The sweet potato (Ipomoea batatas, Lam.) and the yams (genus Dioscorea) are root crops that today nurture millions of people within the world’s tropics. Moreover, they are plants whose origin and dispersals may help in an understanding of how humans manipulated and changed specific types of plants to bring them under cultivation. Finally, these cultivars are important as case studies in the diffusion of plant species as they moved around the world through contacts between different human populations.
This chapter reviews the questions surrounding the early dispersals of these plants, in the case of the sweet potato from the New World to the Old, and in the case of yams their transfers within the Old World. In so doing, the sweet potato’s spread into Polynesia before European contact is documented, and the issue of its penetration into Melanesia (possibly in pre-Columbian times) and introduction into New Guinea is explored. Finally, the post-Columbian spread of the sweet potato into North America, China, Japan, India, Southeast Asia, and Africa is covered. In addition, a discussion of the domestication and antiquity of two groups of yams, West African and Southeast Asian, is presented, and the spread of these plants is examined, especially the transfer of Southeast Asian varieties into Africa.
The evidence presented in this chapter can be viewed fundamentally as primary and secondary. Primary evidence consists of physical plant remains in the form of charred tubers, seeds, pollen, phytoliths, or chemical residuals. Secondary evidence, which is always significantly weaker, involves the use of historical documents (dependent on the reliability of the observer), historical linguistics (often impossible to date), stylistically dated pictorial representations (subject to ambiguities of abstract representation), remanent terracing, ditches or irrigation systems (we cannot know which plants were grown), tools (not plant specific), and the modern distribution of these plants and their wild relatives (whose antiquity is unknown).