One of the enduring mysteries of world history is whether the Americas had any contact with the Old World before Columbus, apart from the brief Viking settlement in Newfoundland. Many aspects of higher civilisation in the New World, from the invention of pottery to the building of pyramids, have been ascribed to European, Asian or African voyagers, but none has stood up to scrutiny.

The one convincing piece of evidence for pre-Hispanic contact has been the humble sweet potato, which is of tropical American origin but widely cultivated across the Pacific islands. Until a few years ago it was assumed that this was the result of Spanish transmission, dating to the early colonial period, but archaeological discoveries in the Cook Islands show this to be wrong: excavations at Mangaia yielded carbonised remains of sweet potato (Ipomoea batatas) dating to AD1000, five centuries before Europeans entered the Pacific Ocean.

The question then arose as to whether the diffusion of this useful crop was the result of Amerindians sailing west to Polynesia, as the late Thor Heyerdahl always claimed, or whether it came about because Polynesians exploring on “the road of the winds” beyond Easter Island came to the South American mainland, and took back with them the hardy and nutritious root crop which is today fifth in importance in developing countries.