Why 5-10% of Neolithic skulls across both the Old and New Worlds between 12 ka BP and 5 ka BP have drilled holes with post-operative bone healing, despite no contact between continents and no antibiotics:

Trepanation is an effective treatment for epidural hematoma: a patient ruptures a blood vessel in their skull, filling the cranial cavity with blood, and drilling a hole releases the blood and saves the patient’s life. In a prehistoric culture where people hit their neighbors with flint axes, hematoma rates were far beyond any in recorded history, and dramatic recoveries—a comatose person on the edge of death suddenly waking up—drove overgeneralization of the procedure to migraines and other ailments. Primitive surgeons survived without sterilization because they stopped at the dura partway between the scalp and brain, the scalp has unusually high blood flow, and primitive bacteria lacked the virulence of modern hospital-adapted strains. As head trauma rates declined with civilization, miraculous hematoma recoveries became rare and the practice died out.