Defining the history of early mankind has long been a desire for archaeologists and others who study ancient artifacts of civilization. How we spread over time and to where and how our cultures interacted with each other proves a source of fascination and knowledge on when we developed technologies and especially agricultural techniques. The trip to the Americas was believed to have happened fairly early on in our hunter-gatherer lifestyle and many questions have been raised over time on what contact our relatives there had with the rest of the world.

Across The Pacific Ocean

The pieces of evidence that such contact existed, specifically between American and Polynesian tribal peoples, were based on three factors: the leftover genetics of the humans themselves, chickens, and sweet potatoes. There have been a number of fights over whether these are reliable pieces of evidence ever since the 1800s and many researchers settled on three decided interactions between those peoples and called them the “tripartite hypothesis”. This included the two well known and documented engagements with Polynesians by Spanish and Portuguese settlers in recent times and a third claimed incident far back before Europe was ever involved.

You see, there were several incongruencies between the American and Polynesian tribes. Sweet potatoes, originally found only in the Americas, was shown to have also existed in Polynesia before Europeans and their ships were involved. The same was seen for chickens and what appeared to be shared genetic relationships between the two peoples. These three sources were what held up the claims of this pre-Columbian interactions across the Pacific Ocean.

The latter two pieces didn’t hold up, however. After further research with more modern techniques, the chickens were found to have likely arrived by an alternative method and the DNA sharing became highly questionable. So all that left the ancient strand of the tripartite with was how sweet potatoes got to Polynesia. Either this was a true example of exchange between continents or it was just the final leg of the chair waiting to be broken.

A Genetic Debunking

A collaboration between the University of Oxford and the Oregon State University has finally finished debunking this last part of the debate. This was done using genomic analyses to break down the connections between sweet potatoes, their relatives, and their common ancestor. The researchers began by collecting 199 specimens from the Ipomoea genus, commonly known as the morning glories, that the sweet potato is a member of. Twenty five different species were represented, including ancient germplasm samples, all supplied by the International Potato Center in Peru.

The DNA samples that were ultimately collected ranged in age from 1769 to 2014, with only three rare and likely uninvolved species being left out. With the DNA, they set up probes targeting 605 shared gene regions, along with a comparison to the regular potato species. The purpose of all of this was to then develop phylogenetic trees that could explain the ancestry of the sweet potato, whether it evolved only once or multiple times independently. Since past tree constructions have come up with confusing, conflicting results.

The results they obtained is that the Ipomoea Batatas species, that being sweet potatoes, appears to have had a single origin, even when comparing both the nuclear and chloroplast genomes. However, the sister relatives differed between these two sources. But they did both determine that the Ipomoea trifida species was the closest living relative of the sweet potato. This also helped them finally figure out and reconcile the conflicting studies showing sweet potato having both a single and multiple points of origin.

A Complicated Evolution

It appears that sweet potatoes first evolutionarily diverged from the common ancestor with I. trifida long in the past and then, once they had each become their own species, sweet potatoes had a partial hybridization event with I. trifida. This involved only capturing the chloroplast genome from its sister species, while not sharing any nuclear DNA between them. This is a known, but rare, occurrence in plant hybridization.

Thus, this explains why it appears that sweet potatoes have multiple divergence events, while really only having one and then picking up that confounding chloroplast DNA at a later point. This also allowed for more accurate measurements of the divergence times of sweet potatoes and when they would have begun spreading as their own species in the wild.

First, the morning glory’s Convolvulaceae family split from its sister potato family Solanaceae around 1.5 million years ago. Then, sweet potatoes became their own family separate from I. trifida around 800,000 years ago. Lastly, the chloroplast capture event appears to have happened somewhere in the realm of 56,000 years after the speciation event.

With the phylogeny explained, the researchers finally turned to showing examples of how sweet potatoes are capable of dispersing. Two other sister species were shown to be capable of dispersing quite far on their own by having such a buoyancy that they can float across the ocean and remain viable. Ipomoea littoralis appears to have done so at a point before it speciated, as it is the only member of Ipomoea to be found in Polynesia and not in the Americas. The only real explanation that fits is thorough sea dispersal. Meanwhile, I. tuboides has been found to be endemic to Hawaii, but is part of a genetic clade of Mexican species, which can only mean that dispersal from Mexico 1.1 million years ago is how they reached the islands.

The Hypothesis Finally Cracked

Since both of these incidents show that sea dispersal of seeds, even across the entire Pacific Ocean, is possible, that makes long-distance dispersal the most logical option for how sweet potatoes came to Polynesia as well. This hypothesis was backed up by samples from the earliest collected sweet potatoes in Polynesia by European explorers during the expedition of Captain Cook in 1769. The sequenced genomes of both the nucleus and chloroplast show that these belong to a chloroplast lineage that existed before the I. trifida chloroplast capture.

That can only mean that these sweet potatoes came to Polynesia before that event happened hundreds of thousands of years ago. This seems to pretty concisely and concretely settle the question of how sweet potatoes came to the Pacific islands and it wasn’t due to any involvement of humans. Thus, the “tripartite hypothesis” appears to officially have become the bipartite hypothesis. That version, at least, seems to have legs to stand on.

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Photo CCs: Ipomoea batatas 006 from Wikimedia Commons

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