Dr. Stephen Oppenheimer

Your Questions Answered

How is it that every human alive today is able to trace their mitochondrial DNA to that of a single prehistoric woman? And how can we know for sure what route our ancestors took out of Africa?

Hundreds of you had questions for Oxford University's Stephen Oppenheimer, featured in the program The Real Eve (click here to see his bio). And he had the answers. Check them out below

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Q: I have been hearing about this for years, and finally The Real Eve gives me some kind of detail and confirmation. What I still don't understand is how humans physically changed during the migration from Africa to the Americas. How could we physically and genetically change from dark hair and dark skin to finer hair and paler skin?

A: In evolutionary terms, changes of skin and hair color are relatively minor events and can take place in a population over a period as short as 20,000 years. Other animal species can also show enormous differences in skin and hair pigmentation; for instance, the black panther is just a black or "melanistic" leopard. Our skin color is controlled by rather few genes. These produce melanin, a brown pigment. The normal or original state for modern humans is probably black because melanin protects against the harmful effects of the sun in the tropics. The melanin genes can mutate in various ways that stop our skin from making effective pigment. In northern Europe and northern Asia, this is beneficial, because too much melanin in a low-sun environment prevents production of vitamin D. Over thousands of years, people with paler skin survive better in the north and are "naturally selected" to survive. This is the most likely reason Europeans and northern Asians are pale-skinned.

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Q: All people who left Africa are closely related because they have a common mother. What about those who stayed in Africa? They cannot trace their mitochondrial DNA to the same woman, unless their ancestors migrated back to Africa? Does that mean that, genetically, Africans are a separate evolutionary branch of humans?

A: No, Africans are part of the same closely related tree of modern humans. They are not a separate branch. Under the skin we are all very alike. The common mtDNA ancestor of all modern humans lived in Africa over 150,000 years ago. In evolutionary terms this was very recent, a mere eye-blink. Her mtDNA line was the common ancestor of all modern humans. Before the exodus from Africa around 80,000 years ago, the African mtDNA tree already had a number of branches. Just one of these branches made it out of Africa and survived to give rise to mtDNA of all non-Africans. This single mtDNA branch, called L3, still has some descendants in Africa as well. This shows that Africa was where L3 originally came from.

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Q: I am still a little confused as to whether "mitochondrial Eve" refers to a single line of ancient humans or to a single woman within the single line of ancient humans. I am inclined to the single-woman theory, since it seems reasonable to extrapolate that the single line must have had a beginning that in turn must have been a single and unique woman. Is my inclination really correct or am I indeed still confused?

A: The confusion is to see genetic lines too literally as representing individual humans. The so-called "genetic Eve" was the ancestral mitochondrial genetic line for all modern living humans. Obviously it was carried ultimately by one real woman over 150,000 years ago. But she was only the common ancestor for mitochondrial DNA. She did not carry all the rest of our ancestral genes. We have 30,000 functioning genes and they could each have had a different individual ancestor living at different times in different places.

The mitochondrial Eve was therefore one woman among thousands living over 150,000 years ago. Our other genes derive from our members of that ancestral population. The real importance of the mitochondrial genetic tree is that it gives a clear line of descent that can be used as a trail marker of our spread round the world. But it is only one particular tiny part of our huge human genome.

How and where could I get my mitochondrial DNA tested?

A: There are a number of private firms that can test mtDNA in the United States now. These can be found on the Internet. I believe one company, for instance, was set up by well-known American geneticist Dr. Rick Kittles, to test Americans of African ancestry wishing to know their mtDNA type. I should warn that the lab work is not a cheap process.

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Q: It was my impression that the Oxford group had identified several (four to seven) African, female progenitors. Is this incorrect, or does new data suggest that these all descended from one female?

A: Your second statement is correct. I think the several African "progenitors" you are referring to are the various surviving branches of the mitochondrial tree in Africa. By sequencing and comparing their mtDNA, these branches can be joined together as a bush or tree and lead back to a common root called, by some, the African mitochondrial Eve, or the most recent common ancestor of all modern humans.

Just one of these branches, labeled L3, also spread out of Africa I believe around 80,000 years ago. The root of L3 was the ancestral mtDNA line for all non-Africans. In other words L3 was the out-of-Africa Eve.

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Q: Is your theory of a single migration out of Africa across the Red Sea supported by Y-chromosome trees?

A: In my view, yes. One branch of the African Y-chromosome tree, defined by a marker called M168, gave rise to all non-African Y chromosomes.

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Q: Have the details of the single female progenitor been published?

