Tracing Ancestry with MtDNA
by Rick Groleau
In 1987, three scientists announced in the journal Nature that they had found a common ancestor
to us all, a woman who lived in Africa 200,000 years ago. She was given the name "Eve," which
was great for capturing attention, though somewhat misleading, as the name at once brought to
mind the biblical Eve, and with it the mistaken notion that the ancestor was the first of our
species—the woman from whom all humankind descended.
The "Eve" in question was actually the most recent common ancestor through matrilineal descent
of all humans living today. That is, all people alive today can trace some of their genetic heritage
through their mothers back to this one woman. The scientists hypothesized this ancient woman's
existence by looking within the cells of living people and analyzing short loops of genetic code
known as mitochondrial DNA, or mtDNA for short. In recent years, scientists have used mtDNA
to trace the evolution and migration of human species, including when the common ancestor to
modern humans and Neanderthals lived—though there has been considerable debate over the
validity and value of the findings.
In reproduction, the nuclear DNA of one parent mixes with the nuclear DNA of the other.
MtDNA, on the other hand, passes on from mother to offspring unaltered.
Nuclear DNA vs mitochondrial DNA
When someone mentions human DNA, what do you think of? If you know a little about the
topic, perhaps you think of the 46 chromosomes that inhabit the nucleus of almost every cell that
comprises your body. These chromosomes hold the vast bulk of genetic information that you've
inherited from your parents.
Outside the nucleus, but still within the cell, lie mitochondria. Mitochondria are tiny structures
that help cells in a number of ways, including producing the energy that cells need. Each
mitochondrion—there are about 1,700 in every human cell—includes an identical loop of DNA
about 16,000 base pairs long containing 37 genes. In contrast, nuclear DNA consists of three
billion base pairs and an estimated 70,000 genes. (This estimate has been revised upward several
times since the announcement that the human genome had been decoded, and likely will be
again.)
Inheriting mtDNA
Whenever an egg cell is fertilized, nuclear chromosomes from a sperm cell enter the egg and
combine with the egg's nuclear DNA, producing a mixture of both parents' genetic code. The
mtDNA from the sperm cell, however, is left behind, outside of the egg cell.
So the fertilized egg contains a mixture of the father and mother's nuclear DNA and an exact
copy of the mother's mtDNA, but none of the father's mtDNA. The result is that mtDNA is
passed on only along the maternal line. This means that all of the mtDNA in the cells of a
person's body are copies of his or her mother's mtDNA, and all of the mother's mtDNA is a copy
of her mother's, and so on. No matter how far back you go, mtDNA is always inherited only
from the mother.
If you went back six generations in your own family tree, you'd see that your nuclear DNA is
inherited from 32 men and 32 women[1]. Your mtDNA, on the other hand, would have come
from only one of those 32 women.
Defining mitochondrial ancestors
Let's get back to "Eve." The ancestor referred to in the 1987 Nature article can be more precisely
stated as "the most recent common ancestor through matrilineal descent of all humans living
today." In other words, she is the most recent person from whom everyone now living on Earth
has inherited his or her mtDNA. This certainly does not mean that she is the ancestral mother of
all who came after her; during her time and even before her time there were many women and
men who contributed to the nuclear genes we now carry. (To see how this can be, check out
Tracing Ancestry.) It also does not mean that the mtDNA originated with this "Eve"; she and her
contemporaries also had their own "most recent common ancestor though matrilineal descent," a
woman who lived even further into the past who passed on her mtDNA to everyone living during
"Eve's" time. (We get our mtDNA from that same, older ancestor. She's just not, to us, the most
recent common ancestor.)
So what about all of the mtDNA of the other women who lived during "Eve's" time? What
happened to it? Simply this: Somewhere between now and then, they had female descendants
who had only sons (or no children). When this happened, the passing on of their mtDNA halted.
Finding mitochondrial ancestors
Even though everyone on Earth living today has inherited his or her mtDNA from one person
who lived long ago, our mtDNA is not exactly alike. Random mutations have altered the genetic
code over the millennia. But these mutations are organized, in a way. For example, let's say that
10,000 years after the most recent common ancestor, one of the mtDNA branches experienced a
mutation. From that point on, that line of mtDNA would include that alteration. Another branch
might experience a mutation in a different location. This alteration would also be passed on.
