On a visit to Peru during the 19th century, the U.S. Commissioner there was shown a skull found in an Incan cemetery dating to pre-Columbian times. What was odd about the skull was the rectangle-shaped hole in its top.

The diplomat, E.G. Squier, who was also an expert in archeology, took the skull to New York in 1865 and presented it to the New York Academy of Medicine. Squier was convinced the hole represented prehistoric brain surgery. What amazed him even more were the signs of healing in the skull that suggested the patient lived at least a few weeks.

However, others were not convinced. So Squier took the skull to the famed French surgeon and anthropologist, Paul Broca, who was able to confirm Squier's suspicions. In fact, Broca had already found an archaeological site in France dated 6500 BCE with forty instances of trepanation.

Trepanation is drilling a hole in the skull without damaging the brain, or by modern terms, a craniotomy. It's actually among the oldest surgeries, which the ancient Greeks and Romans used to relieve pressure and remove skull fragments from head wounds.

As Mr. Squier found with the Peruvian skulls, many trepanned skulls show signs of healing that allowed some to survive for years. Fossils around the world show the practice may have originated in the late Paleolithic period, around 2.5 million years ago, and lasted to around 10,000 BCE.

The techniques and tools differed. In South America, a ceremonial knife called a tumi was used, while in Europe tools were made of flint or obsidian, and sea shells were used in the South Pacific. For some skulls, the reason for the trepanation is obvious, to treat a traumatic head wound, while others remain a mystery.