In 1867, an archaeologist and diplomat named Ephraim George Squier sought out the help of Paul Pierre Broca, the esteemed anatomist and surgeon. He was trying to solve a mystery about an ancient Incan skull that had been given to him by a wealthy artifact collector in Peru. In addition to its age, the Neolithic skull had a unique feature: on the top of the cranium a rectangular piece of bone had been removed. The presence of several cross-cuts surrounding the hole suggested that it was not a simple battle wound, but instead the result of a surgical procedure known as trepanation.

This alone would have made the skull an interesting relic, but what really sparked a scientific controversy about the skull was that many who examined it believed the surgery had been performed some time before the individual's death, as the bone seemed to show evidence of healing after the cuts had been made. While it was conceivable that Neolithic Peruvians could have performed such an operation as part of some after-death ritual, it was hard for many in Squier's time to believe these ancient peoples possessed the surgical acumen necessary to excise part of the skull of a living patient without causing death in the process. After all, the survival rate for surgical trepanation in the 1800s seldom reached 10% in the best hospitals of the day. Being unable to elicit a consensus view on the timing of the surgery from the members of the New York Academy of Medicine, Squier sent the skull to France to get an opinion from Broca, who was a distinguished expert in the study of the human skull.

At the time, Broca had already made the key discovery that would cause him to be a household name among psychologists and neuroscientists: that there was a region of the frontal lobe (now known as Broca's area) that seemed to be involved specifically in the production of language. He was still in the midst of vigorously defending this hypothesis (as he would continue to do for years to come), but he immediately developed a great interest in the skull Squier sent to him.

After examining the skull, Broca also was convinced that the opening was evidence of a surgical procedure done while the patient was still alive; Broca believed the patient survived for up to two weeks after trepanation. Doubts among the rest of the scientific community remained, however, until a collection of skulls was unearthed from a Neolithic grave site in central France several years later; a number of the skulls also had holes in them and the healing observable on these skulls made a more convincing argument for the idea that the holes were made well before death. In many cases, in fact, it seemed years may have passed between surgery and death.

Why trepanation?

The discovery of the French skulls helped to convince many of Broca's contemporaries that Neolithic peoples had the ability to perform trepanation on the living in such a way that the patient could often survive, but major questions remained as to how and why they did it. After Broca's interest had been piqued by Squier's skull he pursued answers to these other questions with characteristic determination. In fact, Broca ended up writing more papers on the reasons for prehistoric trepanation than he did on Broca's area and language.

To answer the question about how trepanation was done, Broca tried using simple tools that were available to Stone Age peoples (like flint) to scrape holes in the crania of recently-deceased individuals. He found that, although it took him 50 minutes to scrape through an adult skull (counting time spent taking breaks to rest his tired hand), it could be accomplished with these crude instruments. Now we know that this scraping method was only one of several different primitive approaches to trepanation. Others included making intersecting cuts in the skull and then removing a rectangular portion of the bone (this was what was seen in Squier's skull), or making a circular cut and then removing a disc of skull.

It's unclear if anesthesia was used during the operation when conducted in ancient times. Some have suggested Peruvians may have used coca (the plant cocaine would later be isolated from), as it can act as a local anesthetic. Others have hypothesized ancient peoples used substances like alcohol or opium to reduce pain associated with the procedure. It's also very possible, however, that no anesthesia was used; studies of Oceanic and African cultures that still practiced trepanation in the 20th century found that many of them did so without any type of anesthesia.

But the biggest mystery about trepanation is why the procedure was done. Broca thought and wrote extensively about this subject, eventually coming to favor a hypothesis that the practice was rooted in superstition. According to his view, Stone Age peoples did not understand the physiological basis of disorders like epilepsy, and thus were inclined to believe they were due to mystical events like demonic possession. Trepanation, Broca thought, may have been a way of treating these intractable mental disorders by creating a hole in the head through which demonic spirits could escape.

Although there are some aspects of Broca's original hypothesis that have become discredited (such as his belief---formed due to how long it took him to scrape through an adult skull---that the procedure was conducted only on children), it is still considered by many to be a valid explanation for why trepanation was done in the ancient world. Others, however, like Broca's colleague P. Barthelemy Prunieres, argued that trepanation had a more practical justification. Prunieres reasoned that the procedure grew out of the attempted treatment of cranial fractures, which would likely have involved efforts to remove pieces of fractured bone from the site of the injury. In some cases, head injuries can cause the accumulation of blood within the cranium, which may lead to a potentially life-threatening increase in intracranial pressure; this pressure can sometimes be partially relieved by trepanation. Thus, the primitive surgery may have produced a real benefit for some patients. If trepanation appeared to lead to an improvement in the condition of some patients, this may have caused the procedure to be utilized more frequently even if the true reasons for the improvements were not fully understood.

The perspectives of Broca and Prunieres represent two general views of ancient trepanation that each continue to receive support today: one that contends trepanation was done due to the influences of mysticism, another that argues it was a prehistoric attempt at rational surgery. It is likely, however, that different groups in different geographical areas had different reasons for performing the procedure, as trepanation was not a practice confined to one region or culture. Indeed, studies of 20th-century African tribes who still use the procedure found that reasons for trepanning varied among tribes, with some using it to treat cranial injuries and others using it to expel evil spirits.

Trepanation beyond the Stone Age

Trepanation did not begin and end with ancient Stone Age peoples. It was advocated by the famous Greek physician Hippocrates to allow for the drainage of blood after a cranial injury. Galen, the preeminent surgeon of the Roman Empire, also promoted the use of the procedure for blood drainage, but added to his recommendations a discussion of its beneficial effects on intracranial pressure. In the process, Galen provided an explanation of the potential palliative effects of trepanation that crudely resembles a contemporary understanding of them. The ancient Greeks and Romans also began developing more modern tools to use in trepanation; in the 1600s a three-pronged device for drilling through the skull was invented; it was called a tre fines, from the Latin for three ends. This led to the term trephination becoming a synonym for trepanation.

Trepanation continued to be used up through the 1800s for the treatment of head injuries as well as for epilepsy and other mental illnesses. Gradually, however, the practice fell out of favor in the 19th century. The mortality rates for trepanation at the time were very high, and it came to be recognized that any benefits it might offer were significantly outweighed by the risk of death associated with the surgery. Today similar procedures like craniectomy, which also involves removing part of the skull, are sometimes used to treat instances of increased intracranial pressure caused by major head trauma.

We will likely never be certain of the reasons Neolithic peoples practiced trepanation. Perhaps it was due to primitive beliefs in demonic possession, or maybe it was an attempt to protect the brain from the pressure created by intracranial bleeding. Then again, it may be that both of these explanations are erroneous. We can, however, feel fairly confident that trepanation was one of the first common surgical procedures and likely the first attempt at any intervention that could be considered remotely neurosurgical. And it likely will forever remain one of the longest-standing mysteries of neuroscience, due both to its origins in ancient human prehistory as well as to the improbability of its mystery ever being fully solved.