What we would eventually end up with are some descendants who have mtDNA that is exactly
or very much like that of some people's, somewhat like that of others, and less like that of yet
others. By looking at the similarities and differences of the mtDNA of all of these individuals,
researchers could try to reconstruct where the branching took place.
This is what some researchers have done. For the original 1987 Nature article, the three authors
(Rebecca Cann, Mark Stoneking, and Allan Wilson) looked at the mtDNA of 147 people from
continents around the world (though for Africans, they relied on African Americans[2]). Later,
with the help of a computer program, they put together a sort of family tree, grouping those with
the most similar DNA together, then grouping the groups, and then grouping the groups of
groups. The tree they ended up with showed that one of the two primary branches consisted only
of African mtDNA and that the other branch consisted of mtDNA from all over the world,
including Africa. From this, they inferred that the most recent common mtDNA ancestor was an
African woman.[3]
Dating mitochondrial ancestors
The three researchers went even further—they estimated the age of the ancestor. To get the
estimate, they made the assumption that the random mutations occurred at a steady rate. And
since they now had an idea of how much the mtDNA had changed from the ancestor's, all they
needed was the mutation rate to determine the age of the ancestor. For instance, if they took the
mutation rate to be one in every 1,000 years and knew that there was a difference of 10
mutations between the mtDNA of people living today and the mtDNA of an ancestor who lived
long ago, then they could infer that the ancestor lived 10,000 years ago.
Cann, Stoneking, and Wilson estimated the mutation rate by looking at the mtDNA of groups of
people whose ancestors migrated to areas at known times. One group was Australian aborigines,
whose ancestors moved to the island-continent a then-calculated 30,000 years ago.[4] Since the
three then knew how long it took for that group's mtDNA to diverge as well as how much it
diverged, they determined the mutation rate. Using this rate, they determined that the most recent
common ancestor lived 140,000 to 290,000 years ago (which they roughly averaged to 200,000
years ago). That was back in 1987. Since then, researchers have updated the estimate to 120,000
to 150,000 years ago. However, the margin for error for this estimate and the previous one are
significant—when all of the variables are taken into account, the current range is more like
50,000 to 500,000.
Neanderthals and mtDNA
Finding out about our most recent common ancestor relies solely on inferences from the mtDNA
of people living today. What if we could actually compare our mtDNA with mtDNA of a distant
ancestor? This, in fact, has been done, with mtDNA from the bones of Neanderthals. Comparing
mtDNA of these Neanderthals to mtDNA of living people from various continents, researchers
have found that the Neanderthals' mtDNA is not more closely related to that of people from any
one continent over another. This was an unwelcome finding for anthropologists who believe that
there was some interbreeding between Neanderthals and early modern humans living in Europe
(which might have helped to explain why modern Europeans possess some Neanderthal-like
features); these particular anthropologists instead would have expected the Neanderthals'
mtDNA to be more similar to that of modern Europeans than to that of other peoples. Moreover,
the researchers determined that the common ancestor to Neanderthals and modern Homo sapiens
lived as long as 500,000 years ago, well before the most recent common mtDNA ancestor of
modern humans. This suggests (though it does not prove) that Neanderthals went extinct without
contributing to the gene pool of any modern humans.
Final note
There are many variables that can affect the mutation rate of mtDNA, including even the
possibility that mtDNA is not always inherited strictly through maternal lines. In fact, recent
studies show that paternal mtDNA can on rare occasions enter an egg during fertilization and
alter the maternal mtDNA through recombination. Such recombination would drastically affect
the mutation rate and throw off date estimates.
Not surprisingly, there is currently a heated debate over the value of "mitochondrial Eve"—
especially between history-hunting geneticists and some fossil-finding paleoanthropologists.
According to these anthropologists, even if we could accurately gauge the age of the ancestor,
that knowledge is meaningless because all she really is is the woman whose mtDNA did not die
out due to random lineage extinctions. Furthermore, her status as the most recent common
ancestor doesn't mean that she and her contemporaries were any different from their ancestors.
(Remember, she and all of her contemporaries had their own mitochondrial Eve.)
Perhaps the most valuable finding regarding the "most recent common ancestor" is that she
probably lived in Africa—a finding that supports the most popular theories about the worldwide
spread of hominids.
Questions:
1.) What is the significance of the findings presented in this article?
2.) The article goes into explaining how we got these findings at length, in your own words,
summarize this process.
3.) Nuclear DNA and mtDNA are completely separate entities. Explain the differences in the
passing along of these two forms of genetic information from generation to generation.