Sigh. Maybe a sweet potato *doesn’t* mean that prehistoric Polynesians visited South America after all.

The Guardian has some sweet potato evolutionary research that unintentionally drives a wedge in the idea that the presence of the humble yam in both places indicates that prehistoric Polynesian sailors didn’t really have to have visited South America to get their sweet potatoes:

The research reveals that the sweet potato evolved just once, probably in central or northern South America, and originated from a single ancestor. What’s more, an analysis of part of a 250-year-old sweet potato plant collected during Captain Cook’s voyage to the South Pacific on HMS Endeavour suggests the spuds arrived in Polynesia by means of ocean currents.

“I think we have proved that there is no need to claim human transportation of the sweet potato across the Pacific – there is much more powerful evidence for naturally driven dispersal,” said Pablo Muñoz-Rodríguez, a PhD student and first author of the research from the University of Oxford.

To delve into the sweet potato’s past, Muñoz-Rodríguez and colleagues analysed genetic material from almost 200 specimens of the plant and its 14 closely related wild species to reconstruct a sort of “family tree”.

The results reveal that the sweet potato is more closely related to one wild species, known as Ipomoea trifida, than any other, suggesting that both evolved from a common ancestor.

“All the evidence points to Ipomoea trifida being the only species with a role in the origin of the sweet potato,” said Muñoz-Rodríguez.

The team add that this branch in the family tree occurred long before humans walked the Earth, with one estimate putting it at least 800,000 years ago.

While some have suggested that the plant’s presence points to communication between inhabitants of the two regions, the latest study suggests it is more likely that seeds of the sweet potato simply floated across the Pacific on sea currents.

That, they write, is backed up by the fact that a wild relative of the sweet potato found in Polynesia, but not America, appears to have split from American species more than a million years ago – ruling out human transport. The seed pods of this plant, the authors note, are very similar to those of sweet potatoes, suggesting that it, too, could have travelled around the world on the waves.

Further evidence for the idea came from a sweet potato plant collected by the naturalist Joseph Banks during Captain Cook’s voyage to Polynesia in 1769. “[It is] the oldest sweet potato specimen known from Polynesia,” said Muñoz-Rodríguez.

This variety is only found in Polynesia, but the genetic data shows that it split from its closest relative, an American variety, more than 100,000 years ago – long before modern humans colonised the islands.

“It removes the last remaining potential evidence for contact,” said Prof Alan Cooper, director of the Australian Centre for Ancient DNA, who was not involved in the study. “Of course, that doesn’t mean that no contact occurred – just that there is no evidence for the idea.”