Mitochondrial Eve


Mitochondrial Eve
Haplogroup Modern humans

Early diversification.PNG

Possible time of origin 152,000 - 234,000 BP [1]
Possible place of origin East Africa
Ancestor n/a
Descendants Mitochondrial macro-haplogroups L0, L1, and L5
Defining mutations None

In the field of human genetics, Mitochondrial Eve refers to the matrilineal "MRCA" (most recent common ancestor). In other words, she was the woman from whom all living humans today descend, on their mother's side, and through the mothers of those mothers and so on, back until all lines converge on one person. Because all mitochondrial DNA (mtDNA) is generally passed from mother to offspring without recombination, all mitochondrial DNA (mtDNA) in every living person is directly descended from hers by definition. Mitochondrial Eve is the female counterpart of Y-chromosomal Adam, the patrilineal most recent common ancestor, although they lived thousands of years apart.

Each ancestor (of people now living) in the line back to the matrilineal MRCA had female contemporaries such as sisters, female cousins, etc. and some of these female contemporaries may have descendants living now (with one or more males in their descendancy line). But none of the female contemporaries of the "Mitochondrial Eve" has descendants living now in an unbroken female line.

Mitochondrial Eve is estimated to have lived around 200,000 years ago,[2] most likely in East Africa,[3] when Homo sapiens sapiens ("anatomically modern humans") were developing as a population distinct from other human sub-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.

Contents

Female and mitochondrial ancestry

Through random drift or selection the female-lineage will 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 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.[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 3,500 years.[7][8][9] 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 180 kya. In 1987, Cann et al. suggested that mitochondrial Eve may have lived between 140-280 kya.

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.[10][11] Nuclear DNA studies indicate that the size of the ancient human population never dropped below tens of thousands. There may have been many other women alive at Eve's time with descendants alive today, but sometime in the past, those lines of descent included at least one male, who do not pass on their mother's mitochondrial DNA, thereby breaking the line of descent. By contrast, Eve's lines of descent to each person alive today includes precisely one purely matrilineal line.[10]

Not a contemporary of "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 mitochondrial "Eve", Y-chromosomal "Adam" probably lived in Africa; however, this "Eve" lived much earlier than this "Adam" – perhaps some 50,000 to 80,000 years earlier.[12]

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 newest to oldest can list various important points in the ancestry of modern human populations:

  • The Human MRCA. All humans alive today share a surprisingly recent common ancestor, perhaps even within the last 5,000 years, even for people born on different continents.[13]
  • The Identical ancestors point. Just a few thousand years before the most recent single ancestor shared by all living humans was the time at which all humans who were then alive either left no descendants alive today or were common ancestors to all humans alive today. In other words, "each present-day human has exactly the same set of genealogical ancestors" alive at the "Identical ancestors point" in time. This is far more recent than Mitochondrial Eve.[13]
  • "Y-Chromosomal Adam", the most recent male-line common 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.
  • Mitochondrial Eve, the most recent female-line common ancestor of all living people.

Implications of dating and placement of Eve

Initially there was a lot of resistance against the Mitochondrial Eve hypothesis. This resistance was due, in part, to the popularity of the Multiregional Evolution hypothesis amongst 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 BP 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 that for 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 a previous mitochondrial DNA studies, including one by some of the same authors which associated the origins of mitochondrial haplogroups L0 and L1 with "click languages" in southern and eastern Africa.[14]

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 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 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.

In popular science

  • Newsweek 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.[15]
  • 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.

See also

Evolutionary tree of Human mitochondrial DNA (mtDNA) haplogroups

  Mitochondrial Eve (L)    
L0 L1-6
L1 L2 L3   L4 L5 L6
  M N  
CZ D E G Q   A S   R   I W X Y
C Z B F R0   pre-JT P  U
HV JT K
H V J T

References

  1. ^ Pedro Soares et al 2009, Correcting for Purifying Selection: An Improved Human Mitochondrial Molecular Clock. and its Supplemental Data. The American Journal of Human Genetics, Volume 84, Issue 6, 740-759, 04 June 2009
  2. ^ University of Leeds - New ‘molecular clock’ aids dating of human migration history
  3. ^ 'Your Genetic Journey' - The Genographic Project
  4. ^ Endicott, P; Ho, SY; Metspalu, M; Stringer, C (September 2009), "Evaluating the mitochondrial timescale of human evolution", Trends Ecol. Evol. (Amst.) 24 (9): 515–21, doi:10.1016/j.tree.2009.04.006, PMID 19682765 
  5. ^ Giles, Richard E; H Blanc, H M Cann, and D C Wallace (1980), "Maternal inheritance of human mitochondrial DNA", PNAS 77 (11): 6715–6719, doi:10.1073/pnas.77.11.6715, PMC 350359, PMID 6256757, http://www.pnas.org/content/77/11/6715.abstract 
  6. ^ Birky, C. William (2008), "Uniparental inheritance of organelle genes", Current Biology 18 (16): R692–R695, doi:10.1016/j.cub.2008.06.049, PMID 18727899, http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VRT-4T96DJ4-B&_user=10&_coverDate=08%2F26%2F2008&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1232071752&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b73147dffdd2ca6a010de407176efd84 
  7. ^ Soares, P; Ermini, L; Thomson, N; Mormina, M; Rito, T; Röhl, A; Salas, A; Oppenheimer, S et al. (June 2009), "Correcting for purifying selection: an improved human mitochondrial molecular clock", American Journal of Human Genetics 84 (6): 740–59, doi:10.1016/j.ajhg.2009.05.001, PMC 2694979, PMID 19500773 
  8. ^ Gibbons, A (January 1998), "Calibrating the Mitochondrial Clock", Science 279 (5347): 28–29, doi:10.1126/science.279.5347.28, PMID 9441404, http://www.sciencemag.org/content/279/5347/28.full 
  9. ^ There are sites in mtDNA (such as: 16129, 16223, 16311, 16362) that evolve more rapidly, have been noted to change within intragenerational timeframes - Excoffier & Yang (1999).
  10. ^ a b Takahata, N (January 1993), "Allelic genealogy and human evolution", Mol. Biol. Evol. 10 (1): 2–22, PMID 8450756, http://mbe.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=8450756 ""
  11. ^ Dawkins, Richard (2004), The ancestor's tale: a pilgrimage to the dawn of evolution, Boston: Houghton Mifflin, ISBN 0-618-00583-8 
  12. ^ Mitochondrial Eve and Y-chromosomal Adam The Genetic Genealogist
  13. ^ a b Rohde, DL; Olson, S; Chang, JT (September 2004), "Modelling the recent common ancestry of all living humans", Nature 431 (7008): 562–6, doi:10.1038/nature02842, PMID 15457259 
  14. ^ Tishkoff et al. (2007); also see Behar et al. (2008) for example.
  15. ^ Oppenheimer, Stephen (2004), The Real Eve: Modern Man's Journey Out of Africa, New York: Carroll & Graf, ISBN 0-7867-1334-8 

Further reading

External links


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