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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
