Kava is sterile. It does not set viable seed, it is propagated only from cuttings, and no wild ancestral population has ever been found, though Piper wichmannii in Melanesia is the probable progenitor. Every kava plant in existence was planted by a person, and the species as such is a human artefact.
Two things follow. The first is a dating argument: producing a sterile crop with many named cultivars, selected for potency and effect and distinguished by growers, takes a very long time, and kava's distribution across the Pacific tracks the Austronesian voyages that carried it. The plant is a record of those voyages. The second is that kava depends on continuous cultivation. A tradition that lapsed for a generation would lose its cultivars permanently.
Its modern history has a sharp episode. In the late 1990s and early 2000s kava extracts sold as herbal supplements in Europe were linked to cases of severe liver injury, and several countries banned it. The bans were economically devastating for Pacific growers. Subsequent review found the picture considerably more complicated: the traditional preparation is a water extraction of the peeled root, while several implicated products used solvent extraction or included aerial parts and peel, which carry compounds the traditional method leaves out. Most of the bans were eventually lifted or narrowed. The episode is a clean example of a traditional preparation being safer than the industrial version marketed in its name.