Many botanists have claimed that humans must have brought the precious base in the Pacific of South America, a hidden thing chapter of human history. Not so, according to a new study.
Of all the plants that humanity has turned into crops, no one is more disconcerting than the sweet potato. The natives of Central and South America cultivated it on the farms for generations, and the Europeans discovered it when Christopher Columbus arrived in the Caribbean.
In the eighteenth century, however, Captain Cook stumbled back among sweet potatoes – over 4,000 miles away, on remote Polynesian islands. The European explorers subsequently found them elsewhere in the Pacific, from Hawaii to New Guinea.
The distribution of the plant has baffled the scientists. How could sweet potatoes be born from a wild ancestor and then end up being scattered over such a wide range? It was possible that unknown explorers brought him from South America to countless Pacific islands?
An in-depth analysis of the DNA of sweet potatoes, published Thursday in Current Biology, comes to a controversial conclusion: humans have nothing to do with this. The cumbersome sweet potato spread all over the world long before humans could play a part: it is a natural traveler.
Some agricultural experts are skeptical. “This paper does not solve the issue,” said Logan J. Kistler, curator of archeogenomics and archaeobotany at the Smithsonian Institution.
The alternative explanations remain on the table, because the new study did not provide sufficient evidence to understand exactly where the sweet potatoes were tamed and when they arrived in the Pacific. “We do not have a smoking gun yet,” said Dr. Kistler.
The sweet potato, Ipomoea batatas, is one of the most valuable crops in the world, providing more nutrients per acre grown than any other staple food. He has supported human communities for centuries. (In North America, it is often called a yam, in fact the yams are a different species native to Africa and Asia.)
Scientists offered a series of theories to explain the broad distribution of I. batatas. Some scholars proposed that all sweet potatoes originate from the Americas and that, after Columbus’s journey, they were spread by Europeans to colonies like the Philippines. The Pacific Islanders have acquired the crops from there.
As it turned out, however, the inhabitants of the Pacific Islands had cultivated the crop for generations when Europeans showed up. On a Polynesian island, archaeologists have found sweet potato remains dating back over 700 years.
A radically different hypothesis emerged: the inhabitants of the Pacific islands, masters of oceanic navigation, collected sweet potatoes traveling to the Americas, long before Columbus arrived. The evidence included a suggestive coincidence: in Peru, some natives call the sweet potato coot. In New Zealand, it’s kumara.
A potential link between South America and the Pacific was the inspiration for Thor Heyerdahl’s famous 1947 trip aboard the Kon-Tiki. He built a raft, which then successfully sailed from Peru to the Easter islands.
The genetic tests only complicated the picture. Examining the plant’s DNA, some researchers concluded that sweet potatoes were born only once by a wild ancestor, while other studies have indicated that it happened at two different points in history.
According to these latest studies, the South Americans domesticated sweet potatoes, which were then bought by the Polynesians. The Central Americans domesticated a second variety that was later taken up by the Europeans.
Hoping to shed light on the mystery, a team of researchers recently undertook a new study – the largest DNA investigation of sweet potatoes yet. And they came to a very different conclusion.
“We find very clear evidence that sweet potatoes could arrive in the Pacific by natural means,” said Pablo Muñoz-Rodríguez, botanist at the University of Oxford. He believes that wild plants have traveled thousands of miles across the Pacific without the help of humans.
Mr. Muñoz-Rodríguez and his colleagues visited museums and herbariums all over the world to collect samples of varieties of sweet potatoes and wild relatives. Researchers used a powerful DNA sequencing technology to collect more genetic material from plants as much as possible in previous studies.
Their research has indicated only a wild plant as an ancestor of all sweet potatoes. The closest wild relative is a herbaceous flower called Ipomoea trifida which grows around the Caribbean. Its pale purple flowers look very much like those of the sweet potato.
Instead of a thick and tasty tuber, I. trifida cultivates only a thick pencil root. “We have nothing to eat,” said Muñoz-Rodríguez.
The ancestors of the sweet potatoes separated from I. trifida at least 800,000 years ago, they calculated the scientists. To investigate how they arrived in the Pacific, the team headed to the Museum of Natural History in London.
The sweet potato leaves collected by Captain Cook’s crew in Polynesia are kept in the museum’s closets. The researchers cut pieces of leaves and the DNA extracted from them.
Polynesian sweet potatoes proved to be genetically unusual – “very different from anything else,” said Muñoz-Rodríguez.
The sweet potatoes found in Polynesia were separated over 111,000 years ago from all the other sweet potatoes studied by the researchers. Yet humans arrived in New Guinea some 50,000 years ago, and only reached remote islands in the Pacific over the last few millennia.
The age of the sweet potatoes of the Pacific made it unlikely that any human, Spanish or Pacific on the island would transport the species from the Americas, said Muñoz-Rodríguez.
Traditionally, researchers have been skeptical that a plant like a sweet potato could travel through thousands of miles of ocean. But in recent years, scientists have discovered that many plants have traveled, floated on water or carried in pieces by birds.
Even before the sweet potato made the journey, its wild relatives traveled to the Pacific, scientists discovered. One species, the Hawaiian crescent moon, lives only in the dry forests of Hawaii, but its closest relatives all live in Mexico.
Scientists estimate that the Hawaiian moon moon has separated from its relatives and made its journey across the Pacific over a million years ago.
But Tim P. Denham, an archaeologist from the Australian National University who was not involved in the study, found it difficult to digest this scenario.
It would suggest that the savage ancestors of sweet potatoes spread across the Pacific and were then domesticated many times, while still remaining the same. “This would seem unlikely,” he said.
Dr. Kistler claimed that it was still possible for Pacific Islanders to travel to South America and return with the sweet potato.
A thousand years ago, they could have met many varieties of sweet potatoes on the continent. When Europeans arrived in the 1500s, they probably wiped out much of the genetic diversity of the crop.
As a result, Dr. Kistler said, the surviving sweet potatoes of the Pacific seem only distantly related to those of the Americas. If the scientists had done the same study in the 1500s, the sweet potatoes of the Pacific would have adapted perfectly to other South American varieties.
Dr. Kistler was optimistic that the debate over sweet potatoes would be resolved one day. The herbals of the world contain a vast number of varieties that have yet to be genetically tested.
“There are more than we could see in a lifetime,” said Dr. Kistler.
For his part, Mr. Muñoz-Rodríguez plans to look for other wild relatives of sweet potatoes in Central America, hoping to obtain further clues about how exactly a thin grass gave rise to an invaluable crop.
Developing the history of crops like this could do more than satisfy our curiosity about the past. Wild plants contain many genetic variants lost when domesticated crops are domesticated.
Researchers can find plants that can hybridize with domesticated sweet potatoes and other crops, equipping them with genes for disease resistance or to withstand climate change.
“Essentially, it is preserving the genetic heritage that feeds the world,” said Dr. Kistler.