Geneticists have previously estimated mutation rates by comparing the human genome with the sequences of other primates. On the basis of species-divergence dates gleaned -- ironically -- from fossil evidence, they concluded that in human DNA, each letter mutates once every billion years. "It's a suspiciously round number," says Linda Vigilant, a molecular anthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. The suspicion turned out to be justified. In the past few years, geneticists have been able to watch the molecular clock in action, by sequencing whole genomes from dozens of families5 and comparing mutations in...