The islands of Polynesia have long been a source of mystery and speculation for armchair scientists1. The origin of the Easter Island statues, the abandonment of the so-called 'Mystery Islands' and the ultimate origins of the Polynesian people are well-known enigmas. However, perhaps the greatest mystery of them all is that of the sweet potato.
Vikings of the Pacific
Polynesian colonists spread quickly over the entirety of Polynesia2 in the period 1300 BC to 1100 AD. They travelled in huge double-hulled canoes carrying all the supplies needed for a successful new colony3: stone tools, animals (pig, chicken and dog) and, most importantly, the seeds of the crops on which they survived. From previous experiences they quickly learned that new islands were basically uninhabitable if they were not quickly and drastically modified with outside products. Their crops were varied and often depended on the specific climate of the island; most likely they carried a wide variety in the colony canoe and only those suited to the climate eventually survived. These plants included coconut, yam, taro, banana, breadfruit, and sweet potato.
How Sweet It Is
All the domesticated plants and animals carried by the Polynesians were of Southeast Asian origin with one notable exception: the sweet potato - Ipomoea batatas. Unlike the yam4, sweet potatoes originated in South America, in the Andes Mountains of Peru and Colombia. This origin is undisputed and is based on many of the methods used to determine the origin of any food plant, such as the existence of wild versions of the plant in South America, the greater cultural importance Andean peoples placed on the plant, the fact there are more varieties in South America than in Polynesia, etc. It is also important to note that sweet potatoes in Polynesia grow primarily on the most outlying islands of Hawaii, New Zealand, and Easter Island (Rapa Nui). It was because of the presence of the sweet potato, among other reasons, that the prevailing theories of Polynesian origin for centuries were based on an Andean origin. This theory has been mostly disproven by an array of linguistic, genetic, and archaeological studies, but certain scientists continued to support the theory until very recently; see below. It is also quite certain that the plants did not evolve convergently in the different locations, nor did they drift to Polynesia before the arrival of human beings. For instance, the natives of Peru call the sweet potato Kumar, while the natives of Easter Island call it Kumara.
Thus we have a problem. On the one hand it is at this point indisputable that Polynesians did not originally come from South America - in fact it seems likely that their earliest traceable homeland is Taiwan. On the other hand it is equally indisputable that they raised a crop which originated in South America and had been doing so for at least several centuries before the arrival of the first Europeans5, based on archaeological findings of specially designed sweet potato storage facilites and carbonised remains. As the title of the Entry indicates, there is no clear answer to the question of how this came to be, and it remains a mystery to this day. The problem was made more complicated in 1974 when Dr Douglas Yen, using complicated mathematical models, determined that, within Polynesia, the sweet potato did not first arrive at Easter Island, the closest point to South America, but in the Marquesas Islands6 and that they arrived shortly after the intial settlement of those islands around 300 AD.
Of course, with such a big problem looming over the field, almost everyone who studies Polynesia has come up with some sort of an answer. Here are a few examples.
Return to the Out-of-South America Theory
Thor Heyerdahl (1914-2002), a rather famous Norwegian researcher, was the last to seriously advocate the South American origin theory, based on his personal journey across the Pacific on a raft he named the Kon-Tiki (never mind that no Polynesian society has ever been seen using rafts to travel). As stated before, no one today seriously believes this particular theory, although Heyerdahl was very well respected for other advancements in Polynesian anthropolgy.
Accidental Drift Theory
Some anthropologists theorise a raft floating off from the South American coast carrying a band of locals who were for some reason accompanied by enough sweet potatoes7 to cross the 4,000km gap (from the South American coast to the closest Polynesian island, Easter Island), sweep onward past Easter all the way to the Marquesas without stopping (another 4,000km or so), and still have some viable sweet potatoes left over to plant. In this theory, these individuals were not the colonisers of Polynesia but arrived after the Polynesians were already settled in and were merely responsible for introducing the sweet potato. This seems unlikely, both because it is highly improbable, as stated, that they would be carrying such a large number of sweet potatoes at any given time, and also because currents and winds along the South American coast very rarely produce the right conditions to wash a raft out to sea. Even Thor Heyerdahl's raft had to be dragged out some 100km before he could begin his drifting journey.
International Trade Theory
Others speculate that Polynesian traders may have been able to cross the same 4,000km gap against the prevailing head winds in wooden canoes and then return safely. This would suppose that they could build canoes capable of doing a two-way journey across the 8,000km from the Marquesas to South America, have enough left over when they arrived to trade for the sweet potatoes, recognise the plant as a food crop capable of growing in their homeland8, pick up enough to survive the journey back and still have enough to plant, finding their way back over such a vast distance, etc.
The Chinese Intervention Theory
Gavin Menzies, in his controversal book '1421: The Year China Discovered the World', published in 2002, proposed that 15th-Century Chinese traders sailed between China and South America, passing though the Polynesian islands on the way. This author is neither an anthropologist nor a historian and this claim is an incidental part of his greater claim for a huge around-the-world journey of Chinese trading vessels in the year 1421. Besides the still-disputed nature of his whole theory, this ignores the fact that sweet potatoes probably first arrived in Polynesia somewhere between 300 and 700 AD.
None of these answers seems very conclusive, and the answer remains a mystery.
Some Suggestions for Further Reading
Bellwood, Peter The Polynesians: Prehistory of an Island Culture
Diamond, Jared The Third Chimpanzee
Heyerdahl, Thor Kon-Tiki
Kirch, Patrick Feathered Gods and Fishhooks
Kirch, Patrick On the Road of the Winds
Menzies, Gavin 1421: The Year China Discovered America