What, if anything, is a Mitochondrial Eve?

by
Krishna Kunchithapadam

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).

Krishna.

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.

Two excellent discussions of the Mitochondrial Eve are to be found in:

Richard Dawkins, River out of Eden, Basic Books, 1995.
Daniel C. Dennett, Darwin's Dangerous Idea, Simon and Schuster, 1995.

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 could be further from the truth.
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 ME lived).
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 DNA.
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 in question.
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 contemporaries).
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 book).
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 DNA.
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 change hands.
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 descent.

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

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