Mitochondrial Eve (mtMRCA) is the name given by researchers to the woman who is defined as the matrilineal most recent common ancestor (MRCA) for all living humans. Passed down from mother to offspring, all mitochondrial DNA (mtDNA) in every living person is directly descended from hers. Mitochondrial Eve is the female counterpart of Y-chromosomal Adam, the patrilineal most recent common ancestor, although they lived thousands of years apart.

Mitochondrial Eve is generally estimated to have lived around 200,000 years ago, most likely in East Africa. Mitochondrial Eve lived during a period of time when Homo sapiens sapiens were developing as a species separate from other human species.

Mitochondrial Eve lived much earlier than the out of Africa migration that is thought to have occurred between 95,000 to 45,000 BP.^[1] The dating for 'Eve' was a blow to the multiregional hypothesis, and a boost to the hypothesis that modern humans originated relatively recently in Africa and spread from there, replacing more "archaic" human populations such as Neanderthals. As a result, the latter hypothesis is now the dominant one.

[edit] Female and mitochondrial ancestry

Through random drift or selection the female-lineage may trace back to a single female, such as Mitochondrial Eve

Without a DNA sample, it is not possible to reconstruct the complete genetic makeup (genome) of any individual who died very long ago. By looking at descendant's DNA, however, parts of ancestral genomes are estimated by scientists. Mitochondrial DNA (mtDNA) and Y chromosome are commonly used to trace ancestry in this manner. MtDNA is generally passed un-mixed from mothers to children of both sexes, along the maternal line, or matrilineally. Matrilineal descent goes back to our mothers, to their mothers, until all female lineages converge.

Branches are identified by one or more unique markers which give a mitochondrial "DNA signature" or "haplotype" (e.g. the CRS is a haplotype). Each marker is a DNA base-pair that has resulted from a SNP mutation. Scientists sort mitochondrial DNA results into more or less related groups, with more or less recent common ancestors. This leads to the construction of a DNA family tree where the branches are in biological terms clades, and the common ancestors such as Mitochondrial Eve sit at branching points in this tree. Major branches are said to define a haplogroup (e.g. CRS belongs to haplogroup H), and large branches containing several haplogroups are called "macro-haplogroups".

The mitochondrial clade which Mitochondrial Eve defines is the species Homo sapiens sapiens itself, or at least the current population or "chronospecies" as it exists today. In principle, earlier Eve's can also be defined going beyond the species, for example one who is ancestral to both modern humanity and Neanderthals, or, further back, an "Eve" ancestral to all members of genus Homo and chimpanzees in genus Pan. According to current nomenclature, Mitochondrial Eve's haplogroup was within mitochondrial haplogroup L because this macro-haplogroup contains all surviving human mitochondrial lineages today.

The variation of mitochondrial DNA between different people can be used to estimate the time back to a common ancestor, such as Mitochondrial Eve. This works because, along any particular line of descent, mitochondrial DNA accumulates mutations which survive at least until the next generation approximately once every 3624 years.^[2][3][4] A certain number of these new variants will survive into modern times and be identifiable as distinct lineages. At the same time some branches, including even very old ones, come to an end, when the last family in a distinct branch has no daughters.

Mitochondrial Eve is the most recent common matrilineal ancestor for all modern humans. Whenever one of the two most ancient branches dies out, the MRCA will move to a more recent female ancestor, the first branch point in the surviving branch. The amount of mutations which can be found distinguishing modern people is determined by two criteria: firstly and most obviously, the time back to her, but secondly and less obviously by the varying rates at which new branches have come into existence and old branches have become extinct. By looking at the number of mutations which have been accumulated in different branches of this family tree, and looking at which geographical regions have the widest range of least related branches, the region where Eve lived can be proposed.

The date when Mitochondrial Eve lived is estimated by determining the MRCA of a sample of mtDNA lineages. 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. For a more detailed discussion of the methods used to determine when mitochondrial Eve lived see Human mitochondrial molecular clock

[edit] Solitary female Eve as a misconception

One of the misconceptions of the heralding of mitochondrial Eve in the late 1980s was that since all females descended matrilineally from a single female that there was only one female alive at the time.^[5][6] While mtDNA studies cannot absolutely rule out this possibility, studies at other genetic loci suggest that the ancient human population carried longstanding population 10,000s of individuals in size.^[5]

[edit] Implications of dating and placement of Eve

The first studies suggesting a recent common ancestor for humans within Africa came a time when a hypothesis for human evolution, known as the Multiregional Evolution hypothesis was popular among some leading Paleoanthropologist (e.g. Milford Wolpoff). The impetus for this hypothesis comes from the belief that humans left Africa first about 2 million years ago and spread globally, these humans were similar to modern humans in many ways. Other biases indicated the temporal (time) difference between Homo erectus and modern Homo sapiens was too short to allow for another new species, and many authors perceived regional evolution from Archaic forms into modern forms. Consequently, the finding of a recent maternal ancestor for all humans in Africa created an extended controversy. Over a decade the MREH theory retracted to theories of admixture (Such as proposed by Eric Trinkhaus) to mostly recent Africa origin hypotheses.

