From this PBS article:
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"...
From today's 'Archeological evidence' posting:
They subsequently genetically engineered our species, 200,000
years ago...
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http://www.pbs.org/wgbh/nova/neanderthals/mtdna.html
Scientists explore humankind's past with mtDNA.
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.
Mitochondrial DNA is extracted from the bones of Neanderthals
and compared to the mtDNA of living Homo sapiens.
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.
Rick Groleau is managing editor of NOVA Online.
Notes
1. Unless two or more of those 64 married each other and bore
children from which you are descended. For example, your
great-great-grandfather on your mother's side might have
married and had children with your great-great-grandmother on
your father's side. In that case, the number of your ancestors
in this example would drop to 63.
2. Although the original study was criticized for using
African Americans instead of native Africans, a subsequent
study in which the researchers used mtDNA from native Africans
came up with similar results.
3. Other researchers later showed that the computer program
could come up with other variations of the tree, some of which
did not place an African at the root of the tree. This study,
then, cannot be viewed as definitive proof that the ancestor
lived in Africa. However, it does still suggest that humans
originated in Africa, a hypothesis that other, more recent
studies support.
4. The date for the migration to Australia is now estimated to
be 50,000 to 60,000 years ago.
Sources
"Human Evolution." Svante Pääbo. Trends in Genetics. 15(12):
M13-M16, 1999.
"Neanderthal DNA Sequences and the Origin of Modern Humans."
Matthias Krings, et al. Cell , July 11, 1997.
"Mitochondrial DNA and Human Evolution." Rebecca L. Cann, Mark
Stoneking, Allan C. Wilson. Nature , January 1, 1987.
"The Case of Mitochondrial Eve." Frank R. Zindler. American
Atheist , February 1988.
Shreeve, James. The Neanderthal Enigma: Solving the Mystery of
Modern Human Origins. New York: Avon Books, 1995.
Stringer, Christopher; Clive Gamble. In Search of the
Neanderthals: Solving the Puzzle of Human Origins. New York:
Thames and Hudson, Inc., 1993.
©| Updated January 2002
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