Humankind has suffered from illnesses since humans first appeared on Earth millions of years ago. But in the absence of medical knowledge, which only began about 5,000 years ago, ailments and sickness were assumed to have been brought on by angered gods or evil spirits. Even today, some primitive groups believe they must appease the gods and spirits through offerings and sacrifices in order to stave off or rid themselves of illness; these are practices carried out by witch doctors or healers. In prehistoric times such superstitious beliefs resulted in the first “medical” procedure, in which a hole was drilled into the patient’s skull—this practice was believed to allow evil spirits to escape the body and thereby rid the body of sickness. Fossil findings showing this method of treatment date back about 10,000 years. One vestige of ancient treatments of illness that still has relevance today is the use of plants and plant substances for medicinal purposes.