[edit] Geographic and Temporal constraints of early modern humans

The strict Out of Africa and Multiregional Evolution controversy revolved around where to best place the evolution of anatomically modern humans over evolutionary timescales.

Cann, Stoneking & Wilson (1987) placement of a relatively small population of humans in sub-saharan Africa, lent appreciable support for the recent Out of Africa hypothesis. The current concept places between 1,500 and 16,000 effectively interbreeding individuals (census 4,500 to 48,000 individuals) within Tanzania and proximal regions. Later, Tishkoff using data from many loci has extrapolated origins to the Angola-Namibia border region near the Atlantic Ocean (although this region has poor genetic definition), whereas Behar et al. 2008 places an ancestral population in Ethiopia. These opinions all point toward a sub-Saharan origin. More recent literature on languages and pygmy phenotype indicate that L0 and L1 were carried by click-speaking pygmies from SE Africa to Central and Western Africa, therefore explaining much of the genetic diversity in those regions. Consequently, more recent studies have tended to push the cradle of humanity more toward the South or East of Africa.

To some extent the studies have already revealed that the presence of archaic homo sapiens in Northwest Africa, Jebel Irhoud, were not likely part of the contiguous modern human population. In addition, the older remains at Skhul and Qafzeh are also unlikely part of the constrict human population, evidence currently indicates humans expanded in the region no earlier than 90,000 BP.^{[citation needed]} Tishkoff argues that humans might have migrated to the levant before 90 Ka, but this colony did not persist in SW Asia.^{[citation needed]} Better defined is the genetic separation between Neanderthals, Flores hobbit, Java man, Peking man. In 1999 Krings et al., eliminated problems in molecular clocking postulated by Nei, 1992 when it was found the mtDNA sequence for the same region was substantially different from the MRCA relative to any human sequence. Currently there are 6 fully sequenced Neanderthal mitogenomes, each falling within a genetic cluster less diverse than humans, and mitogenome analysis in humans has statistically markedly reduced the TMRCA range so that it no longer overlaps with Neandertal/human split times. Of all the non-African hominids European archaics most closely resembled humans, indicating a larger genetic divide with other hominids.

Since Multiregional evolution hypothesis (MREH) revolved around a belief that regional modern human population evolved in-situ in various regions (Europe - Neandertals to Europeans, Asia - Homo Erectus to East Asians, Australia - Sumatran erectines to Indigeonous Australians), the demonstrated that a pure MREH hypothesis could not explain one important genetic marker.

[edit] Mitochondrial MRCA and the MRCA of all humans

Mitochondrial Eve is the most recent common matrilineal ancestor, not the MRCA. Since the mtDNA are inherited maternally and recombination is either rare or absent, it is relatively easy to track the ancestry of the lineages back to a MRCA; however this MRCA is valid only when discussing mitochondrial DNA. Ironically mtDNA are not human; they are organelles that live within our cells, so it is better to say these are human-mitochondrial Most Recent Common Ancestor. Despite the recent fixation of the mtDNA genome in humans, other genes have evolved that were broadly selective in the human population, genes that have swept through the human population, two such genes having been identified on the X-chromosome.

Rohde, Olson & Chang (2004) indicate that the overwhelming majority of humans have a recent common ancestor within the last 5000 years (albeit between any two individuals, it may not be the same ancestor), however the genetic relationship between well diverged individuals may not reflect the theoretical relationship, as geographic and cultural barriers may slow gene migration. Gene migration is not fluid in humans, as genes are passed in units called chromosomes, which undergo a limited number of recombination on each unit per generation; therefore a common ancestor genealogically may not indicate the passage of DNA from that ancestor to the two divergent individuals. Whereas since mtDNA does not undergo this dilution via recombination, we can argue that the majority of mtDNA sequence (that which has not undergone mutation) from mtDNA ~16000 nts came from a single individual >150,000 years ago. A more recent common ancestor for all males is the much larger Y chromosome (however it codes for very few genes).

[edit] In popular science and culture

Cover of the January 11, 1988 edition of Newsweek

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.^[7]

Bryan Sykes (2001) 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.
• In the "reimagined" Battlestar Galactica television series, Hera Agathon (the Cylon-human hybrid) is the Mitochondrial Eve as revealed in the series finale.^[8]

[edit] See also

[edit] Footnotes

[edit] References

[edit] External links

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