On isolated Polynesian islands from Easter Island to New Zealand, people grow and eat sweet potatoes. Which is interesting, because sweet potatoes come from the highlands of South America, where they were domesticated around 8000 years ago. It seems highly unlikely that they could have floated across the Pacific, since they are killed by even a short immersion in salt water. So how did they get to Polynesia?
There have long been two theories: one, that the potatoes were taken to the islands by early Spanish or Portuguese mariners in the 1500s, something that had already been forgotten by islanders before Bougainville and Cook made their scientific expeditions 200 years later; two, that the potatoes were brought from South America by Polynesian voyagers who reached the continent in prehistoric times. In recent years the remains of charred sweet potatoes from Polynesian archaeological sites have been radiocarbon dated to as far back as 1000 CE. This is strong evidence that they spread before Columbus, but if you have followed this blog you know that radiocarbon dating is too quirky for archaeologists to take is as proof of anything that can't be readily reconciled with other evidence. There is also some linguistic evidence, that is, the words for sweet potato in some Polynesian languages resemble the Quechua (Inca) word. Against this is the question of why, if Polynesians discovered South America, they then forgot about it. In historic times they had no knowledge of the continent beyond the eastern horizon, which seems a little odd if they had been there. Also, once the more remote Polynesian islands were settled, there is not much evidence of traffic between them and the settlers' home islands -- the Polynesians went out, but rarely back. So even if some Polynesians reached South America, how the sweet potato get back to Tahiti?
Now there is new evidence that weighs on the side of Polynesian, pre-Columbian spread. A team of French scholars investigated the genetics of sweet potatoes and found that those used by the natives in the 18th century were genetically quite distinct from the varieties taken to the Philippines and China by the Spanish. To do this they cleverly went to the herbaria assembled by those 18th-century scientific explorers, who brought back shiploads of specimens, and took samples from those old roots. Some islands in the far west did have varieties related to those carried by the Spanish, so they may have acquired theirs from Europeans, but farther east the potatoes were all of a different strain. By itself this would not prove much, but the converging lines of evidence represented by DNA, radiocarbon dating, and linguistics together make a strong case that Polynesian explorers reach South America and brought back what became their staple crop.