Mitochondrial Eve

Molecular clock

Using molecular clock methods in the late 1970s, Allan Wilson, Mark Stoneking, Rebecca L. Cann and Wesley M. Brown found that mutation in human mtDNA was unexpectedly fast. It exhibited 0.02 substitution per base (1%) in a million years, which is 5-10 times faster than in nuclear DNA. Related work allowed for an analysis of the evolutionary relationships among gorillas, chimpanzees (common chimpanzee and bonobo) and humans. With data from 21 human individuals, Brown published the first estimate on the age of the mt-MRCA at 180,000 years ago. Using a larger, improved sample, Rebecca L. Cann concluded that it was possible to trace back individual female lineages back hundreds of generations reaching the point of a common ancestor. Early studies remained somewhat unconclusive, though. As one main result, a common Africal ancestry of Mitochondrial Eve began to emerge using more elaborate statistical estimates including PAUP and Neighbor joining in addition to Fitch-Margoliash trees.

1987 publication

By 1985, data from the mtDNA of 145 women of different populations, and of two cell lines, HeLa and GM 3043, derived from a Black American and a !Kung respectively, was available. After more than 40 revisions of the draft, the manuscript was submitted to Nature in late 1985 or early 1986 and published on 1 January 1987. The published conclusion was that all current human mtDNA originated from a single population from Africa, at the time dated to 140,000 and 200,000 years ago.

The dating for "Eve" was a blow to the multiregional hypothesis, which was being controversially discussed at the time, and a boost to the theory of the recent origin model.

Cann, Stoneking and Wilson did not use the term "Mitochondrial Eve" or even the name "Eve" in their original paper; it appears to originate with a 1987 article in Science by Roger Lewin, headlined "The Unmasking of Mitochondrial Eve." The biblical connotation was very clear from the start. The accompanying research news in Nature had the title "Out of the garden of Eden." Wilson himself preferred the term "Lucky Mother" and thought the use of the name Eve "regrettable." But the concept of Eve caught on with the public and was repeated in a Newsweek cover story (11 January 1988 issue featured a depiction of Adam and Eve on the cover, with the title "The Search for Adam and Eve"), and a cover story in Time on 26 January 1987.

Criticism and new evidence

Shortly after the 1987 publication, criticism of its methodology and secondary conclusions was published. Both the dating of mt-Eve and the relevance of the age of the purely matrilineal descent for population replacement was controversially discussed during the 1990s; Alan Templeton (1997) asserted that the study did "not support the hypothesis of a recent African origin for all of humanity following a split between Africans and non-Africans 100,000 years ago" and also did "not support the hypothesis of a recent global replacement of humans coming out of Africa."

Although the original research did have analytical limitations, the estimate on the age of the mt-MRCA has proven robust. More recent age estimates have remained consistent with the 140–200 kya estimate published in 1987: A 2013 estimate dated Mitochondrial Eve to about 160 kya (within the reserved estimate of the original research) and Out of Africa II to about 95 kya. Another 2013 study (based on genome sequencing of 69 people from 9 different populations) reported the age of Mitochondrial Eve between 99 to 148 kya and that of the Y-MRCA between 120 and 156 kya.

Female and mitochondrial ancestry

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 DNA 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 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, and she must predate the emergence of L0.

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 3,500 years per nucleotide. 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.

Not the only woman

One common misconception surrounding mitochondrial Eve is that since all women alive today descended in a direct unbroken female line from her, she must have been the only woman alive at the time. However, nuclear DNA studies indicate that the size of the ancient human population never dropped below tens of thousands. Other women living during Eve's time have descendants alive today but not in a direct female line.

Not a fixed individual over time

The definition of mitochondrial Eve is fixed, but the woman in prehistory who fits this definition can change. That is, not only can our knowledge of when and where Mitochondrial Eve lived change due to new discoveries, but the actual mitochondrial Eve can change. The mitochondrial Eve can change, when a mother-daughter line comes to an end by chance. It follows from the definition of Mitochondrial Eve that she had at least two daughters who both have unbroken female lineages that have survived to the present day. In every generation mitochondrial lineages end – when a woman with unique mtDNA dies with no daughters. When the mitochondrial lineages of daughters of mitochondrial Eve die out, then the title of "Mitochondrial Eve" shifts forward from the remaining daughter through her matrilineal descendants, until the first descendant is reached who had two or more daughters who together have all living humans as their matrilineal descendants. Once a lineage has died out it is irretrievably lost and this mechanism can thus only shift the title of "Mitochondrial Eve" forward in time.

Because mtDNA mapping of humans is very incomplete, the discovery of living mtDNA lines which predate our current concept of "Mitochondrial Eve" could result in the title moving to an earlier woman. This happened to her male counterpart, "Y-chromosomal Adam," when older Y lines from Africa were discovered.

Not necessarily a contemporary of "Y-chromosomal Adam"

Sometimes mitochondrial Eve is assumed to have lived at the same time as Y-chromosomal Adam, from whom all living people are descended patrilineally, perhaps even meeting and mating with him. Even if this were true, which is currently regarded as highly unlikely, this would only be a coincidence. Like mitochondrial "Eve", Y-chromosomal "Adam" probably lived in Africa. A recent study (March 2013) concluded however that "Eve" lived much later than "Adam" – some 140,000 years later. (Earlier studies considered, conversely, that "Eve" lived earlier than "Adam".) More recent studies indicate that mitochondrial Eve and Y-chromosomal Adam may indeed have lived around the same time.

Not the most recent ancestor shared by all humans

Mitochondrial Eve is the most recent common matrilineal ancestor, not the most recent common ancestor. 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 newest to oldest can list various important points in the ancestry of modern human populations:

Not the biblical Eve

Owing to its figurative reference to the first woman in the Biblical Book of Genesis, the Mitochondrial Eve theory initially met with enthusiastic endorsement from some young earth creationists, who viewed the theory as a validation of the biblical creation story. Some even went so far as to claim that the Mitochondrial Eve theory disproved evolution. However, the theory does not suggest any relation between biblical Eve and Mitochondrial Eve because Mitochondrial Eve:

Implications of dating and placement of Eve

There was initial resistance among some scientists to the Mitochondrial Eve hypothesis, in part because it challenged the widely accepted multiregional evolution hypothesis held by some leading paleoanthropologists, such as Milford Wolpoff. This prevailing theory held that the evolution of humanity from the beginning of the Pleistocene (2.5 million years ago) to the present day has been within a single, continuous human species, evolving worldwide to modern Homo sapiens. More resistance came from those who argued that there was too little time between Homo erectus and modern Homo sapiens to allow for another new species, and others who argued the regional evolution from archaic hominin forms into modern ones. Consequently, the finding of a recent maternal ancestor for all humans in Africa was very controversial.

Cann, Stoneking & Wilson (1987)'s 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 et al. (2009) using data from many loci (not just mitochondrial DNA) extrapolated that the Angola-Namibia border region near the Atlantic Ocean is likely to be near the geographical point of origin of modern human genetic diversity. In its relatively southern origin proposals, this autosomal study was considered by the authors to be broadly consistent with previous mitochondrial DNA studies, including one by some of the same authors who associated the origins of mitochondrial haplogroups L0 and L1 with "click languages" in southern and eastern Africa.

To some extent the studies have already revealed that the archaic Homo sapiens present in Northwest Africa (Jebel Irhoud) were not likely part of the contiguous modern human population. 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 that 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 wider 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 indigenous Australians), these results demonstrated that a pure MREH hypothesis could not explain one important genetic marker.

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