What, if anything, is a Mitochondrial Eve?
I wrote this essay in December, 1995 for the ERRANCY mailing
list (which is devoted to the discussion and refutation of the doctrine
of biblical inerrancy).
One of the best indications that a scientific and mathematical statement
has not been explained properly is the many different (and incorrect)
ways people interpret it.
Excellent discussions of the Mitochondrial Eve are to be
- Bryan Sykes, The Seven Daughters of Eve, Bantam Press, 2001.
- Richard Dawkins, River out of Eden, Basic Books, 1995.
- Daniel C. Dennett, Darwin's Dangerous Idea, Simon and
Here are some points to note:
- The name Eve, in retrospect, is perhaps the worst possible
name to give to the entity in question. I believe that
this is probably the cause of so much confusion in understanding what
the significance of this entity is. People think that this title has
some deep theological or religious consequences. Nothing of that sort.
Someone you come across who claims that the bible (or the book of Genesis)
has been validated by the discovery of the Mitochondrial Eve, is
talking crap---you should feel free, and even obligated, to tell them so.
- The Mitochondrial Eve of 200,000 years ago (ME for short
henceforth) is NOT our common ancestor, or even common
genetic ancestor. She is the most-recent common ancestor of
all humans alive on Earth today with respect to matrilineal descent.
That may seem like a mouthful, but without even a single one of those
qualifying phrases, any description or discussion of the ME reduces to
a lot of nonsense.
While each of us necessarily has two parents, we get our mitochondria and
mitochondrial DNA from the ovum (and hence from our mothers). Our mothers
got their mitochondrial DNA from their mothers and so on. Thus, while
our nuclear DNA is a mish-mash of the DNA of our four
grandparents, our mitochondrial DNA is an almost exact copy of the DNA of
our maternal grandmother (the match may not be exact due to mutations.
In fact, the mutations in the mitochondrial DNA provide the molecular
clock that allows us to determine how much time has elapsed since the
The ME represents that woman whose mitochondrial DNA (with mutations)
exists in all the humans now living on Earth. That does
not mean that she is our lone woman ancestor. We have ancestors who
are not via matrilineal descent. For example, our father's mother (who
did pass on her mitochondrial DNA to her daughters) is an example of an
ancestor who is not matrilineal to us. However, she did exist at one
time and was probably of the same age as our mother's mother, who is
a matrilineal ancestor of ours and from whom we got our mitochondrial
- The term Mitochondrial Eve itself is a title given
retroactively to a woman. Often (and as is certainly the
case with the ME that we are discussing) the conferring of the title
occurs many hundreds of thousands of years after the death of the woman
- ME lived with many other humans (men and women); she was certainly not
alone. When she was alive, she was most certainly NOT
the Mitochondrial Eve. The title at that time was held
by a distant ancestor of hers (and of the many humans who were her
- The existence of the Mitochondrial Eve is
NOT a theory; it is a mathematical fact (unless something
like a multiple-origins theory of human evolution i.e. the human species
arose independently in different geographically separated populations,
and that the present-day ease of interbreeding is the result of a
remarkable convergent evolution, is true. Few people subscribe to the
multiple-origins theory, and the Mitochondrial Eve observation
is a refutation of multiple-origins).
- The proof for the existence of a Mitochondrial Eve is as
follows (based on an argument by Daniel Dennett in the above mentioned
Consider all the humans alive today on Earth. Put them into a set S.
Next, consider the set of all those women who were the mothers of the
people in the set S. Call this set S'. A few observations about this
new set S'. It consists of only women (while set S consists of both
men and women)---this is because we chose to follow only
the mother-of relationship in going from set S to set S'.
Also note that not every member of set S' needs to be in set S---set
S consists of all people living today, while some of the mothers of
living people could have died, they would be in set S' but not in set S.
Third, the size of set S' is never larger than the size of set S. Why?
This is because of the simple fact that each of us has only
one mother. It is however overwhelmingly more likely that the size of
set S' is much smaller than that of set S---this is because each woman
usually has more than one child.
Repeat the process of following the mother-of relationship with
set S' to generate a new set S''. This set will consist of only women,
and will be no larger (and very likely smaller) than set S'.
Continue this process. There will come a point when the set will
consist of smaller and smaller number of women, until we finally come
to a single woman who is related to all members in our original set
via the transitive-closure of the mother-of relation. There is
nothing special about her. Had we chosen to follow the father-of
relation, we would have hit the Y-chromosome Adam (more on him
later). Had we chosen to follow combinations of mother-of and
father-of relations, we would have hit some other of our common
ancestors. The only reason why the mother-of relationship seems
special is because we can track it using the evidence of mitochondrial
Thus there must exist a single woman whose is the
matrilineal most-recent common ancestor of every in set S.
