MadSci Network: Evolution

Re: So we have a genetic Eve, but what about Eve's ancestors?

Date: Wed May 20 22:58:34 2009
Posted By: Steve Mack, Assistant Staff Scientist, Molecular and Cell Biology
Area of science: Evolution
ID: 1242857189.Ev

I have been researching our supposed genetic Eve from Africa that all of mankind is related to through mtDNA. It seems a small possibility that everyone descended from one woman, even though there were other women alive and procreating at the same time but not altogether impossible. However, what I want to know is who were Eve's parents, grandparents, great grandparents and so on? If we are all descendents of her then we are all descendents of her mother and so on and so forth. If we trace this back will we eventually see that we all all descended from one single gene involved in the Big Bang? This is technically impossible right now but I want to know why this woman is our Eve and not her ancestors.

Hi Megan,

We have quite a few answers in our archives that discuss the so-called "Mitochondrial Eve", the most recent common female ancestor of all modern humans. So, I am going to refer you to those answers in the archives, as well as to the Talk Origins Archive for additional background on "Mitochondrial Eve"

As you will see if you follow those links, so-called Mitochondrial Eve is the most recent common female ancestor of all modern humans, and is not the only female ancestor of all humans. What the term most recent common ancestor tells us is that of course, there were other older ancestors, but that the one we are talking about is the very last one held in common. Since we are talking about mitochondrial DNA, we are only talking about the most recent common female ancestor of all modern humans; as you will see from the rest of my answer, we have inherited nuclear genes from that woman's relatives and ancestors.

When we consider the theory of evolution (all species are descended from older species) and Rudolf Carl Virchow's observation that "Omnis cellula e cellula" (all cells come from cells), we can conclude that all life on Earth is descended from one single common ancestor -- the first cell, which existed billions of years ago.

So, we have always known that all humans shared a common ancestor; until 1987, the question facing human evolutionary biology was "how long ago did the most recent common ancestor of humans live?" Prior to the 1980s, it was assumed that the most recent common human ancestor was a Homo erectus or a member of an even older species, and that humans in different parts of the world had evolved largely independently of each other for millions of years. In 1987, Rebecca Cann, Mark Stoneking and Allan Wilson determined that the mitochondrial chromosomes of modern humans were extremely similar; so similar that they all have to had been descended from an ancestral mitochondrial chromosome that was only 200,000 years old. Because we inherit our mitochondrial chromosomes only from our mothers, that meant that our maternal lineages all led back to one woman who lived approximately 200,000 years ago*.

What Cann, Stoneking and Wilson did not say was that that woman was the only woman who was alive at that time. And of course, there were many women alive at that time (the effective (i.e., breeding) population size of that ancestral human population is estimated to be around 10,000 individuals), and we are descended from them. Most of the genes in the nucleus are inherited from both parents, and we have all inherited nuclear genes from members of that ancestral population. Because we have twice as many nuclear genes as mitochondrial genes, the most recent common ancestor for any nuclear gene (except for Y-Chromosome genes) is much older than the most recent common ancestor of the mitochondrial chromosomes. So, the nuclear genes that we all have today are likely inherited from those ancestors of "Mitochondrial Eve" that you were asking about.

Primary Reference:
Cann RL, Stoneking M, Wilson AC (1987) Mitochondrial DNA and human evolution. Nature. 325 (6099):31-6.

Historical Footnote: I was a graduate student of Allan Wilson's (in whose laboratory the initial work identifying the most recent common female ancestor of all modern humans as having lived in Africa approximately 200,000 years ago was done), and can say that he really objected to the use of the name 'Eve' in "Mitochondrial Eve" or "African Eve" as it distracted from the central issue of the work, and left the incorrect impression that there was only one single woman alive at the time.

Factual Footnote: Because of the uni-parental mode of inheritance for the mitochondrial chromosome, mitochondrial lineages (mitochondrial chromosomes with a particular DNA sequence) die out at a particular (stochastic) rate; because not every woman gives birth to daughters. So, the age of the most recent common mitochondrial ancestor is something of a function of the size of the human population (the number of women of reproductive age); the more women of reproductive age there are in a given generation, the lower the chance that a particular lineage will not be passed on to the next generation. Therefore, the concept of a most recent common ancestor is something of a moving target; in the future, the most recent common female ancestor of all (future) humans will be a different more recent common female ancestor than our current most recent common female ancestor.

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