When people think of genes and you think of the sort of traits that the genes influence in humans,
like our eye color, our hair color, our skin color, our stature, our height, our weight:
those are all influenced by genes that are carried on chromosomes within the nuclei of our cells,
and you get half of those genes from your mother and half from your father.
But, in addition to that genetic material, we have a special kind of DNA called mitochondrial DNA,
which is found outside the nucleus of the cells in structures called mitochondria,
which produce the energy needed for the cell.
And that DNA is particularly useful for our sorts of studies
because you don't get part of it from your mother and part from your father.
You get all of your mitochondrial DNA only from your mother,
and she in turn got all of her mitochondrial DNA only from her mother.
So when we look at mitochondrial DNA variation,
we're basically getting a picture of the maternal history of a population or of a species.
And what's particularly nice then about mitochondrial DNA,
and the fact that it's only inherited through the maternal line, then,
is that the only source of new variation in mitochondrial DNA are new mutations.
So if I compare my mitochondrial DNA to your mitochondrial DNA,
and I can count the number of mutational differences there are,
the more mutational differences there are,
the further back in time we last shared a common maternal ancestor.
So we can use this estimate of the number of mutations to take
all of the mitochondrial DNA types in a sample of individuals and estimate their genealogy.
What ended up being one of the more controversial aspects of the work was this idea
that all of the variation that we see today among mitochondrial DNAs of living people
traces back to a single common ancestor--meaning a single individual
who lived at a particular time and a particular place.
If you start with the idea that all life has a single origin on this planet,
and every thing alive today is descended from that single origin of life,
then it has to be the case that all of the variation in any molecule that one looks at--
whether it's mitochondrial DNA, whether it's a gene in the nucleus of the DNA--
whatever DNA one looks at has to at some point in the past trace back to a single common ancestor.
The fact that mitochondrial DNA is maternally inherited,
so it's traced from mother to mother to mother,
means that the common ancestor of all mitochondrial DNAs in humans today
would have to have been a woman living some time in the past.
There was a lot of confusion about whether or not was this the first modern human.
Was this an Eve in the biblical sense, that this was the only woman who was alive at that time?
And so now when we talk about the work, especially to a public audience,
we try to be careful to point out that this is just the fact that we traced the variation
back to a single individual at some point in the past.
This is just a natural consequence of evolutionary theory,
and there's nothing particularly special about this.
We can, in principle, do this for any of our genes.