Study of sweet potato raises questions about early contact between Polynesia and the Americas

New research challenges the citing of sweet potatoes in Polynesia as evidence of pre-European contact between South America and Polynesia, according to a scimex report (HERE).

Genetic evidence indicates the plant species is at least 800,000 years old — far older than even early humans – and the researchers suggest the sweet potato was dispersed naturally around the Pacific. Hence it was already there when humans arrived.

But New Zealand experts approached by the Science Media Centre question the conclusions of the study and call for more robust evidence.

Scimex quotes from a media release from Cell Press, which draws attention to a paper published in Current Biology (see HERE).

 The evidence in the paper suggests sweet potatoes were growing long before there were any humans around to eat them, Cell Press says.

It also suggests the sweet potato crossed the ocean from America to Polynesia without any help from people.

“Apart from identifying its progenitor, we also discovered that sweet potato originated well before humans, at least 800,000 years ago,” says Robert Scotland from the University of Oxford.

“Therefore, it is likely that the edible root already existed when humans first found this plant.”

Scotland and colleagues set out to clarify the origin and evolution of the sweet potato, which is one of the most widely consumed crops in the world and an important source of vitamin A precursors.

They also aimed to explore a question that has been of interest for centuries: how did the sweet potato, a crop of American origin, come to be widespread in Polynesia by the time Europeans first arrived?

The researchers combined genome skimming and target DNA capture to sequence the whole chloroplasts and 605 single-copy nuclear regions from 199 specimens representing the sweet potato and all of its crop wild relatives.

The data strongly suggest that sweet potato arose after a genome duplication event. Its closest wild relative is Ipomoea trifida. The findings confirm that no other extant species was involved in the sweet potato’s origin.

Phylogenetic analysis of the DNA sequences produced conflicting family trees. However, the researchers report, those conflicting patterns can be explained by a dual role for I. trifida. Sweet potato arose from I. trifida and later hybridised with I. trifida to produce another, independent sweet potato lineage.

“We demonstrate that the existence of those two different lineages is the result of an ancient hybridisation between sweet potato and its progenitor,” says Pablo Munoz-Rodriguez, first author of the paper.

“We conclude that sweet potato evolved at least 800,000 years ago from its progenitor, and then after the two species became distinct, they hybridised.”

The findings come as good news for the future of the sweet potato, Cell Press says. That’s because the loss of genetic diversity in crops is a major threat for food security.

One way to improve or reinforce desirable properties in food crops is to cross them with their closest wild relatives. So, Scotland says, the identification of the sweet potato’s progenitor opens the door to a more accurate understanding of its potential role in sweet potato breeding.

The new view on sweet potato history also has major implications for understanding human history.

“Our results challenge not only the hypothesis that the sweet potato was taken to Polynesia by humans, but also the long-time argued existence of ancient contacts between Americans and Polynesians,” Munoz-Rodriguez says.

“These contacts were considered as true based on evidence from chickens, humans, and sweet potato. Evidence from chickens and humans is now considered questionable, and thus sweet potato was the remaining biological evidence of these alleged contacts. Therefore, our results refute the dominant theory and call into question the existence of pre-European contacts across the Pacific.”

But the Science Media Centre question has published a joint statement (HERE) from Professor Lisa Matisoo-Smith and Dr Michael Knapp, both from the Department of Anatomy at the University of Otago.

They say:

“The paper addresses two fundamental questions regarding the origin and dispersal of the sweet potato (Impomoea batatas):

  1. Did the sweet potato evolve once or multiple times and what species were involved in its origin?
  2. How did the sweet potato, a crop of American origin, come to be widespread in Polynesia before European arrival?

“As far as we can tell, the authors and their data answer the first question well – they have found reasonable DNA evidence suggesting that the sweet potato likely evolved once, from the wild Ipomoea trifida. This is indeed an important finding, as the authors point out, as it has implications for food security and the ability to look to appropriate wild sources from which to introduce new genetic variation into this important food crop.

“Unfortunately, it seems that they thought that, with the inclusion of one additional sample, they might be able to address question number two, and it is here that this paper runs into problems. The data used to address the issue of how the sweet potato got to the Pacific consisted of chloroplast and nuclear DNA obtained from a plant specimen collected in 1769 in Tahiti by Joseph Banks and Daniel Solander, who accompanied Captain James Cook during his Endeavour expedition.

“This was a sample held in the Herbarium at the British Museum. While technically only 250 years old, this specimen is not fresh tissue and will have degraded DNA (DNA begins to break down as soon as an organism dies). Any DNA obtained from this sample needs to be treated with all of the precautions of ancient DNA. There are well accepted and published protocols for the extraction and sequencing of ancient DNA (the most basic of which include working in a spatially isolated and dedicated ancient DNA facility, running negative controls and undertaking independent replication of results) – none of which appear to have been followed by these researchers.

“Not surprisingly, they found an unusual number of DNA mutations in the Banks and Solander sample and from this, they calculated that the Polynesian sample must have been isolated from its source population for over 100,000 years. Humans have only been in East Polynesia for just over 1,000 years, so the authors conclude that the sweet potato dispersed to Polynesia naturally (carried by birds or floating on weed mats, for example). From there, they conclude that there is no evidence for pre-Columbian contacts between Polynesia and the Americas.

“So, the data that the whole argument is based upon is DNA from one sample that was produced without using standard ancient DNA protocols and analysed without using standard ancient DNA authentication methods. The data presented, to an ancient DNA researcher, ring alarm bells and appear very much like damaged and/or contaminated DNA. I would want to at least see replication before considering using it in further analyses. There are also technical issues with the actual phylogenetic analyses that would have a major impact on the date of divergence.

“In addition, much of the discussion regarding the likelihood of the sweet potato being introduced to Polynesia due to human contact as opposed to naturally (and the topic has been debated amongst anthropologists and archaeologists for decades) is the strong linguistic evidence. The Polynesian word, kumara, is derived from the Quechuan word, cumar (traced particularly to the Gulf of Guayaquil, in Ecuador, one of the centres of sweet potato domestication). Such linguistic evidence requires human contact and the authors mention this information but they do not account for it in their argument for natural dispersal. They also lightly brush off or do not mention a significant amount of other anthropological evidence for pre-Columbian contacts.

“While natural dispersal of the sweet potato is certainly possible, the data and argument presented are not convincing. Can their DNA result be replicated in an ancient DNA facility? There are multiple samples from that same Banks and Solander collection, including plants collected from New Zealand, held in the British Museum Herbarium – do they show the same pattern of genetic divergence? Is there any evidence of sweet potato starch, phytoliths or pollen found in soils that are over 10,000 years old, or even more than 1500 years old in Polynesia?

“We would like to see more robust data, ideally from multiple sources, presented before we can accept the data and reconsider the current interpretation that the sweet potato was brought to Polynesia by humans at some point around 1000-1200 AD.”

Researchers from the University of Oxford, Oregon State University and Duke University produced the article published in Current Biology.

Their work was primarily supported by the Leverhulme Trust and the University of Oxford through the John Fell Fund.

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