Mitochondrial Eve

Mitochondrial Eve is the name given by researchers to the woman who is the most recent common matrilineal ancestor of all living humans. We know about Eve because of mitochondria organelles that are only passed from mother to offspring. Each mitochondrion contains Mitochondrial DNA (mtDNA), and the comparison of DNA sequences from mtDNA reveals a phylogeny. Based on the molecular clock technique of correlating elapsed time with observed genetic drift, Eve is believed to have lived in a population of humans about 150,000 years ago in Africa.

Matrilineal descent

Although Mitochondrial Eve was named after Eve of the Genesis creation story, this 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). Rather, at all times during humanity's existence there has been a large population of humans. Many women alive at the same time as Mitochondrial Eve have descendants alive today. However, only Mitochondrial Eve produced an unbroken line of daughters that persists today — each of the other matrilineal lineages was broken when a woman had only sons, or no children at all.

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 the lineages will converge as sisters share the same mother. The further back in time one goes, the fewer lineages there will be until only one lineage is left — this is the common matrilineal ancestor of all the humans we started with, 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 around 150,000 years ago, 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 was the most recent matrilineal ancestor of humans alive today. However, at times in the past, as certain lineages died out, the status of common matrilineal ancestor would have been passed to a different woman. For example, the common matrilineal ancestor of the population alive at the time of our Mitochondrial Eve would have lived still further back in time.

The smaller a population, the more quickly matrilineal lineages converge.

Most recent common ancestor

Mitochondrial Eve is the most recent common matrilineal (female-lineage) ancestor, 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 lineage, 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 within historical times ( 3000 BC - 1000 AD), though the MRCA was probably less recent than that when accounting for long isolated peoples.

Mitochondrial DNA

We know about Eve because of mitochondria organelles that are passed only from mother to offspring. Each mitochondrion contains Mitochondrial DNA (mtDNA), and the comparison of DNA sequences from mtDNA 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).

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 the most recent patriarch of all humanity is much more recent than the most recent matriarch, so 'Adam' and 'Eve' were not alive at the same time.

Academic investigation

Allan Wilson, Rebecca Cann, Steven Carr, M. George Jr., U. B. Gyllensten, K. Helm-Bychowski, R. G. Higuchi, Stephen Palumbi, E. M. Prager, R. D. Sage, and Mark Stoneking laid out the theoretical background for the analysis of mitochondrial DNA in a 1985 paper. Cann, Wilson and Stoneking then proposed the concept of Mitochondrial Eve in a 1987 paper in Nature. Cann et al used restriction mapping on 147 persons from five separate populations to derive their data. Gradually mitochondrial DNA sequence data from more people around the world were collected, giving a better picture.

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 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; the current debate focuses more on questions of dating an event that is generally considered proven. 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.

In popular culture

Bryan Sykes has written a popular science book entitled The Seven Daughters of Eve.
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.