A couple of months ago I read Sea People, an engaging popular survey of the scholarship on the historical orgins of the Polynesian peoples, and their subsequent exploration and settling of the Pacific islands. I recently recalled this book due to a discussion at Language Log that intersects with my South Americanist interest, touching on the long-standing question of how sweet potatoes were introduced into the Pacific. While it appears fairly clear that they must have come from South America, the question is whether humans were the vector for their introduction (i.e. Polynesians traveling to South America or South Americans to Polynesia) or whether sweet potatoes dispersed via a natural mechanism that did not involve humans.
Note that the latter hypothesis is not at all implausible. Darwin, for example, devotes an extensive section of the On the Origin of Species (a previous summer book of mine) to mechanisms for the geographic dispersal of plants and animals (floating on water, on natural rafts, stuck to the feet of birds, who can get blown around by storms, etc.). But there is one reason that makes some people think that humans were the vector for the transmission of sweet potatoes to the Pacific, namely, that the word used by Polynesians for this cultigen is very similar to that used by Indigenous peoples in western South America: kumara. There are, to be fair, some linguist details to be nailed down: were there actually truly coastal people who cultivated sweet potato and used this word? Is the form of the Polynesian word what one would expect of the phonological form of the word used on the west coast of South America plus the phonological adaptation processes that would have been involved in Polynesians borrowing the word?
Notwithstanding these issues, I think that the preceding lexical borrowing hypothesis is more plausible than the alternative, namely, that sweet potatoes were introduced to the Pacific via a non-human vector, and Polynesians by chance developed a word almost identical to that of the South American term. Note, crucially, that it is not just a question of the similarity of the form of the two words: as any historical linguist can tell you, surprising coincidences crop up all the time when you compare the lexicons of multiple languages. Rather, it is the similarity in question combined with nature of the lexicon innovation event. We know that sweet potatoes were introduced to the Polynesian around 1300 CE, which would have presented the Polynesians with a linguistic problem to solve, namely, what to call the new cultigen. There are basically three options: 1) they could have borrowed a word; 2) they could have extended the reference of an existing word to include the new referent; or 3) they could have coined a new word. Focusing on the latter two possibilites, the extension mechanism (plus a disambiguating strategy) is something that was employed by Indigenous peoples in the Americas when Europeans introduced new animals and plants. The word for ‘dog’ in many Amazonian languages, for example, is the word that was historically applied to jaguars and other wild cats (e.g. yawara in many Tupi-Guarani langauges). Subsequently a disambiguation strategy was employed when needing to clarify reference to jaguars versus to dogs (e.g., yawara-usu, where -usu is an augmentative suffix). I haven’t heard this extension strategy being mentioned as a possible source for Polynesian kumara, but perhaps it would be work ruling that out, if someone hasn’t already done so.
The other non-borrowing strategy is coining a new word. Here I go out on a bit of a limb and propose that in cases of coining new words for new referents, the new coined word is overwhelmingly likely to be etymologically transparent. For example when the French opted to coin a word for potato instead of borrowing from an American language via Spanish, English, or another language, they coined pomme de terre (lit. ‘earth apple’), and English itself, instead of borrowing, created the compound sweet potato. Similarly, Máíhùnà, who I have worked with, generally do not borrow words from Spanish, but instead use the vast classifier system of Máíhɨ̃̀kì, and the nominalizing properties of classifiers, to coin vast numbers of words: toya-tɨka (write-CL:stick) for pencil, aga-seu (call-CL:root) for telephone, kio-ro (metal-CL:concavity) for metal cooking pot, etc. Although I’m sure such cases exist, I’m not aware of any cases where, when faced with a new referent, speakers of the language have coined a new word that has no semantic basis (whether via extension or compounding) in extant words in their language. If it is the case, then, that kumara is etymologically non-transparent in Polynesian languages, I think this strongly suggests that it was a borrowing.
In short, the linguistic fact that Polynesians use the word kumara for a recently introduced cultigen strongly suggests, in my view, that the introduction of sweet potatoes into the Pacific involved a human vector: either a Polynesian visit to South America, or a visit by South Americans to the Pacific.