A few others points to keep in mind. One might say that if each woman
has only a single daughter (and however many sons), the size of the sets
will be the same as we extrapolate backwards. But also note that this
backwards mathematical extrapolation is an extrapolation into the past.
This process cannot be continued indefinitely because the age of the
Earth, life on Earth, and the human species is finite (this argument
comes from Dawkins).
Also important to keep in mind is that while the final set S'* has only
one member (the Mitochondrial Eve), she was by no means the only living
woman on Earth during her lifetime. Many other women lived with her,
but they either did not leave descendents or did not leave descendents
via the matrilineal line, who are still alive today.
- Let us now see how the title of Mitochondrial Eve can
Consider an extremely prolific woman living today. She has many
daughters and takes a vacation to a remote Carribean island for a week.
During the same week a plague of a mutated Ebola virus sweeps the Earth
and drastically decreases the fecundity of all living women. Not only
that, the viral infection also changes the genome of these women so that
the daughters they give birth to will inherit this reduced fecundity.
This means that far more than average of their fetuses will undergo
abortions (or, in a somewhat kinder scenario, their female
fetuses will be aborted more often than male ones).
Only this one woman and her daughters who were off in this Carribean
island are safe from the viral plague. Also assume that the viral plague
consumes itself within that fateful week. This woman and her daughters
are now free to breed in a world where their reproductive potential far
outstrips that of every other woman alive (and to be born of these women).
Soon, almost every one on Earth will be related in some fashion to this
one woman. Finally, when the last woman who was born to one of the
matrilineal descendents of an infected woman dies, the non-infected
Carribean tourist takes on the title of the new Mitochondrial
Eve. Every human alive on Earth at that point in
time is now related via the mitochondrial line to her.
But consider this new twist. Suppose a group of astronauts (men and
women) were sent off into space during the infection week, and were
thus not infected themselves. After many centuries in a Moon or Mars
colony, they returned to Earth. At that time, suddenly, the title of
Mitochondrial Eve would revert back to our own ME. The humans
alive on the Earth at that time would all share their mitochondrial DNA
with an earlier common ancestor.
- Thus the title of Mitochondrial Eve depends very critically
on the present human population of the Earth. As people die or are
born, the title can change hands. Once a ME is established (via the
death of a matrilineal line), further births cannot change the title.
Further deaths can, however, transfer the title to a more recent woman.
The older ME is still the common ancestor of all humans alive
today on Earth with respect to matrilineal descent, but she is not the
most-recent .... This is part of the reason why I said that
each and every word of that definition was important.
As an exercise, try to eliminate just one phrase of the definition of
the ME and see what happens. The key terms are most-recent,
common ancestor, humans alive today, matrilineal
I mentioned the Y-chromosome Adam (YcA for short) earlier
in discussing patrilineal descent. The YcA has also been identified
(by the careful sequencing of a small region of the Y-chromosome that
all men carry) and has been dated considerably more recent than the ME
(yet another slap-in-the-face for bibliolaters---their Adam
and Eve lived many tens of thousands of years apart).
The YcA is not as special as the ME because only men carry the
Y-chromosome, whereas all humans, men and women, carry mitochondria and
mitochondrial DNA. So the YcA would not leave the same kind of trace in
women living today as the ME did. However, the existence of the YcA is
as mathematically necessary as the existence of the ME (use the earlier
set argument, but now with the father-of relationship).
While the existence of the ME and the YcA are mathematical, I am more
interested in the point in time when the titles were conferred on
the particular ME and YcA were are talking about today. These people
have held their respective titles for perhaps many centuries, but the
really tantalizing question is when they qualified. Was the original
human population (from which we all descended) so small that our ME was
identified very quickly after her death or did the death of an old woman
in a remote village in Southern Africa during the time that the Pharohs
ruled in Egypt represent that critical demise of the last matrilineal
line not connected with our ME. Similar arguments hold for the YcA.
A final note. The techniques of DNA sequencing, DNA-relatedness
comparisons, and the calibration of the molecular clock have been
improving dramatically over the past few years. The existence of the
Mitochondrial Eve and the Y-chromosome Adam are no longer in any doubt
(remember, both are mathematical necessities)---what is still being
discussed is the estimation of how long ago they lived. Determining their
ages requires an accurate calibration of the molecular clock and there
is some disagreement here.
Copyright © Krishna Kunchithapadam
Last updated: Mon May 1 16:46:06 PDT 2000
Mirrored from: http://www.geocities.com/krishna_kunchith/misc/eve.html
This page is part of the Fossil Hominids FAQ at the talk.origins Archive.
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