Kofyar yam harvest, Plateau State Nigeria.

Yams

Several species in the genus Dioscorea were domesticated in Asia and Africa. The most important one in Africa today is D. rotundata (left). This is often called the White Yam, but there is also a Yellow Yam -- very similar except for the different colored flesh, and generally now considered the same species. In other words, White Yams may be white or yellow. They are starchy, non-sweet tubers, with flesh somewhat like a potato but slightly fibrous like a sweet potato. But African yams are totally unrelated to the sweet potato (Ipomoea batatas) and the potato (Solanum tuberosum).

It is claimed that the confusion of sweet potatoes often being called "yams" in the American south may have started with slaves calling sweet potatoes yams -- this is plausible. But it leads to the fairly common belief that Africans grow big sweet tubers.

It is commonly eaten as "pounded yam:" the tubers are cut into large chunks and boiled, then skinned and beaten senseless is a large wooden mortar, often with two women working together with enormous wooden pestels (thump-thump...thump-thump...)  This breaks down the chunks of tuber and also develops the gluten, somewhat like the kneading of bread dough.  When the consistency is right, it is taken out and rolled into serving-size balls (typically about the size of an orange), and served along with a separate bowl of sauce.

Christopher Ehret judged that the words for "cultivation" and "yam" in proto-Niger-Congo date back to at least 8000 B.C.E. (Christopher Ehret, "Historical/Linguistic Evidence for Early African Food Production," in From Hunters to Farmers, ed. J. D. Clark and S. A. Brandt (Berkeley: University of California Press, 1984), pp. 26–39).