Humanity’s oldest form of surgery is also one of its most gruesome. As far back as 7,000 years ago, civilizations around the world engaged in trepanation—the practice of boring holes in the skull as a means of curing illnesses. Researchers can only speculate on how or why this grisly form of brain surgery first developed. A common theory holds that it may have been some form of tribal ritual or even a method for releasing evil spirits believed to possess the sick and mentally ill. Still others argue that it was a more conventional surgery used to treat epilepsy, headaches, abscesses and blood clots. Trepanned skulls found in Peru hint that it was also a common emergency treatment for cleaning out bone fragments left behind by skull fractures, and evidence shows that many of the patients survived the surgery.