The origin of Oxalis corniculata L.

BACKGROUND: Oxalis corniculata L. is a weed with a world-wide distribution and unknown origin. Though it belongs to a section of the genus from South America, the evidence that this species came from there is weak. METHODS: We reviewed the evidence for the origin of O. corniculata using herbarium specimens, historic literature and archaeobotanical research. We also summarized ethnobotanical literature to understand where this species is most used by humans as a medicine. RESULTS: Despite numerous claims that it is native to Europe there is no strong evidence that O. corniculata occurred in Europe before the 15th century. Nor is there reliable evidence that it occurred in North or South America before the 19th century. However, there is direct archaeobotanical evidence of it occurring in south–east Asia at least 5,000 years ago. There is also evidence from historic literature and archaeobotany that it reached Polynesia before European expeditions explored these islands. Examination of the traditional use of O. corniculata demonstrates that is most widely used as a medicine in south–east Asia, which, while circumstantial, also points to a long association with human culture in this area. DISCUSSION: The most likely origin for O. corniculata is south–east Asia. This is consistent with a largely circum-Pacific distribution of section Corniculatae of Oxalis. Nevertheless, it is likely that O. corniculata spread to Europe and perhaps Polynesia before the advent of the modern era through trade routes at that time.