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lion years ago, as Sarich's work had shown. We then calculated how much humans had diverged from one another relative to how much they had diverged from chimpanzees—that is, we found the ratio of mitochondrial DNA divergence among humans to that between humans and chimpanzees.

Using two different sets of data, we determined that the ratio was less than 1:25. Human maternal lineages therefore grew apart in a period less than 1/25 as long as five million years, or less than 200,000 years. With a third set of data on changes in a section of the mito-chondrial DNA called the control region, we arrived at a more ancient date for the common mother. That date is less certain, however, because questions remain about how to correct for multiple mutations that occur within the control region.

One might object that a molecular clock known to be accurate over five million years could still be unreliable for shorter periods. It is conceivable, for example, that intervals of genetic stagnation might be interrupted by short bursts of change when, say, a new mutagen enters the environment, or a virus infects the germ-line cells, or intense natural selection affects all segments of the DNA. To rule out the possibility that the clock

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