The first doctors were more like healers, with medical books that came in the form of cave paintings radiocarbon-dated as far back as 27,000 years ago. They told of medicinal plants that were used for pain relief, and were passed down through tribes. Each year, the knowledge developed further. It eventually became the first reference for medical purposes, according to The Evolution of Modern Medicine. Trepanation, for example, was performed thousands of years ago to relieve pain by puncturing the skull. Success varied, but the trial-and-error of such practice is invaluable to medicine’s beginnings.