Last updated August 1, 2018 at 10:18 am
The timeline of the world’s favourite staple food gives new insights into human migration.
Botanists have shown that the humble sweet potato is vastly older than modern humans, challenging current theories of early human migration.
Originating in Central and South America, sweet potatoes are one of the most widely consumed crops in the world.
Scientists have long thought that their existence in Polynesia meant that ancient seafaring Polynesians must have made the 8,000-kilometre trek across the Pacific to South America long before Europeans arrived, bringing back the delicious vegetable.
But a new study, led by Pablo Muñoz-Rodríguez from the University of Oxford, has found that the plant species is at least 800,000 years old, four times as old as the earliest-discovered human fossils. The research suggests that sweet potatoes could have been naturally dispersed around the Pacific well before any humans were around to eat them.
The Polynesian link
The team, which involved botanists from the UK, US and Peru, set out to clarify the origin and evolution of the sweet potato and in particular figure out how this root vegetable came to be in Polynesia as early as 1,000 years ago.
To do this, they used advanced techniques to sequence the DNA of 199 specimens of sweet potatoes and their wild relatives, both living and historic. This resulted in a comprehensive genomic dataset that allowed the botanists to address important unanswered questions.
The dataset indicates that the sweet potato arose 800,000 years ago after a genome duplication event in a species called Ipomoea trifida — essentially, new genetic material was generated during molecular evolution, spawning a new species. In a complex turn of events, the research further suggests that sweet potatoes and I. trifida hybridised again later on, producing another independent lineage.
The discovery has powerful implications for understanding human history.
Travels from the Americas
“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,” Muñoz-Rodríguez 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.”
If the sweet potato has been around for so many millennia, it is likely to have been able to traverse the ocean from America to Polynesia with the help of birds, winds or the ocean — no humans required.
One of the many specimens studied was the first sweet potato known from Polynesia, which was collected in 1769 during Captain Cook’s voyage on the Endeavour.
The team found that this variety split off from American varieties about 100,000 years ago, putting another nail in the coffin of current migration theories.
These findings also mean good news for the survival of sweet potato crops.
The loss of genetic diversity in crops is a big challenge to food security, and a way to combat this and reinforce desirable traits is to interbreed crops with their wild relatives.
By identifying the progenitor of sweet potatoes, I. trifida, this study could lead to improved crops and keep this ancient veggie on our plates.
The research is published in Current Biology.