After examining nearly perfect holes in the teeth of skulls found in a Pakistan graveyard(墓地), researchers concluded that dental drilling dates back 9,000 years. Primitive dentists drilled the teeth of live patients between 5500 B.C. and 7000 B.C., in a practice that probably evolved from intricate(複雑な) ornamental(装飾の) bead(ビーズ) drilling. The researchers believe the drilling was likely done to reduce the pain of cavities(虫歯).