First, a fun fact: the sweet potato (Ipomoea batatas) is only very distantly related to the potato (Solanum tuberosum), though both are from the New World and both were brought back to Europe by the Spaniards. The potato was called pata, while what we call "sweet potatoes" were called batatas, and it is easy to see how the confusion followed. I. batatas is also distinct from what westerners call "yams," which were domesticated independently in S. America, Africa, and Asia, and all of which belong to the genus Dioscorea.

Sweet potatoes (batatas) originated in the New World, probably originally in the Yucatan and/or the northern end of the S. American continent, and were widespread there by the end of the Neolithic. Examples of fossilized sweet potatoes from the Chilca Canyon in Peru date as far back as 6,000 BCE.(1) The writings of early Spanish explorers of the New World indicate that by the 15th century CE, cultivation of the sweet potato was fairly common. It appears in Europe at this time. The sweet potato also appears in Oceania, but it is not clear by what means. It was already well-established and common in Hawaii and Tahiti by the arrival of James Cook in the latter half of the 18th century CE. It is hypothesized that the sweet potato was brought from S. America in prehistoric times, either by Polynesian or by ancient Peruvian voyages.(3) In the 16th century CE, Portuguese explorers brought the sweet potato from the West Indies to Africa, India, and the East Indies, and it became a common crop in all these places. Although attempts were made to cultivate the plant in England, Belgium, and Holland, the temperatures there are (or used to be!) too chilly for widespread success. Fukienese sailors had brought it to southern China from the Philippines by the early 17th century, and thence to Taiwan and Japan by the end of that century.

By the 19th century, the sweet potato was under cultivation pretty much everywhere on the planet where climate permitted. China produces more sweet potatoes than all the other countries on the planet combined by a factor of nearly 10:1, representing 85% of the total crop. Taking away China, Nigeria leads the rest, with Uganda, Indonesia, Japan, Tanzania, and then the United States after.

For a full rundown of the sweet potato in all its glory, see Vincent Lebot, Tropical Root and Tuber Crops, 2009.

(1) Yen, D.E. 1974. The sweet potato and Oceania, an essay in ethnobotany. Bernice P. Bishop Museum Bulletin 236. Bishop Museum Press. Hawaii.

(2) Woolfe, J.A. .1992. Sweet Potato, An Untapped Food Resource. Cambridge; Perry, L. .2002. "Starch granule size and the domestication of manioc (Manihot esculenta) and sweet potato (Ipomoea batatas)." Economic Botany 56: 335–349.

(3) Yen, D.E. 1982. "Sweet potato in historical perspective" in Villareal and Griggs, (eds) Sweet Potato. Proceedings of the First International Symposium. AVRDC Publication No. 82-172, Taiwan, 17–30; Ballardet al. (eds) 2005. The Sweet Potato in Oceania: A Reappraisal. Ethnology Monographs 19, Oceania Monograph 56, The University of Sydney.