Mitochondrial Eve

In the field of human genetics, Mitochondrial Eve refers to the most recent common matrilineal ancestor (MRCA) from whom all living humans are descended. 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,^[2] most likely in East Africa,^[3] 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.^[4] 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.

Female and mitochondrial ancestry

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

Simplified Human mitochondrial phylogeny

Without a DNA sample, it is not possible to reconstruct the complete genetic makeup (genome) of any individual who died very long ago. By analysing descendants' 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.^[5][6] 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 an 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 Eves 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 at the rate of approximately one every 3500 years.^[7][8] 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 branch lines dies out, the MRCA will move to a more recent female ancestor, always the most recent mother to have more than one daughter with living maternal line descendants alive today. The number of mutations that 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. In 1980, Brown 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.

Common fallacies

Not the only woman

One of the misconceptions of mitochondrial Eve is that since all women alive today descended in a direct unbroken female line from her that she was the only woman alive at the time.^[9][10] However nuclear DNA studies indicate that the size of the ancient human population never dropped below some tens of thousands;^[9] there were many other women around at Eve's time with descendants alive today, but somewhere in all their lines of descent there is at least one man (and men do not pass on their mothers' mitochondrial DNA to their children). By contrast, Eve's lines of descent to each person alive today includes at least one line of descent to each person which is purely matrilineal.

Not alive at the same time as "Adam"

Sometimes mitochondrial Eve is assumed to have lived at the same time as Y-chromosomal Adam, perhaps even meeting and mating with him. Like Eve, "Adam" probably lived in Africa; however, Eve lived much earlier than Adam – perhaps some 50,000 to 80,000 years earlier than Adam – due to the greater variability in male fecundity.^[11]

Not the most recent ancestor shared by all humans

Mitochondrial Eve is the most recent common matrilineal ancestor, not the most recent common ancestor (MRCA). Since the mtDNA is 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. An approximate sequence from youngest to oldest can list various important points in the ancestor of modern human populations:

• The Human MRCA. All humans alive today share a surprisingly recent common ancestor, perhaps even within the last 5000 years, even for people born on different continents.^[12]
• The Identical ancestors point. Just a few thousand years before the most recent single ancestor shared by all living humans comes the time at which all humans who were alive either left no descendents or are common ancestors to all humans alive today. In other words, from this point back in time "each present-day human has exactly the same set of genealogical ancestors". This is far more recent than Mitochondrial Eve.^[12]
• "Y-Chromosomal Adam", the most recent male-line ancestor of all living men, was much more recent than Mitochondrial Eve, but is also likely to have been long before the Identical ancestors point.

Implications of dating and placement of Eve

The first studies suggesting a recent common ancestor for humans within Africa came at time when a hypothesis for human evolution, known as the Multiregional Evolution hypothesis, was popular among some leading paleoanthropologists (e.g. Milford Wolpoff). The impetus for this hypothesis comes from the belief that humans first left Africa about 2 million years ago and spread globally; these humans were similar to modern humans in many ways. Other biases indicated the temporal 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 ones. 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 racial admixture, such as proposed by Eric Trinkhaus, to mostly recent Africa origin hypotheses.

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. Tishkoff argues that humans might have migrated to the Levant before 90 Ka, but this colony did not persist in SW Asia. Better defined is the genetic separation among Neanderthals, Flores hobbit, Java man, and 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 that for 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 the Multiregional evolution hypothesis (MREH) revolved around a belief that regional modern human populations evolved in situ in various regions (Europe - Neandertals to Europeans, Asia - Homo Erectus to East Asians, Australia - Sumatran erectines to Indigeonous Australians), these results demonstrated that a pure MREH hypothesis could not explain one important genetic marker.

In popular science and culture

Popular science

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.^[13]

The Seven Daughters of Eve 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.

NOVA on PBS had an episode "Children of Eve" (about 1987).

Popular culture

• In his short story Mitochondrial Eve, Australian science fiction writer Greg Egan imagines the social consequences of the desire to identify, and identify with, one's haplogroup.
• The main villainess of the Parasite Eve series is named Mitochondria Eve.
• In the reimagined Battlestar Galactica, Hera Agathon (the Cylon-human hybrid) is the Mitochondrial Eve as revealed in the series finale.^[14]

See also

References

Further reading

External links

• Krishna Kunchithapadam, "What, if anything, is a Mitochondrial Eve?" a simple explanation