Why did ancient civilizations drill holes in living people's skulls?

The answer

Trepanation - drilling or scraping a hole in the skull - is the oldest known surgical procedure, with evidence dating back 10,000 years. Over 1,500 trepanned skulls have been found worldwide, from Peru to France to China. The Inca civilization achieved remarkable success rates: a 2018 study in World Neurosurgery analyzing 800 Peruvian trepanned skulls found survival rates exceeding 80% by the 1400s CE, compared to roughly 50% during the American Civil War. Ancient purposes likely included treating head trauma, relieving intracranial pressure, and spiritual rituals to release evil spirits. Skulls show varied techniques including scraping, grooving, boring, and cutting, with some individuals surviving multiple procedures - one Peruvian skull had seven healed trepanation holes.