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Mitochondrial Eve

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Mitochondrial Eve (mt-mrca) is the name given by researchers to the woman who is defined as the matrilineal most recent common ancestor (MRCA) for all currently living humans. Passed down from mother to offspring, her mitochondrial DNA (mtDNA) is now found in all living humans: every mtDNA in every living person is derived from hers. Mitochondrial Eve is the female counterpart of Y-chromosomal Adam, the patrilineal most recent common ancestor, although they lived at different times - 100,000 years apart.

Mitochondrial Eve is believed to have lived about 170,000 years ago, or roughly 8,000 generations ago. This places her shortly after the (assumed in RAO) speciation of homo sapiens sapiens and the emergence of the first anatomically modern humans. Mitochondrial Eve would have been roughly contemporary with humans whose fossils have found in Ethiopia near the Omo river and at Hertho. Mitochondrial Eve lived significantly earlier than the out of Africa migration which occurred some 60,000 years ago. According to multiregional evolution the genetic history recorded in mtDNA is only one history of many genetic histories revealing in whole human genome.[1]

Time breadth of mtDNA genetic history is limited. The high frequency mutation and monophiletic inheritance limit mtDNA genetic memory to 100 ka.[1] Mitochondrial Eve is the MRCA of all humans via the mitochondrial DNA pathway, not the unqualified MRCA of all humanity. All living humans can trace their ancestry back to the MRCA via at least one of their parents, but Mitochondrial Eve is defined via the maternal line. Therefore, she necessarily lived at least as long, though likely much longer, ago than the MRCA of all humanity.

The existence of Mitochondrial Eve and Y-chromosomal Adam does not imply the existence of population bottlenecks or a first couple. They each may have lived within a large human population at different times.

Matrilineal descent

For more information on the principles involved, see matrilineal descent and genetic genealogy (matrilineal).

To find the Mitochondrial Eve of all living humans, one can start by tracing a line from every individual to his/her mother, then continue those lines from each of those mothers to their mothers and so on, effectively tracing a family tree backward in time based purely on mitochondrial lineages. Going back through time these mitochondrial lineages will converge when two or more women have the same mother. The further back in time one goes, the fewer mitochondrial ancestors of living humans there will be. Eventually only one is left, and this one is the most recent common matrilineal ancestor of all humans alive today, i.e. Mitochondrial Eve.

It is possible to draw the same matrilineal tree forward in time by starting with all human female contemporaries of Mitochondrial Eve. Some of these women may have died childless. Others left only male children. For the rest who became mothers with at least one daughter, one can trace a line forward in time connecting them to their daughter(s). As the forward lineages progress in time, more and more lineage lines become extinct, as the last female in a line dies childless or leaves no female children. Eventually, only one single lineage remains, which includes all mothers, and in the next generation, all people, and hence all people alive today.

Date

Gonder et al. (2007)[2] date the most ancient mtDNA lineage L0d to 106,000 BP. This is consistent with Ingman et al. (2000)[3], who date Mitochondrial Eve to 171,500 ± 50,000 BP. Ingman et al. sampled 53 persons, 32 of whom were Africans from different regions of sub-Saharan Africa. They sequenced the complete mtDNA but excluded the evolutionarily unpredictable D-loop in the analysis.

The results by Gonder et al. and Ingman et al. confirm the less precise result found originally by Cann et al. (1987) [4], whose estimate ran to 215,000 ± 75,000 BP.

Probabilistic studies[5], place her around 140,000 BP, in the lower range of the error margin given by both genetic studies mentioned above.

Genome sequence

Simplified Human mitochondrial phylogeny

Mitochondrial Eve's sequence is typically represented as the top root node of the human mitochondrial phylogenetic tree. The first split in the tree is found between macrohaplogroup L0 and macrohaplogroup L1. From this it can be inferred that Mitochondrial Eve had at least two daughters who survived to have their own children[6]. Today L0 is restricted to African populations, whereas L1 is the ancestral haplogroup of all non-Africans, as well as most Africans. Mitochondrial Eve's sequence can be approximated by comparing a sequence from macrohaplogroup L0 with a sequence from macrohaplogroup L1. By reconciling the mutations in L0 and L1, a putative ancestral sequence can be inferred. Usually sequences of chimpanzees and bonobos are used as outgroups to determine the most likely original state of a particular base pair in the human sequences. The mtDNA sequences of contemporary human populations will generally differ from Mitochondrial Eve's sequence by about 50 mutations.[7][8][9]

