Talk:Mitochondrial Eve/MW Version

Mitochondrial Eve is the name given to the most recent common ancestor of all living humans by matrilineal descent. Every human inherits their mitochondrial DNA (mtDNA) from their mother, but not their father. This means that a person not only possesses their mother's mtDNA, but their mother's mother's mtDNA and their mother's mother's mother's mtDNA and so on. By tracing mitochondrial DNA lineages back in time, scientists discovered that all mtDNA lineages converge on a single woman who they dubbed "Mitochondrial Eve". The existence of such a person means that all humans alive today have inherited the mitochondrial DNA of this Eve.

Mitochondrial Eve is believed to have lived approximately 200,000 years ago, most likely in East Africa. She was not the only woman alive at the time, but only her mitochondrial lineage is found in all living humans. Mitochondrial Eve is the female counterpart of Y-chromosomal Adam, the patrilineal most recent common ancestor, although they almost certainly lived thousands of years apart.

The development of the theory of Mitochondrial Eve in 1987 was a pivotal event in the study of human origins for a number of reasons. Firstly, the placement of Eve in Africa lent support to a recent and exclusive African origin of modern humans. Secondly, the fact that only one lineage from about 200,000 years ago is found among all humans today may indicate that there were very few humans alive during the time of Eve. Some unknown events may have led to a population bottleneck, and this bottleneck appears to have been contemporaneous with the emergence of the first anatomically modern humans as indicated by the human fossil record.

Mitochondria
Found within most Eukaryotic cells, mitochondria are organelles that produce energy. They are thought to be the remnants of a simple single-celled organism that was engulfed by an early ancestor of Eukaryotic cells. This engulfed single-celled organism later entered into a symbiotic relationship with its host. Most Eukaryotes, including all animals, plants, and fungi possess mitochondria which supports the theory that they are all descended from the single cell that engulfed the ancestor to mitochondria several hundreds of millions of years ago.

Mitochondrial DNA
As mitochondria may have started out as separate and independent organisms, they have their own DNA that is separate from DNA found in the nucleus of a cell. Consequently Mitochondrial DNA has numerous advantages for studying phylogenetic relationships
 * Mitochondrial DNA is relatively short and easy to sequency - MtDNA is about 16,000 base pairs long compared with 3.1 billion base pairs of Nuclear DNA.
 * Mitochondrial DNA do not undergo recombination- Nuclear DNA, with the exception of the y-chromosome, go through a process of genetic recombination during sexual reproduction. humans possess chromosomes in pairs, each pair consisting of one chromosome from each parent. During gamete formation, each pair of chromosomes goes through a process of recombination during which the pairs exchange segments of DNA. This shuffling of genetic material is a mechanism of producing genetic variation to increase fitness of the organism. However recombination makes it impractical to determine long term phylogenetic relationship because of the entanglement of DNA segments.

Maternal inheritance
Egg cells or oocytes produced by females are significantly larger than sperm cells as they form they bulk of the fertilized egg that will develop into an embryo. Consequently they are packed with mitochondria. Sperm on the other hand are designed mainly to deliver their genetic material to the egg, they are much smaller and only have a few mitochondria. During fertilization the sperm penetrates the oocyte and transfers its genetic material to the oocyte's nucleus, the rest of the sperm including its mitochondria are destroyed by the oocyte. Consequently the developing zygote only contains mitochondria from its mother and not its father. By this mechanism, an unbroken matrilineal line of descent is created between all humans and Mitochondrial Eve

Molecular clock
According to the molecular clock theory, random mutations occurring within a particular genetic system can be thought of as occurring at a uniform rate. By counting the mutations that differentiate two genetic lineages, one can then estimate when the two lineages diverged from a common ancestor. For example, according to Hart et al. humans accumulate a mutation in their mtDNA every 3800 years. On average human mtDNA sequences have accumulated about 50 mutations since the time of Mitochondrial Eve. This would place Eve as having lived 170,000 years ago. In practice, complex statistical techniques and computer simulations process large amounts of genetic data to determine the time to the most recent common ancestor(TMRCA) of different genetic lineages.

