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and twenty-five thousand years ago.
    One reason for this drastic downward revision is that problems have been found in the method used to calculate how much time has elapsedsince two animals had a common ancestor. The DNA most commonly used for this purpose comes not from the nucleus but from the mitochondria (whose genetic content is abbreviated as mtDNA). Very occasionally, only once every few thousand years, mitochondrial DNA mutates, such that mother and daughter, who would otherwise have identical mtDNA, exhibit sequences that differ at just one location (this applies only to mothers—fathers do not pass on any mtDNA to their offspring, male or female). Unlike other kinds of mutation, these changes have no effect on the health or fecundity of the animal, and so are passed on “silently” down the generations, spreading throughout all the daughter’s descendants. By counting the number of differences between two individual animals’ mtDNA, scientists can estimate the amount of time the two individuals’ lineages have been diverging—and can thus form an idea of how long ago their most recent shared female ancestor lived. The bigger the number of distinct mutations, the older the two animals’ joint lineage must be.
    Errors slip into this sort of mtDNA dating when scientists, having determined how many unshared genetic mutations exist between two individuals, attempt to figure out just how often these mutations may have occurred in both animals. The regularity of these mutations varies from one kind of animal to another. However, scientists know from the fossil record and from carbon dating that the dog’s ancestor, the wolf, diverged from the coyote about 1 million years ago. A simple comparison between the number of differences between dog and wolf, and the number between wolf and coyote, suggests that the dog and the wolf had been separated for about one-tenth of that time—in other words, for about a hundred thousand years. This calculation, however, relies on the mutations in mtDNA occurring at the same rate in domestic and wild animals. Since the 1997 study, it has become apparent that mtDNA mutations occur more frequently in domesticated animals than in wild ones. The same comparative method used in the 1997 study has consistently overestimated the time since domestication for virtually every animal to which it has been applied: For example, the DNA of the pig, probably first domesticated nine thousand years ago, suggests a domestication of sixty thousand to five hundred thousand years ago; and the horse, more than three hundred thousand years agoinstead of about six thousand. The mutation rate must therefore be faster during domestication than in the wild, speeding up the rate at which mtDNA changes from once every few thousand years to once every few hundred. Studies of other species suggest that this accelerated rate is a side effect of chronically high levels of stress hormones, caused by living in crowded conditions and in close proximity with man. Thus the estimate of a hundred thousand–plus years is highly likely to be an overestimate, perhaps by a factor of five or more, bringing the interval since the dog’s domestication down to a much more realistic twenty thousand years or so.
    In addition to comparing the dog’s DNA with that of the wolf, scientists can examine how much variation there is between different types of dog, as a way of determining how long they have been around. However, this procedure, too, superficially seems to suggest that dogs were domesticated much earlier than twenty thousand years ago. A recent analysis of the DNA that codes for the dog’s immune system has produced an estimate of several hundred thousand years since domestication—a figure even more unlikely than the hundred thousand years indicated by the mtDNA, since it predates the evolution of our own species. On the other hand, such an estimate assumes that mutation