Misconceptions

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.[10] 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[4] 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.[11]

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.[5][12]

Mitochondrial DNA

Main article: Mitochondrial DNA

Mitochondrial organelles, which contain mitochondrial DNA (mtDNA), are passed only from mother to offspring. A comparison of DNA sequences from mtDNA in a population reveals a molecular phylogeny. Unlike mtDNA, which is outside the nucleus, genes containing nuclear DNA become recombined after being inherited from both parents, and therefore we can be statistically less certain about nuclear DNA origins than we can for mtDNA, which is only inherited from the mother. mtDNA also mutates at a higher rate compared to nuclear DNA, so it gives researchers a more useful, magnified view of the diversity present in a population, and its history.[13][14]

Just as mitochondria are inherited matrilineally, Y-chromosomes are inherited patrilineally.[15] Thus it is possible to apply the same principles outlined above to men. The common patrilineal ancestor of all humans alive today has been dubbed Y-chromosomal Adam. Importantly, the genetic evidence suggests that the most recent patriarch of all humanity is much more recent than the most recent matriarch, suggesting that 'Adam' and 'Eve' were not alive at the same time. While 'Eve' is believed to be alive 140,000 years ago, 'Adam' lived only 60,000 years ago.[5]

Mitochondrial clock

In order to determine the timing of an evolutionary event such as the origins of the common ancestor of human beings living today for which the fossil record is incomplete, Zuckerkandl and Linus Pauling devised the hypothesis of the molecular clock, which dates events based on analysis of change of molecular traits.[16]

An initial assumption for the rate of mutation was made based on the fossil record of human-chimpanzee divergence of 5 million years ago. [citation needed] This rate was then applied to date the mitochondrial eve sequence, placing it between 120,000 to 220,000 years ago. [17] In the 1990s, scientists made a startling discovery. Mitochondrial DNA appeared to mutate faster than expected, which raised troubling questions concerning the dating of evolutionary events. The discovery was made upon comparing the noncoding mitochondrial DNA sequence of the remains of the Russian tsar Nikolai II to that of his relatives. Using this new calibrated clock puts Mitochondrial Eve at a mere 6,000 years ago. [18]. A subsequent study that used a far greater area of the mitochondrial sequence does not confirm the high rate of mutation, and instead dates Mitochondrial Eve at 170,000 (+/-50,000) years [19]

Some have suggested a varying mutation rate or an inconsistent molecular clock. [20]

Eve and the Out-of-Africa theory

Main article: Out of Africa hypothesis

Homo sapiens is assumed to have speciated from Homo heidelbergensis in the period of 200–160 kya. The fact that Mitochondrial Eve happens to be dated to precisely this period has been taken as evidence of a population bottleneck (e.g. Toba catastrophe theory) giving rise to the human species. There are, however, many ways such family trees can be constructed. A tree can be constructed based on any gene, not just the mitochondrial DNA. When different such trees including the mtDNA tree are compared, no population bottleneck is found because different trees show different coalescent points. The inconsistencies between coalescent points indicate that there had been numerous gene interchanges between population groups around the world, even after the first exodus out of Africa. This idea forms the basis of Alan Templeton's 'Out of Africa Again and Again' theory.[5][21]

The Mitochondrial DNA provides another support for the Out of Africa hypothesis in the form of gene diversity. One finding not subject to interpretation is that the greatest diversity of mitochondrial DNA sequences exists among Africans. This diversity is believed to have accumulated because humans have been living longer in Africa than anywhere. Family trees (or "phylogenies") constructed on the basis of mitochondrial DNA comparisons show that the living humans whose mitochondrial lineages branched earliest from the tree (L0) are prevalent among the San and the Mbuti people.[22] The subsequent branches of L1, L2 and L3 are also largely confined to Africa, while only the macrogroups M and N, descended from L3, participated in the ancient migration(s) out of Africa.

File:Adam and eve newsweek cover.jpg
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 record number of copies.[23]

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.

  • 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.[24]
  • 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 descended from the artificial cylons and humans from the Twelve Colonies.