There are two reasons, however, why I have some doubts about the Polynesian visit hypothesis. First, there appears to be no oral history record of this voyage. Keep in mind that for Polynesians to have made it to South America, they would have had to gone against the prevailing winds in the Pacific for what would have been the longest of all the Polynesian voyages. In short, it would have had to have been a deliberate trip, not a question of someone getting blown off course. And at the same time, this voyage would have yielded a a set of tremendous discoveries: a new continent, new peoples, and among other things, a new cultigen which would quickly become one of the staples of the Polynesian peoples. The idea that such a deliberate and momentous voyage of discovery would have not become an important part of the oral history of the people who carried the voyage strikes me as extremely implausible. The history of Polynesian exploration makes clear that they would certainly have been capable of a voyage like this, but that same history suggests that it would be remembered in oral history — somewhat subject to processes of mythification, perhaps — but certainly not forgotten.
Second, it puzzles me that if Polynesians had indeed made it to the western coast of South America, that sweet potatoes would have been the only cultigen that they brought back. Why not also corn, squash, manioc, chili peppers, tobacco, or cotton, just to name a few? It strikes me as implausible that Polynesians would have made a journey of this magnitude and not brought back other cultigens that would have been at least as easy to transport as sweet potatoes, if not easier.
If the above thinking is on the right track, but we do believe that humans were the vector for the introduction of sweet potatoes into the Pacific, that leaves South Americans going to Polynesia. The historical record mentions that South Americans employed ocean-going rafts equipped with sails in coastal trade, so they would have had vessels that could have made the trip (as Thor Heyerdahl and many others have shown). And such a trip would have been in the direction of the prevailing winds, favoring the South Americans, who, as far as we know, were nowhere near as talented open ocean sailors as the Polynesians. In fact, once away from the mainland, the South Americans would probably have had no choice but to continue sailing with the prevailing winds, given the balky nature of their vessels. Whether such a trip was deliberate or accidental (imagine traders being blown out to sea), the fact that they would not have returned would have made it unlikely for others to have emulate the trip, or for South American oral history to have recorded the trip, if it had been deliberate.
As the discussion at Language Log illustrates, this is clearly an issue that inspires some strong feelings, and I suspect that for a consensus to emerge, additional findings, whether from ethnography, archaeology, or human genetics, will need to be added to the linguistic and ethnographic facts we now have at our disposal.
Amazonian Linguistics 
Hi Lev,
About the linguistic points, kumara is indeed non-analyzable in Polynesian. Richard Scaglion has found a mention of the word comal ‘sweet potato’, in a 16th century record of Cañari, which was spoken around the Gulf of Guayaquil. See, e.g., his article in the volume Polynesians in America, ed. Terry Jones et al. (2011).
Computer simulations of voyaging from East Polynesia to the east also support a coastal landing in Ecuador or Peru.
Every aspect of the topic has generated a lot of literature for more than a century, some of it good.
Thanks for your comment, Yoram. What’s your take on the lack of mention of this discovery in Polynesian oral history? And the fact that sweet potatoes were the only cultigen to be introduced into the Pacific from the Americas?
My uderstanding is that there are some suggestive oral histories which could refer to South American contact, but that they are too vague to be definitive. Even histories of inter-island voyages are often expressed in high poetic language, which makes them hard to interpret nowadays.
As to other cultigens, who knows? I read once a suggestion that sweet potatoes would have been appealing as a similar but tastier substitute for yams. Maybe basic foodstuffs was all the voyagers were interested in? Maybe other cultivars did not survive? Negative evidence is hard to work with, except by speculation.
Well, indeed, and absence of evidence is not evidence of absence, after all. On the topic of oral history and what it does and does not preserve, the variation is striking. In northwest Amazonia, for example, oral history preserves quite a lot of detail about the migrations of the ancestors of the modern Piapoco and Achagua people, which occurred about a thousand years ago. In contrast Iquito oral history does preserve information about migration by a group Iquito people from the headwaters of the Pintuyacu to the Amazon proper in the late 18th or early 19th century, but does not appear to preserve any information about Iquito interactions with the Jesuits during the early 18th century.
Lev, a 2020 article in Nature lends support to your theory – called Native American gene flow into Polynesia predating Easter Island, by Ioannidas, A. G. et al. (8 July, 2020)
Their genome study suggests a single contact event in Eastern Polynesia around 1200AD or so. This article prefers the theory that the Polynesians came to South America and brought back some people from there (along with the sweet potato), but what they say about it does not negate the possibility of the contact being initiated the other way.
Best, Leanne