A: Yes. The details are a bit technical: There are a number of scientific papers that show a single line coming out of Africa now. The important ones as far as mitochondrial DNA is concerned are:

* Watson E., Forster P., Richards M., Bandelt H.-J. (1997). "Mitochondrial Footprints of Human Expansions in Africa." American Journal of Human Genetics. 61: 691-704

* Richards M., and Macaulay V. "Genetic Data and Colonization of Europe: Genealogies and Founders," in Archaeogenetics: DNA and the Population Prehistory of Europe (eds. C. Renfrew and K. Boyle). MacDonald Institute Monograph, pp.139-141

This last paper had a tree that showed that the L3 line had two genetic daughters who jointly colonized every corner of Asia and Australia/New Guinea. One of the two daughters also gave rise to all Europeans and the Near East.

Q: Can you recommend a book that covers this migration theory and includes a thorough explanation of the underlying mitochondrial DNA analysis?

A: Well, I would like to recommend my own book, The Peopling of the World, written for The Real Eve film and coming out later this year.

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Q: Do you feel this information on the "real Eve" will be accepted by the anthropological community at large, and how long do you think it will take Western educational systems to begin to disseminate this knowledge as historical fact?

A: I hope so. The first important out-of-Africa genetic papers were published in 1986/1987 respectively. That general view is so widely accepted now that it is the orthodoxy in the anthropological and archaeological community. I feel the further logic of the single southern exodus through Yemen toward India, subsequently giving rise to all non-Africans, including Europeans, is compelling and will also become generally accepted by the scientific community.

Dissemination in education systems is another matter. Different Western countries and their regions have differing views on what should be taught in their schools. The word "fact" is also difficult and sometimes inappropriate to use in reconstructions of prehistory. Such reconstructions are usually approximate; they are not always accepted by everybody, and they may change as new evidence is found.

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Q: Since we are so closely related, does this not make modern humans vulnerable to an aggressive disease? AIDS being one example.

A: This is an important point. I discuss it in my forthcoming book The Peopling of the World. Lack of diversity in a species may increase susceptibility to certain infections, especially those coming from other animal reservoirs. Humans may be numerous, but they lack diversity. Overcrowding and rapid worldwide travel and communication increase the risk of spread of exotic infectious disease.

Q: Does the theory of the "real Eve" complement or contradict any linguistic theories that seek to show evolution/migration of man?

A: This is a controversial field. In my view, however, the "real Eve" cannot complement or contradict any linguistic theories that seek to show the evolution/migration of man. This is because the time scale we deal with in The Real Eve is only brought up to about 10,000 to 15,000 years ago. There are disagreements between linguists on how far back they can trace language families. Most historical linguists would agree, however, that gradual language change prevents them from tracing family trees much further back than around 7,000 to 8,000 years ago. This does not mean that some linguists have not tried to push the curtain further back, and I deal with this in my book.

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Q: How do you respond to the work of Parsons, Gibbons, Loewe and others, which suggests that mtDNA mutates at a much higher rate than the rates determined based on evolutionary assumptions? [Parsons, T.J. et al. (1997). "A High Observed Substitution Rate in the Human Mitochondrial DNA Control Region." Nature Genetics Vol. 15: 363 - 368; Gibbons, A. (Jan. 2, 1998). "Calibrating the Mitochondrial Clock." Science 279 (5347): 28-29; Loewe, L. and Scherer S. (Nov. 1997). "Mitochondrial Eve: The Plot Thickens." Trends in Ecology and Evolution, 12(11): 422-423.]

A: There are two broad issues here. One is that individual mtDNA sites and even whole regions within the molecule mutate at different rates, and the other is how to calibrate the mtDNA clock. Briefly, the problem with different site rates is largely overcome in the method of "Rho" calculation by averaging out the different rates over the same stretch of mtDNA for all comparisons. The calibration of the clock can be made using prehistoric events as independent markers. These events can be in the very distant past, like the split between the ancestors of chimps and those of ourselves, and more recent events like the big thaw after the last ice age, which allowed people to expand north again both in the Americas and in Eurasia. Different calibrations can be used to cross-check each other. Such efforts to calibrate the genetic clock are under constant review, and it should be realized that genetic methods of dating remain somewhat imprecise. The various objections on rates are dealt with admirably in Macaulay V. et al (1997). "mtDNA rates — No Need to Panic." American Journal of Human Genetics Vol. 61: 983-986.

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Q: Since modern humans supplanted the Neanderthals in the Middle East and later through Europe, could this "rapid" replacement have been caused by a disease to which the Neanderthals had never been exposed because of their long isolation? After all, Neanderthals had 250,000 years of success on the planet before being introduced to modern humans.

A: It is possible, but there is no proof that I know of. The other problem with that view is that imbalance in survival between the two peoples resulting from different initial exposure to disease might largely have evened out over the thousands of years that Neanderthals and modern humans coexisted in Europe.