See also

References

  1. ^ a b Schaffner, Sf (2004). "The X chromosome in population genetics" (PDF). Nature reviews. Genetics. 5 (1): 43–51. doi:10.1038/nrg1247. ISSN 1471-0056. PMID 14708015. {{cite journal}}: Unknown parameter |month= ignored (help); Unknown parameter |qoute= ignored (help)
  2. ^ Gonder MK, Mortensen HM, Reed FA, de Sousa A, Tishkoff SA. (2007). "Whole-mtDNA genome sequence analysis of ancient African lineages". Mol. Biol. Evol. 24 (3): 757–68. doi:10.1093/molbev/msl209. PMID 17194802. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  3. ^ Cite error: The named reference name=Ingman00 was invoked but never defined (see the help page).
  4. ^ a b Cann RL, Stoneking M, Wilson AC (1987). "Mitochondrial DNA and human evolution". Nature. 325 (6099): 31–6. doi:10.1038/325031a0. PMID 3025745.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ a b c d Dawkins, Richard (2004). The Ancestor's Tale, A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company. ISBN 0-618-00583-8.
  6. ^ Dennett. Darwin's Dangerous Idea.
  7. ^ Behar; et al. (2008). "The Dawn of Human Matrilineal Diversity". {{cite journal}}: Cite journal requires |journal= (help); Explicit use of et al. in: |last= (help)
  8. ^ Gonder; et al. (2006). "Whole-mtDNA Genome Sequence Analysis of Ancient African Lineages". {{cite journal}}: Cite journal requires |journal= (help); Explicit use of et al. in: |last= (help)
  9. ^ A Suggested Genome for “Mitochondrial Eve”
  10. ^ Rohde DL, Olson S, Chang JT (2004). "Modelling the recent common ancestry of all living humans". Nature. 431 (7008): 562–6. doi:10.1038/nature02842. PMID 15457259. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  11. ^ See the chapter All Africa and her progenies in Dawkins, Richard (1995). River Out of Eden. New York: Basic Books. ISBN 0-465-06990-8.
  12. ^ Rohde, DLT , On the common ancestors of all living humans. Submitted to American Journal of Physical Anthropology. (2005)
  13. ^ Wilson AC, Cann RL, Carr SM; et al. (1985). "Mitochondrial DNA and two perspectives on evolutionary genetics". Biol J Linn Soc Lond. 26 (4): 375–400. doi:10.1111/j.1095-8312.1985.tb02048.x. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  14. ^ Sykes, Bryan (2001). The Seven Daughters of Eve. New York: Norton. ISBN 0-393-02018-5.
  15. ^ Sykes, Brian D. (2003). Adam's curse: a future without men. London: Bantam. ISBN 0-593-05005-3.
  16. ^ Zuckerkandl, E. and Pauling, L.B. (1962). "Molecular disease, evolution, and genetic heterogeneity". In Kasha, M. and Pullman, B (editors) (ed.). Horizons in Biochemistry. Academic Press, New York. pp. 189–225. {{cite book}}: |editor= has generic name (help)CS1 maint: multiple names: authors list (link)
  17. ^ Hedges SB, Kumar S, Tamura K, Stoneking M (1992). "Human origins and analysis of mitochondrial DNA sequences". Science. 255 (5045): 737–9. PMID 1738849. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  18. ^ Gibbons A (1998). "Calibrating the mitochondrial clock". Science. 279 (5347): 28–9. PMID 9441404. {{cite journal}}: Unknown parameter |month= ignored (help)
  19. ^ Ingman M, Kaessmann H, Pääbo S, Gyllensten U (2000). "Mitochondrial genome variation and the origin of modern humans". Nature. 408 (6813): 708–13. doi:10.1038/35047064. PMID 11130070. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  20. ^ Ho SY, Larson G (2006). "Molecular clocks: when times are a-changin'". Trends Genet. 22 (2): 79–83. doi:10.1016/j.tig.2005.11.006. PMID 16356585. {{cite journal}}: Unknown parameter |month= ignored (help)
  21. ^ Templeton A (2002). "Out of Africa again and again". Nature. 416 (6876): 45–51. doi:10.1038/416045a. PMID 11882887. {{cite journal}}: Unknown parameter |month= ignored (help)
  22. ^ Chen YS, Olckers A, Schurr TG, Kogelnik AM, Huoponen K, Wallace DC (2000). "mtDNA variation in the South African Kung and Khwe-and their genetic relationships to other African populations". Am. J. Hum. Genet. 66 (4): 1362–83. doi:10.1086/302848. PMC 1288201. PMID 10739760. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  23. ^ Oppenheimer (2004). "Out of Africa". The Real Eve. ISBN 0786713348. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
  24. ^ Review of Greg Egan story, Mitochondrial Eve
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Phylogenetic 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   O A S R   I W X Y
C Z B F R0   pre-JT   P   U
HV JT K
H V J T
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