Chapter 63:

"He does have other avenues to life, should he perceive this one blocked," Snape said dryly, uncapping a fourth bottle. "And before you ask, it must be the original grave, the place of first burial, the bone removed during the ritual and not before. Thus he cannot have retrieved it earlier; and also there is no point in substituting the skeleton of a weaker ancestor. He would notice it had lost all potency."