The sweet potato was one of a number of crops domesticated in the Andes and, like many of the rest, it became a global crop in the colonial era. But there were some hints that the sweet potato may have already started its global sweep before the Europeans ever took a bite out of one. Some of the early European explorers, including Captain Cook, reported finding it in places like Hawaii. All of which implies that the Polynesians, who managed to spread widely across the Pacific, had made it all the way to South America.

But it was difficult to be sure, given that European travelers later enhanced its spread within the Pacific and elsewhere. This has also created a complex genetic legacy that obscures its origins. Now, researchers have gone back and obtained DNA from museum samples, including some collected by Cook's crew, and find that the DNA indicates that Polynesians made it as far as South America.

Archeological remains appear to place sweet potato cultivation in the core of Polynesia by the year 1200, and it spread with further migrations to places like New Zealand and Hawaii. It's possible that the plant had naturally spread as seeds across the ocean and the Polynesians learned to cultivate it independently. One of the arguments against this is the fact that the Polynesian terms for the crop appear to be closely related to its name in Quechua, the language of the Peruvian Andes. ("Kuumala" and derivatives vs. "kumara" and relatives.)

That, and the fact that the Polynesians made it most of the way across the Pacific, clearly getting as far as Easter Island. Still, reaching South America and then returning with crops is quite a significant step beyond that.

Trying to piece together a model of the sweet potato's spread based on things like historical reports, known human travels, and genetic information, however, has been quite a mess. Even the simplest ideas that are consistent with the data involve the sweet potato entering the Pacific through three different routes, and then potentially hybridizing with the strains already in residence.

It may be complicated, but the new genetic study provides support for this three-pronged (tripartite) assault on the Pacific. The researchers start with a detailed study of the cultivars found in South America, Central America, and the Caribbean. The researcher team, based in France, found that there are two distinct populations: a northern strain, found in Mexico and the Caribbean, and a southern one, in the Andes, where the crop was first domesticated. (There's also a region of overlap and intermingling in between the two.)

Genetic relatives of the northern strain entered the Pacific via two routes. In the first, trade with the Philippines during the Spanish colonial era brought it there; from that site, it diffused into mainland Asia and to some island strains. But the Caribbean strain made its way across the Atlantic, through Madagascar, and into Indonesia. Its arrival, likely in the 1700s, transformed the agriculture of the New Guinea highlands, which had previously been focused on taro. Cultural changes followed in its wake.

Most modern cultivars are descended from some combination of these two invasions. But samples from museums, including some collected by the earliest European voyagers, show that this wasn't likely to be the case prior to the late 1800s. Before then, most strains were spread directly from initial sources, with very little interbreeding.

The other thing that museum samples indicate is that, prior to extensive European travels in the Pacific, there was another, genetically distinct form of the sweet potato present in Eastern Polynesia. And that appears to have originated from the southern population, focused in the Andes. It was already present in Polynesia before colonists left this region to settle in Hawaii and New Zealand.

Given that we already know Polynesians had the technology to engage in long-distance voyages across the Pacific, the simplest explanation for this is that they did make it to South America, probably somewhere around the year 1000. And, most strikingly, some of them apparently turned around and traveled back halfway across the Pacific.

This, combined with last week's results that showed travelers from India made it to Australia over 4,000 years ago, provides impressive evidence of just how mobile some of our ancestors have been. And they managed to travel all that distance with technologies we'd generally consider primitive. But it's important to consider that, just because you wouldn't hop into an open canoe and head out into the Pacific, it wouldn't have meant certain death for anyone with the right skills.

PNAS, 2013. DOI: 10.1073/pnas.1211049110 (About DOIs).