DNA can be used to understand the evolution of modern humans, trace migrations of people, identify individuals, and determine the origins of domestic plants and animals. DNA analysis, as one scholar put it, is "the greatest archaeological excavation of all time." Because ancient DNA molecules are normally so few and fragmented, and preserved soft tissues so rare, scientists had little hope of finding and analyzing it. But two breakthroughs have made this possible: the polymerase chain reaction (PCR), a method for copying any fragment of DNA, and the successful recovery of DNA from preserved hard tissues, bones and teeth, that...