Mitochondrial Eve



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 150,000 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. As a result, only Eve's mitochondria have descendants in the cells of living humans, and only from Eve do all living people descend along their maternal lines.

Chain of events

This surprising circumstance 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 matralineal ancestries of all humans alive today would converge on Eve (or any one contemporary of Eve's).

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


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 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 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.



Y-chromosomal Adam


In human genetics, Y-chromosomal Adam is the male counterpart to mitochondrial Eve: a real or hypothetical single male human ancestor from whom all male Y chromosomes are descended. Unlike other genes, those of the Y chromosome are passed exclusively from father to son, just as mitochondrial DNA is passed to children exclusively by their mothers.

If such a person existed, he probably lived between 35,000 and 90,000 years ago years ago, judging from molecular clock studies. While their descendants certainly became close intimates, Y-chromosomal Adam and mitochondrial Eve themselves never met: rather, they lived many thousands of years apart. They are named after the characters called "Adam" and "Eve" in Genesis, but should not be identified with them. Based on the DNA of peoples living in Africa today, both Y-chromosomal Adam and mitochondrial Eve are believed to have lived in Africa.

More on the process by which many lineages winnow down to one can be found in the article on mitochondrial Eve, which also elaborates on how Adam and Eve relate to the Out-of-Africa theory of human evolution.





Mitochondrial genome


The mitochondrial genome is the genetic material of the mitochondria. The mitochondria are organelles that reproduce themselves semi-autonomously when the eukaryotic cells that they occupy divide.

The genetic material forming the mitochondrial genome is similar in structure to that of the prokaryotic genetic material. It is formed of a single circular DNA molecule.

The mitochondria of a sexually-reproducing animal comes only from the mother's side. The mitochondrial DNA of a human being is essentially the same as that of his or her mother.

In this way, mitochondrial genetic diseases can affect both males and females, but can only be transmitted by females to their offspring.

Compared to the nuclear genome, the mitochondrial genome possesses some very interesting features:

  • All the genes are carried on a single circular DNA molecule.
  • The genetic material is not bounded by a nuclear envelope.
  • The DNA is not packed with proteins.
  • The genome doesn't contain a lot of non-coding (junk DNA) areas.
  • Some codons do not follow the universal rules in translation.
  • Some bases are considered as a part of two different genes: as the last base of a gene and the first base of the next gene.


Mitochondrial DNA


Mitochondrial DNA is DNA which is located not in the nucleus of the cell but in the mitochondria. It has been widely believed that mitochondrial DNA is always passed on to offspring solely by the mother, although it has been reported that it can also occasionally be inherited from the father (Schwartz and Vissing, 2002).

It appears that the mitochondria in mammalian sperm are usually destroyed by the egg cell after fertilization. In 1999 it was reported that paternal sperm mitochondria (containing mt DNA) are marked with ubiquitin to select them for later destruction inside the embryo (Sutovsky et. al. 1999). Occasionally this process goes wrong, for example in inter-species hybrids.

Mitochondrial DNA has been studied to trace lineage far back in time. Svante Pääbo has published studies tracing the ancestry of domestic dogs to 4 individuals. The concept of the mitochondrial Eve is based on the same type of analysis.



Single-origin hypothesis


In paleoanthropology, the single-origin hypothesis (or Out-of-Africa model) is one of two competing accounts of the origin of anatomically modern humans, Homo sapiens sapiens.

Because of the scarcity of fossils and the discovery of important new finds every few years, researchers disagree about the details and sometimes even basic elements of human evolutionary history. While they have revised this history several times over the last decades, currently, researchers agree that the first species of the genus Homo, Homo habilis, evolved in Africa around two million years ago, and that members of the genus migrated "out of Africa" somewhat later. The descendants of these ancient migrants, which probably included Homo erectus, have become known through fossils uncovered far from Africa, such as those of "Peking man" and "Java man." The Neanderthals are also considered the descendants of early migrants.

According to the single-origin model, however, every species of the genus Homo but one was driven extinct: Homo sapiens. This species evolved in Africa between 100,000 and 200,000 years ago and, in a second important exodus from Africa, began colonizing the rest of the world some time afterwards. According to the single-origin model, these more recent migrants did not interbreed with the scattered descendants of earlier exoduses. For this reason, the model is sometimes called the "replacement scenario." In support of it, advocates have drawn from both fossil and DNA evidence, in particular from mitochondrial and Y-chromosome DNA sequences.

The opponents of a single origin argue that interbreeding indeed occurred, and that the characteristics of modern humans, including those that have been and still are perceived by some to distinguish races, reflect genetic contributions from several lineages that evolved semi-independently in different parts of the world. This is the "multiregional model.



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