When scientists announced the discovery of a common female ancestor to every living human, many people had trouble understanding how the scientists came to this conclusion. In reality, Mitochondrial Eve isn't the only ancestor we share, as some have claimed. Nor was she an isolated woman who somehow avoided genes from other people. Mitochondrial Eve was simply someone who contributed a vital cell component – the DNA of the mitochondria – to all of her children, who then passed it to everyone in future generations.
The Genetics of Mitochondrial Eve
All humans have two copies of each chromosome, and therefore each gene, with one copy from the mother and one from the father. But in addition to these chromosomal genes, people also have mitochondrial DNA, small strands of DNA in the cell organelle called mitochondria. These organelles, and the genes in them, are almost exclusively inherited from the mother.
When scientists looked at these mitochondrial genes, they found a high degree of similarity between the genes in all people. They could also trace the changes to specific points in time, based on an analysis of how often mitochondrial DNA mutates and calculating backwards to determine how long ago each mutation must have occurred.
Who Was Mitochondrial Eve?
Scientists don't know a lot about the woman who contributed mitochondrial DNA to everyone. One of the things they do know is that she lived in sub-Saharan Africa about approximately 170,000 years ago, about the time period when Homo sapiens split off from other Homo genus groups.
The discovery of Mitochondrial Eve lends support to the theory that modern humans developed in Africa before spreading across the globe (as opposed to the idea that modern humans developed spontaneously in many places around the world.)
Is Mitochondrial Eve the Only Common Ancestor?
Many people make the mistake of thinking that because humans all share the same mitochondrial DNA from this mysterious ancient woman, that means she is the only human ancestor. The easiest way to understand why this isn't the case is by looking at families today. Most people have cousins, individuals who share a grandparent. This doesn't mean that only that one grandparent contributed to their genes, however.
A grandmother who passes on her mitochondrial DNA to her three daughters, who then pass it down to their three daughters each, produces nine grand-daughters who all share the same mitochondrial DNA. Of course, these girls also have genes in their chromosomal DNA, which may come from people other than their shared grandmother, including their different fathers (since all three original daughters didn't marry the same man) and their shared grandfather (who didn't give his daughters mitochondrial DNA, but did give them half of their chromosomal DNA.)
In the case of Mitochondrial Eve, these granddaughters continued to have daughters, granddaughters and generations of girls who had children with various other people in the community and the world for almost 200,000 years until their descendants counted in the billions. So the simplest way to think of Mitochondrial Eve is as a many-times great-grandmother of all of humanity. In reality, each person has hundreds or thousands of other great-grandmothers and grandfathers in her history, just as any family tree will show.
For Further Reading:
Sykes, Bryan The Seven Daughters of Eve: The Science That Reveals Our Genetic Ancestry. W.W. Norton, 2001
Dawkins, Richard River out of Eden. New York: Basic Books. 1995
Rebecca L. Cann, Mark Stoneking & Allan C. Wilson "Mitochondrial DNA and Human Evolution," Nature, 325. 1987
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