Yams are so numerous that only the main cultivated species can be mentioned. Most of them are natives of the Old World, from SE Asia, nearby Pacific islands, and Africa; but there is also a small group native to S. America. Yams existed at least as far back as the beginning of the Jurassic era, when dinosaurs had not yet been succeeded by mammals and S. America and Asia were still joined. After the continents separated at the end of the Cretaceous era, the evolution of American yams proceeded separately, but they are still not much different from their Old World relatives. It is true that, as a group, they tend to produce clumps of small tubers rather than single huge ones; but they share this convenient habit with certain Old World yams. The differences between Asian and African yams, which were separated only in historic times by the drying up of the intervening land, Arabia, are also slight. The transfer of useful species by human agency has anyway confused the picture.