A reader writes:
You quoted a reader who wrote, wrongly:
It's more complicated than
population of humans at one point diminished to a point where all humans now living are in fact descended from just one woman in the population, dubbed "mitochondrial Eve." This does not mean that we are all descended from the "first woman," just that we are all descended from the "same woman." Some Jews and Christians who are not literalists will nonetheless take this as a comforting affirmation of the reality of our common humanity, as will some secular humanists. At any rate, it is what it is, an emerging scientific fact.
This is a really silly interpretation of a coalescent date for a genetic locus. It is NOT true that "mitochondrial Eve" means that the population size of our ancestors fell to the point where we all have but a single female ancestor. What actually happens is that in all populations, of whatever size, some individuals leave no or few descendants, while other individuals leave many.
Barring new mutations, each generation would lose a certain amount of its pre-existing genetic diversity and, with enough time, all of the pre-existing genetic diversity is lost and all individuals would carry just one of the pre-existing genetic variants. This is what happened in the case of "mitochondrial Eve," enough time has gone by that all of the other mitochondrial DNA variants present more than 200,000 years ago have disappeared except for one. The only reason we don't actually all now have the same mitochondrial DNA is that during this interval there have been many mutations occurring. So over time, the diversity that had existed is lost, while new mutations generate new diversity within the remaining lineages.
The size of the population can affect how quickly this all happens, and how much diversity a population can "hold on to" but the fact that mtDNA coalesces to a single ancestral "Eve" does not indicate by itself that past populations were particularly small 200,000 years ago. Furthermore, there is also a "Y-chromosome Adam" due to the same process: all living male Y chromosomes can be traced back to a single copy. Not only that, but every one of the 3 billion other base pairs in the genome that is biparentally transmitted can also be traced back to a different common ancestor. So there are very many different "Adams" and "Eves" who were the sources of the "ancestral" copies from which all current diversity is derived, for each of these loci.
These many Adams and Eves may go back to many different points in the past (since the variants are lost and mutations occurs in a somewhat random manner), and there is nothing special about these many many Adams and Eves, they just happened to be the individuals in the past who carried a copy of that particular bit of DNA that was left after all the other lineages for that locus were lost by random processes. Further, if the living humans carrying the most divergent sequence at a given gene die without reproducing, then the most common recent ancestor of that gene become a different human who lived later in time who represents the ancestral variant common to the diminished diversity present in future humans. For example, when Neanderthals were still alive, our common "mitochondrial Eve" was a woman who lived more than 500,000 year ago. The minute that the last Neanderthal died some 30,000 years ago, the designation "mitochondrial Eve" shifted to a different woman who lived 200,000 years ago.
Think about it in the sense that there may be nothing special about the oldest tree in a forest other than by chance it is the one that has survived the longest without getting destroyed by fire, pests, etc. And if that tree gets destroyed, then there will be a different oldest tree, one that germinated more recently in time. But the fact that we can identify an oldest tree in the forest does not mean that there were not many other trees around when the tree that is currently oldest first germinated.
Another expert weighs in:
As a population geneticist, let me clear up the confusion over "mitochondrial Eve." Animals have two kinds of DNA. Nuclear DNA is what we normally think of when we think of DNA. It is the stuff that makes up chromosomes, and it is the source of virtually all of our genetic information – humans have about 3 billion base pairs (i.e. As, Ts, Gs, and Cs) of nuclear DNA. However, our mitochondria, the cell organelles responsible for providing the cell with power, have a small bit of their own DNA – about 16,000 base pairs.
It is true that we can trace the origin of all human mitochondrial DNA back to a common mitochondrial ancestor around 200,000 years ago. However, this is simply the common ancestor of our mitochondrial DNA – a far cry from the common ancestor of all of DNA. All of our other genes would trace back to their own common gene ancestors (most of them much farther back in time). Thus, it is erroneous to claim that the individual carrying our common mitochondrial ancestor was THE common ancestor of humans.
To help clarify the fallacy, imagine a hypothetical person named Joe traced the gene responsible for his hairy ears back to his paternal grandfather. It would be erroneous to claim the paternal grandfather as THE ancestor of Joe. Joe had four grandparents that each contributed aspects of his genome. In the same way, "mitochondrial Eve" contributed the mitochondrial DNA to all humans alive today. However, the mitochondrial genome is a very small part of human DNA, and we know that many thousands of other individuals contributed other aspects of our current genetic diversity. Thus, genetics, in no way, supports a literal Adam or a literal Eve. And I say this as a Christian.
(Image of mitochondria via MIT)
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