People around the world in historical times practised trepanation: a crude surgical procedure that involves forming a hole in the skull of a living person by drilling, cutting or scraping away layers of bone with a sharp implement.
To date, thousands of skulls bearing signs of trepanation have been unearthed at archaeological sites across the world.
But despite its apparent importance, scientists are still not completely agreed on why our ancestors performed trepanation.
Anthropological accounts of 20th-Century trepanations in Africa and Polynesia suggest that, in these cases at least, trepanation was performed to treat pain - for instance, the pain caused by skull trauma or neurological disease.
Trepanation may also have had a similar purpose in prehistory.
Many trepanned skulls show signs of cranial injuries or neurological diseases, often in the same region of the skull where the trepanation hole was made.