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2008 HowStuffWorks

­In 1987, a group of genet­icists published a surprising study in the journal Nature.­ The­ researchers examined the mitochondrial DNA (mtDNA) taken from 147 people across all of today's major racial groups. These researchers found that the lineage of all people alive today falls on one of two branches in humanity's family tree. One of these branches consists of nothing but African lineage, the other contains all other groups, including some African lineage.

­Even more impressive, the geneticists concluded that every person on Earth right now can trace his or her lineage back to a single common female ancestor who lived around 200,000 years ago. Because one entire branch of human lineage is of African origin and the other contains African lineage as well, the study's authors concluded Africa is the place where this woman lived. The scientists named this common female ancestor Mitochondrial Eve.

­The researchers got the idea for this project based on a discovery another geneticist made in 1980. Dr. Wesley Brown noticed that when you compare the mtDNA of two humans, th­e samples are much more similar than when the mtDNA of two other primates -- for example, two chimpanzees -- is compared. Brown found, in fact, that the mtDNA of two humans has only about half as many differences as the mtDNA of two other primates within the same species [source: Cann]. This suggests that humans share a much more recent common ancestor than other primates do, an idea tantalizing enough to launch the Nature investigation.

The study's lead author, Rebecca Cann, called her colleagues' and her choice to use Eve as the name "a playful misnomer," and pointed out that the study wasn't implying that the Mitochondrial Eve wasn't the first -- or only -- woman on Earth during the time she lived [source: Cann]. Instead, this woman is simply the most recent person to whom all people can trace their genealogy. In other words, there were many women who came before her and many women who came after, but her life is the point from which all modern branches on humanity's family tree grew.

When the researchers in the 1987 study looked at samples taken from 147 different people and fetuses, they found 133 distinct sequences of mtDNA. A few of the people sampled, it turned out, were recently related. After comparing the number of differences among the mtDNA samples within races, they found that Africans have the most diversity (that is, the most number of differences) of any single racial group. This would suggest that the mtDNA found in Africans is the oldest: Since it has had the most mutations, a process which takes time, it must be the oldest of lineages around today.

The two distinct branches they discovered contained the mtDNA found in the five main populations on the planet: African, Asian, European, Australian and New Guinean. Researchers found that in the branch that was not exclusively African, racial populations often had more than one lineage. For example, one New Guinean lineage finds its closest relative in a lineage present in Asia, not New Guinea. All of the lineages and both of the two branches, however, can all be traced back to one theorized point: Mitochondrial Eve.

So how did Eve end up being humanity's most recent common ancestor? We'll look at that in this article, as well as some arguments lodged against the Mitochondrial Eve theory. But first, what are mitochondria and why do scientists use mtDNA to track lineage?