Of all the plants that humanity has turned into crops, none is more puzzling than the sweet potato.
Indigenous people of Central and South America grew it on farms for generations, and Europeans discovered it when Christopher Columbus arrived in the Caribbean.
In the 18th century, however, Captain Cook stumbled across sweet potatoes again — over 4,000 miles away, on remote Polynesian islands. European explorers later found them elsewhere in the Pacific, from Hawaii to New Guinea.
The distribution of the plant baffled scientists. How could sweet potatoes arise from a wild ancestor and then wind up scattered across such a wide range?
Was it possible that unknown explorers carried it from South America to countless Pacific islands?
An extensive analysis of sweet potato DNA, published on Thursday in Current Biology, comes to a controversial conclusion: Humans had nothing to do with it.
The bulky sweet potato spread across the globe long before humans could have played a part — it’s a natural traveler.
Some agricultural experts are skeptical.
“This paper does not settle the matter,” said Logan J. Kistler, the curator of archaeogenomics and archaeobotany at the Smithsonian Institution.
Alternative explanations remain on the table, because the new study didn’t provide enough evidence for exactly where sweet potatoes were first domesticated and when they arrived in the Pacific.
“We still don’t have a smoking gun,” Dr. Kistler said.
The sweet potato, Ipomoea batatas, is one of the most valuable crops in the world, providing more nutrients per farmed acre than any other staple. It has sustained human communities for centuries.
Scientists have offered a number of theories to explain the wide distribution of I. batatas.
Some scholars proposed that all sweet potatoes originated in the Americas, and that after Columbus’s voyage, they were spread by Europeans to colonies such as the Philippines. Pacific Islanders acquired the crops from there.
As it turned out, though, Pacific Islanders had been growing the crop for generations by the time Europeans showed up. On one Polynesian island, archaeologists have found sweet potato remains dating back over 700 years.
A radically different hypothesis emerged: Pacific Islanders, masters of open-ocean navigation, picked up sweet potatoes by voyaging to the Americas, long before Columbus’s arrival there.
The evidence included a suggestive coincidence: In Peru, some indigenous people call the sweet potato cumara. In New Zealand, it’s kumara.
A potential link between South America and the Pacific was the inspiration for Thor Heyerdahl’s famous 1947 voyage aboard the Kon-Tiki. He built a raft, which he then successfully sailed from Peru to the Easter Islands.
Genetic evidence only complicated the picture. Examining the plant’s DNA, some researchers concluded that sweet potatoes arose only once from a wild ancestor, while other studies indicated that it happened at two different points in history.
According to the latter studies, South Americans domesticated sweet potatoes, which were then acquired by Polynesians. Central Americans domesticated a second variety that later was picked up by Europeans.
Hoping to shed light on the mystery, a team of researchers recently undertook a new study — the biggest survey of sweet potato DNA yet. And they came to a very different conclusion.
Their research pointed to only one wild plant as the ancestor of all sweet potatoes. The closest wild relative is a weedy flower called Ipomoea trifida that grows around the Caribbean.
Its pale purple flowers look a lot like those of the sweet potato.
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