Our African Mother
You've seen her face guessed at on the covers of national news magazines, where she's been headlined as the "African Eve". She lived in Africa, something on the order of 200,000 years ago, and she was everybody's grandmother -- with about ten thousand "greats" ahead of the "grand".
Now, this does not mean that this common ancestress of us all contributed a disproportionate amount to our total ancestry, nor that she was the first human woman, nor that she was the only woman alive at that time who has descendants today. But if you could trace back your family tree from your mother to your mother's mother to her mother and on for somewhere between six thousand and twenty thousand generations, you would eventually find this woman -- and everyone else on earth, if they could trace their female ancestry the same way, would find the same woman. Every human being on earth is related through her.
Why an Eve rather than Adam? The answer comes from the mechanism of sexual reproduction at the cellular level. The cell which will become a new organism, be it a human being or a spruce tree, is made up of a nucleus, which contains most of the hereditary information (packed into molecules of deoxyribonucleic acid, mercifully abbreviated to DNA), and the rest of the cell, which has various bits of cellular machinery such as ribosomes and mitochondria, some of which have their own private strands of DNA. This cell was formed by the fusion of a specialized cell formed by the female parent, which had half of a normal nucleus plus essentially all of the mother's extranuclear cellular machinery, with a cell which was effectively half a nucleus and nothing else, contributed by the male parent. The DNA in the nucleus of the complete cell thus comes equally from both parents, but the DNA in the various organelles outside the nucleus comes only from the mother.
Both nuclear and extranuclear DNA change by mutations, very slowly and -- seen over a very long time -- at roughly constant rates. If we can measure the differences between the DNA in two different individuals or species, we can estimate how long ago they shared a common ancestor. Mitochondrial DNA, one of the types which comes only from the mother, has two advantages for this kind of work. It has a higher mutation rate than does nuclear DNA, which means changes show up more quickly, though "quickly" in this case is still only about 2 to 4 percent in a million years. Furthermore, it does not get scrambled by the random reassortment of nuclear DNA in each generation.
It was a research group at Berkeley that deduced our common African ancestress by comparing mitochondrial DNA from 147 women of different origins: European, African, Asian, Australian and New Guinean. As a general rule, the differences within races were much larger than the differences between them.
When the different DNA's were arranged in a sort of family tree, with the most similar DNA's treated as having the most recent common ancestor, the oldest fork led to a split between a group found only in Africa and another group with representatives from all of the five groups (including Africa). This very strongly suggests that the first split took place in the only place descendants of both groups survived: Africa.
The total amount of difference among the 147 women suggests a common ancestress who lived between 140,000 and 290,000 years ago. Since some of her descendants seem to survive only in Africa, that is most likely where she lived.
