Sweet Potatoes May Have Originated in Asia

57-milion-year-old leaf fossils from eastern India suggest that the worldwide-distributed morning glory family (Convolvulaceae), which includes sweet potatoes and many other plants, originated in the late Paleocene epoch in the East Gondwana land mass that became part of Asia.

The sweet potato (Ipomoea batatas). Image credit: Llez / CC BY-SA 3.0.

The sweet potato (Ipomoea batatas). Image credit: Llez / CC BY-SA 3.0.

“I think this will change people’s ideas. It will be a data point that is picked up and used in other work where researchers are trying to find the time of the evolution of major groups of flowering plants,” said Professor David Dilcher, from the Department of Geology at Indiana University, Bloomington.

Previous fossil evidence had suggested the Convolvulaceae family may have originated in North America about 35 million years ago.

But molecular analyses had supported the idea that it originated earlier and in the Old World. The new research provides evidence for that conclusion.

The discovery also suggests Convolvulaceae and its sister nightshade family (Solanaceae), which includes potatoes and tomatoes, diverged earlier than previously thought.

Together with the recent, separate discovery of 52-million-year-old nightshade fossils in Argentina, it suggests that morning glories developed in the East and nightshades in the West.

(A) this map shows the modern distribution of the sweet potato family (yellow line) and genus (white line); (B) the fossil leaf of Ipomoea meghalayensis; (C) the modern leaf of Ipomoea eriocarpa, showing a similar size, shape and vein pattern. Image credit: Indiana University.

(A) this map shows the modern distribution of the sweet potato family (yellow line) and genus (white line); (B) the fossil leaf of Ipomoea meghalayensis; (C) the modern leaf of Ipomoea eriocarpa, showing a similar size, shape and vein pattern. Image credit: Indiana University.

The 17 fossils analyzed in the study are the earliest recorded fossils for both Convolvulaceae and the order Solanales, which includes morning glories and nightshades. Morning glory fossils are rare because the plants’ soft structure was not easily preserved in rocks.

Professor Dilcher and his colleagues, Gaurav Srivastava and Rakesh C. Mehrotra of the Birbal Sahni Institute of Palaeosciences in Lucknow, India, found the fossils in the Tura Formation exposed in Nangwalbibra, East Garo Hills District, Meghalaya.

The researchers used microscopic analysis of the shape and structure of the leaves, comparing details of the leaf veins and cells with plants in the genus Ipomoea. This genus includes sweet potato but also hundreds of other plants, most of which don’t produce food for humans.

“We don’t know that these were sweet potatoes. We can’t say there were delicious sweet potatoes there. There may have been, or there may not,” Professor Dilcher said.

The research is published in the Proceedings of the National Academies of Science.

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Gaurav Srivastava et al. Paleocene Ipomoea (Convolvulaceae) from India with implications for an East Gondwana origin of Convolvulaceae. PNAS, published online May 21, 2018; doi: 10.1073/pnas.1800626115

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