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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 his or her mother, this finding implies that all living humans descend from one woman--possibly one pre-human woman--who researchers have dubbed Mitochondrial Eve. Based on the molecular clock technique, Eve is believed to have lived about 200 000 years ago. Family trees suggest she lived in Africa.

Although researchers named her after the Biblical Eve, mitochondrial Eve was not the sole living female of her day. Researchers believe as many as 20 000 individuals of Eve's species may have lived at the same time as her. But of the females of her day, only Eve produced an unbroken line of daughters that persists today. This 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 genetic drift. 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 all humans alive today could trace their descent to Eve (or to one female contemporary of Eve's).

The observation that the mitochondria of living humans point to a common ancestor represents evidence that this ancestor lived in a much smaller breeding pool than humans live in today. 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 the ancient human and hominid remains discovered so far, in fact, the oldest that match the bones of living humans date from around the time that Eve lived.

On the other hand, the most recent common ancestor to father an unbroken line of males, "Y-chromosome Adam," appears to have lived only about half as long ago as Eve. This means that another bottleneck event 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. Some researchers says 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 weakness of the molecular clock technique, which continues to undergo revisions.

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 it turns out that they do, they may not occur frequently enough to make Eve or her identification impossible.

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 past, critics of the Out of Africa model have argued that the mitochondrial evidence can be explained as well or better by trees that associate Eve most closely to the indigenous peoples of other continents. 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 Out of Africa hypothesis may not depend on trees. One finding not subject to interpretation is that Africans living today show the greatest diversity of mitochondrial DNA sequences. 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 corroberated the evidence that mitochondrial DNA has provided for an African origin.