Haplogroups
When a mutation occurs within the mitochondrial DNA of a female, the mutation is passed down to all the descendents of the female. As these mutations occur rarely, they can be used to uniquely identify the matrilineal common ancestry of all the individuals that bear a specific mutation. These unique event polymorphisms define genetic lineages termed haplogroups. Mutations can occur multiple times on a specific genetic lineage creating subclades. Macrohaplogroups are older haplogroups that are found amongst a large number of people and they typically have numerous subclades. Human haplogroups are assigned letters of the alphabet from A to Z except O. The letter L is reserved for the haplogroups most commonly found in Africa and it is L lineages that were used to determine the existence of a Mitochondrial Eve.

Genealogy of Eve
The first split in the human mitochondrial family tree was a mutation that occurred in the DNA of one of Eve's daughters. This mutation differentiated haplogroup L0 from Haplogroup L1. It can be inferred that Mitochondrial Eve must have had at least two daughters who survived to have descendants of their own and all humans alive today are descended from either one these daughters. Haplogroup L0 is found throughout Africa, but its most basal lineages, L0k and L0d are largely restricted to the Khoisan of southern Africa. The restricted distribution of L0k and L0d implies that that the maternal ancestors of the Khoisan diverged from other Africans shortly after the period of mtEve and have remained relatively isolated since. Haplogroup L1 includes most Africans and all non-Africans. It differentiated into haplogroup L2 about 100kya and haplogroup L3 about 80kya. All non African haplogroups are derived from either haplogroup M (mtDNA) or haplogroup N (mtDNA), both of which are subclades of haplogroup L3.

Eve and human origins
The development of the Mitochondrial Eve theory was an important event in the study of human origins as it gave a major boost to the recent African origin (RAO) model of human evolution. According to the RAO theory, anatomically modern humans emerged from an Archaic lineage about 200,000 years ago in Africa. About 50-80kya a small group of humans left Africa either via the horn of Africa or the levant, and it is this small group of humans that went on to populate the rest of the world.

Prior to the discovery of Mitochondrial Eve, many researches subscribed to the multiregional model of human evolution. According to this model, the most recent common ancestor all humans was homo erectus who migrated out of Africa about 1 million years ago. Homo erectus then evolved independently into homo sapiens five separate times on five different continents.

The discovery that mitochondrial Eve lived only 200,000 years ago suggested that humans had a more recent common ancestry than implied by multiregional models of Evolution. Furthermore African populations possessed greater diversity of mtDNA lineages. At least seven ancient lineages, Haplogroups L0-L6 exist in Africa, all non-African haplogroups are subclades of only one of these African lineages, haplogroup L3. This suggested an African origin of all mitochondrial DNA lineages and a subsequent expansion of haplogroups M and N out of Africa 50-80kya. The discovery that Y-chromosomal Adam lived more recently than Eve also lent support to a recent African origin of modern humans

Due to widespread acceptance of the existence of Mitochondrial Eve, traditional theories of Multiregional evolution have fallen out of favour. However, different versions of multiregional evolution persist that incorporate various levels of interbreeding between archaic hominids and the out of Africa migrants. These models suggest that archaic mtDNA lineages that are older than Eve, have not been detected as they have been lost to drift.

Dates
Brown 1980 first proposed that modern humans possessed a mitochondrial common ancestor that may have lived as recently as 180kya. In 1987 Cann et al. suggested that mitochondrial Eve may have lived between 140-280kya.

The Mitochondrial Eve and Most Recent Common Ancestor (MRCA) are the same
Mitochondrial Eve is the most recent common matrilineal ancestor, not the MRCA of all humans. The MRCA's offspring have led to all living humans via sons and daughters, but Mitochondrial Eve must be traced only through female lineages, so she is estimated to have been much older than the MRCA.

