Sweet potatoes are a staple food crop in most of the world today, but they're also a bit of an enigma. We don't know for sure how or when they evolved from their closest wild relatives or whether humans were involved. A new genetic study answers some of those burning questions about the sweet potato's past, and, in the process, it casts some doubt on a popular idea about pre-Columbian sea travel between the Americas and the islands of Polynesia.

Pre-Columbian cultural exchange?

A few tantalizing threads of evidence have emerged over the years to suggest that the people of the Polynesian Islands and the people of the Americas could have maintained at least sporadic contact with each other long before Europeans arrived in either place. None of that evidence has stood up to much scrutiny, though—except for one fact: sweet potatoes, a crop native to Central and South America, had already firmly taken root in the islands of Polynesia by the time Europeans arrived. It seemed logical that someone must have carried them across the Pacific.

But a new study says sweet potatoes actually reached the islands long before there were even people in the Americas—at least 111,500 years ago, and possibly even earlier.

“This finding is likely to be controversial because it calls into question the alleged contacts between Polynesians and Americans in pre-European times,” Oxford University botanist Pablo Muñoz-Rodriguez, who led the study, told Ars Technica. “[The] sweet potato was the only remaining biological evidence of these contacts.”

Muñoz-Rodriguez and his team sampled DNA from 119 specimens of sweet potatoes and all of their wild relatives, including a sweet potato harvested in the Society Islands in 1769 by the Cook expedition. With those samples, Muñoz-Rodriguez and his colleagues built a phylogenetic tree: a family tree that shows evolutionary relationships among organisms based on the differences in their DNA. For plants, researchers often build two phylogenetic trees: one for the DNA stores in the nucleus of the plant's cells and one for the chlorophyll-producing organelles called chloroplasts, which have their own DNA. Genetic material doesn't always get passed on in the same way for both, so it's sometimes useful to compare the two.

The team used the phylogenetic trees to estimate how long ago each branch of the tree split off from the others. It turned out that the Society Islands sweet potato hadn't interbred with Central and South American lines for at least 111,500 years, according to the nuclear DNA, or 139,000 years according to the chloroplast DNA. The first humans didn't arrive in North America until around 13,000 to 14,000 years ago, and the Polynesian culture developed even more recently, so there's no way they could have been carrying produce across the Pacific.

“In fact it was quite surprising to us,” said Muñoz-Rodriguez. “Considering that those ancient contacts would be really difficult to explain, I myself thought that the most likely explanation for the presence of sweet potato in Polynesia was that it had been taken there by Spanish or Portuguese travelers at the beginning of the age of explorations.” Instead, Muñoz-Rodriguez and his colleagues say it's likely that the seeds floated across the Pacific and took root on the islands long before the first people arrived.

Back to its roots

According to Muñoz-Rodriguez’s phylogenetic trees, humans may not have had much of a hand in shaping the sweet potato, either. Sweet potatoes are the only plant in the Ipomoea genus with a tuber—a large, thick root that stores nutrients and produces the buds that grow into new plants. That has often been presented as evidence that human appetites shaped the evolution of the sweet potato.

“We wanted to check whether sweet potato and its tuber evolved in human or pre-human times—that is, whether humans found and domesticated an already edible sweet potato or not,” said Muñoz-Rodriguez.

Sweet potatoes, known scientifically as Ipomoea batatas, diverged from their closest relative, a species called Ipomoea trifida, around 800,000 years ago, according to analysis of the nuclear DNA in the study. Anatomically modern humans didn’t even exist yet. That means human intervention couldn’t be the reason sweet potatoes evolved into a distinct species.

But it doesn't tell us when its key feature, the tuber, first appeared, since that might have happened long after speciation. I. trifida’s pencil-like roots are thicker than any other species in the genus, but only sweet potatoes have proper tubers. The explanation may be that sweet potatoes are a hexaploid species, meaning that its genome contains six copies of each chromosome (for comparison, humans are a diploid species, with just two copies of each chromosome—one from each parent).

“It has been suggested in several studies that polyploidy promotes bigger cells and organs, and so we think the size of the tuber can be a consequence of polyploidy, rather than a consequence of human domestication," Muñoz-Rodriguez told Ars Technica. “Obviously, it is also possible that human domestication made it even larger or more palatable, but I am afraid we do not have the data necessary to answer that question yet, and it needs further research.”

Both hexaploidy and a larger tuber might have helped the species adapt to tougher, more varied environments.

The future looks sweet

All that new information about the sweet potato’s past may also tell us something about how to shape its future. It appears that at some point after sweet potatoes emerged as a distinct species, they hybridized—once more, for old times' sake—with I. trifida.

That's interesting to modern researchers because one of the ways to improve the quality or genetic diversity of a crop is to cross it with its closest wild relative. Species need some diversity in the gene pool in order to have a good chance of adapting to changes in their environments, like climate shifts, new predators, or disease. Now that we know sweet potatoes have already done that once and have confirmed that I. trifida is actually the evolutionary next of kin, that information could someday help breed better, more resilient sweet potato crops.

“If there was a change to which sweet potato could not adapt, and farmers do not have a remedy, the use of crop wild relatives in breeding programs can provide the genetic variability required,” explained Muñoz-Rodriguez.

Current Biology, 2018. DOI: 10.1016/j.cub.2018.03.020 (About DOIs).