that mitochondrial protein variation has no actual effect on an organism's metabolism.
Why might the community of Eve's peers have been so small? One possibility is that the world population of humans in Eve's day passed through a bottleneck. Another is that Eve lived in a subpopulation of humans that came to supplant all others. A still more extreme version of this latter scenario is that Eve lived shortly after whatever isolating event caused the speciation of anatomically modern humans. Of Homo sapiens remains discovered so far, in fact, the oldest that match the bones of living humans date from around the time that Eve lived.
Relation to Adam
On the other hand, there was a most recent man, "Y-chromosomal Adam," who fathered an unbroken line of males that are forebears of all the men on earth. Y-chromosomal Adam appears to have lived only about half as long ago as Eve. This means that another bottleneck, besides the one surrounding Eve, affected the human lineage after her. The fact that the bottleneck in Adam's day appears not to have produced also a matrilineal ancestor of all living humans — a more recent Eve, in other words — illustrates that the branching and disappearance of lineages depends on chance (alternatively, male lineages may dwindle faster, perhaps due to a history of polygamy, which would have allowed only a portion of males to produce offspring). Some researchers say evidence of this second bottleneck exists also in the mitochondrial DNA data. It is also possible that the mismatched dates of Eve and Adam may illustrate the imperfectness of the molecular clock technique, which continues to undergo revisions.
Challenges to the theory
A recent challenge to the Eve theory has been the observation that the mitochondria of sperm are sometimes passed to offspring. Still other evidence suggests that sperm and egg mitochondrial DNA may "recombine", or swap pieces of sequence with each other. So mitochondria may not be so pure a matrilineal marker as they were supposed when the theory was advanced. Depending on how frequently paternal inheritance and recombination occurred, as well as when they occurred, it may be that no Eve even existed. But scientists still disagree on whether these processes do occur, and if they do, whether they occur frequently enough to rule out an Eve. Creationists accept the existence of an Eve, but some (particularly "Young Earth" creationists) do not accept these particular dates, and such creationists emphasize that the evidence is inconclusive. Others retort that inconclusive results only mean more research is necessary before we can have an accurate picture of what happened.
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.
The construction of family trees from DNA data is an inexact science, however. In the 19th century it seemed inconceivable that mankind could have arisen in Africa: as late as 1920, Roy Chapman Andrews was initially drawn to the Gobi Desert in search of the origins of mankind. Critics of the "African genesis" model continued to argue that the mitochondrial evidence can be explained as well or better by trees that associate Eve most closely to the indigenous peoples of Asia. As of 2003, however, following advances in computing power and in methods of tree determination, these criticisms have diminished. In any event, 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. Analysis of Y chromosome sequences have corroborated the evidence that mitochondrial DNA has provided for an African origin for hominids.
See also
- Human evolution
- Neutral theory of molecular evolution
- Mitochondrial genome
- Most recent common ancestor
- Single origin hypothesis
- Y-chromosomal Adam
Further reading
- Cann, R.L., Stoneking, M., and Wilson, A.C., 1987, Mitochondrial DNA and human evolution, Nature 325; pp 31–36
- 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
- Spencer Wells The Journey of Man: A Genetic Odyssey, Princeton University Press, January 2003, hardcover, 246 pages,ISBN 0-691-11532-X
External links and references
- Krishna Kunchithapadam, "What if anything is a Mitochondrial Eve?": a simple explanation
- BBC2's explanation of "one of the most poorly understood scientific ideas of the 20th century"
- Kaessmann, H., and Pääbo, S.: The genetical history of humans and the great apes. Journal of Internal Medicine 251: 1–18 (2002).
- Papers by Svante Pääbo and coworkers
- Science, Volume 278, Number 5339, Issue of 31 Oct 1997, pp. 804–805
- Discovery Channel's The Real Eve
The Japanese horror film and novel 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.