Polynesian navigation snow. In the mid-20th century, Thor Heyerdahl proposed a new theory of Polynesian origins (one which did not win general acceptance), arguing that the Polynesians had migrated from South America on balsa-log boats. The presence in the Cook Islands of sweet potato that is a plant native to the Americas (called "kūmara" in Māori), which have been radiocarbon-dated to 1000 CE, has been cited as evidence that Americans could have traveled to Oceania. The current thinking is that sweet potato was brought to central Polynesia circa 700 CE and spread across Polynesia from there, possibly by Polynesians who had traveled
Thor Heyerdahl also criticised Heyerdahl's theory in his 2009 book "The Wayfinders", which explores the history of Polynesia. Davis says that Heyerdahl "ignored the overwhelming body of linguistic, ethnographic, and ethnobotanical evidence, augmented today by genetic and archaeological data, indicating that he was patently wrong." A 2009 study by Norwegian researcher Erik Thorsby suggested that there was some merit to Heyerdahl's ideas and that, while Polynesia was colonized from Asia, some contact with South America also existed. Some critics suggest, however, that Thorsby's research is inconclusive because his data may have been influenced by recent population contact. However, more recent work indicates