Mitochondrial Eve is estimated to have lived around 120,000 years ago at the latest. The theoretical MRCA could have lived as recently as 3,000 years ago. The most recent common ancestor (MRCA) is the most recent person whom all of humanity can count as one of their ancestors. Because each person's number of ancestors double (ignoring overlaps) with each generation backwards, the MRCA of all humanity occurs relatively recently in history, even when we take into account ancestor overlap. The MRCA answers the question, "Do any of my four grandparents overlap with any of your four grandparents? If not, then do any of my 8 great grandparents overlap with any of your 8 great grandparents?" Between one-hundred and two-hundred generations back, a single person will appear in every living person's family tree. However, each person has only one mitochondrial ancestor with each generation backwards because each person inherits their mitochondria from their mother. The question of "when did Mitochondrial Eve live?" is answering the question of "When does my mother's mother's mother ... overlap with everyone else's mother's mother's mother ... (all the way up the female lineage)?"

Mitochondrial Eve was the only woman alive at that time
Allan Wilson's naming Mitochondrial Eve after Eve of the Genesis creation account has led to some misunderstandings among the general public. A common misconception is that Mitochondrial Eve was the only living human female of her time.

Indeed, not only were many women alive at the same time as Mitochondrial Eve but many of them have living descendants through their sons. While the mtDNA of these women is gone, their Nuclear genes are present in today's population.

What distinguishes Mitochondrial Eve (and her matrilineal ancestors) from all her female contemporaries is that she has a purely matrilineal line of descent to all humans alive today, whereas all her female contemporaries with descendants alive today have at least one male in every line of descent. Because mitochondrial DNA is only passed through matrilineal descent, all humans alive today have mitochondrial DNA that is traceable back to Mitochondrial Eve.

Furthermore, it can be shown that every female contemporary of Mitochondrial Eve either has no living descendant today or is an ancestor to all living people. Starting with 'the' MRCA at around 3,000 years ago, one can trace all ancestors of the MRCA backward in time. At every ancestral generation, more and more ancestors (via both paternal and maternal lines) of MRCA are found. These ancestors are by definition also common ancestors of all living people. Eventually, there will be a point in the past where all humans can be divided into two groups: those who left no descendants today and those who are common ancestors of all living humans today. This point in time is termed the identical ancestors point and is estimated to be between 5,000 and 15,000 years ago. Since Mitochondrial Eve is estimated to have lived more than a hundred thousand years before the identical ancestors point, every woman contemporary to her is either not an ancestor of any living people, or a common ancestor of all living people.

In popular science
Newsweek Magazine reported on Mitochondrial Eve based on the Cann et al. study in January 1988, under a heading of "Scientists Explore a Controversial Theory About Man's Origins". The edition sold a record number of copies.

Bryan Sykes has written a popular science book entitled The Seven Daughters of Eve (2001, ISBN 0-393-02018-5) that presents the theory of human mitochondrial genetics to a general audience.

In River Out of Eden, Richard Dawkins discusses human ancestry in the context of a river of genes and shows that Mitochondrial Eve is one of the many common ancestors we can trace back to via different gene pathways.

The Discovery Channel produced a documentary entitled The Real Eve (or Where We Came From in the United Kingdom), based on the book Out of Eden by Stephen Oppenheimer.

In popular culture

 * The main villainess of the Parasite Eve series is named Mitochondria Eve.
 * Mitochondrial Eve is a science fiction story by Greg Egan that revolves around the concept of Mitochondrial Eve and Y-chromosomal Adam.
 * In the re-imagined Battlestar Galactica series, it is revealed that Hera Agathon, a human/cylon hybrid, is Mitochondrial Eve, implying that all contemporary humans are in part descended from the artificial humanoid cylons, and humans from the Twelve Colonies. However, it falsely describes Mitochondrial Eve as the most recent common ancestor.