A part of it was the keen observation by the medicine men which plants were eaten by animals thus ensuring the same plants, if used by the medicine man would not harm humans.
Origins are varied.
Not documented scientifically by Western medicine but used extensively by the Chinese, and throughout Asia and non-Western countries, the use of herbs has recently gained credibility in the U.S. and other Western countries due to the concern about so many side effects of many drugs. Herbal remedies are usually given in the form of tinctures, infusions, compresses, ointments, lozenges, and extractions from plants, which are compatible with the human body.