Ancient stone tools from a desert shelter in Utah have been found to contain traces of potato flour, the earliest known evidence of potato use in North America, archaeologists say.

The starchy traces were found on a pair of grinding stones from a cliff-side site in south-central Utah known as North Creek Shelter, which had been used intermittently from 11,500 to just 150 years ago.

(Discover an even older Utah site: “12,000-Year-Old Camp Found in Utah May Have Experts ‘Adjusting Their Theories’“)

The stones — including a flat grinding rock known as a metate and a handheld pestle-like tool called a mano — were found in a stratum of the shelter dated to between 10,100 to 10,900 years old, making the particles of potato starch found on them the oldest known on the continent.

“This discovery is the earliest documented use of potatoes in North America, an important energy source that has been largely undervalued or even ignored [in the archaeological record],” the researchers write, in their report on their findings.

Similar evidence was also found on tools that were uncovered in the higher, and more recent, strata of the site, indicating that wild potatoes have been part of the indigenous diet in the Southwest for thousands of years.

“Given this long prehistory and history, the question arises as to whether some [potato] populations could have undergone transport, cultivation, and eventual domestication over such a long period of time,” the researchers write.

In fact, it turns out that the same species of tiny tubers that were used for food nearly 11,000 years ago can still be found in southern Utah, though sparingly.

The tools were unearthed in the early 2000s, when the rockshelter at North Creek was excavated.

Soon after, Dr. Lisbeth Louderback, an anthropologist at the University of Utah, set out to study them in microscopic detail, in order to gain insights into the diet of the site’s many generations of inhabitants.

(Related: “Ice Age Hunting Camp, Replete With Bird Bones and Tobacco, Found in Utah Desert“)

“Grinding plant tissues with manos and metates releases granules that get lodged in the tiny cracks of stone, preserving them for thousands of years,” Louderback said in a press statement.

“Archaeologists can retrieve them using chemicals, modern microscopy and advanced imaging techniques.”

She and her colleagues sieved, washed, and separated residue from the tools and managed to isolate 323 granules of starch, which they then analyzed under a microscope.

All types of starch granules possess a series of concentric circles like growth rings, called hila, which occur in distinctive patterns, the researchers explained.

Potato granules, for example, feature hila that are noticeably off-center. Other hila have fissures running down the middle, some of which branch out to the sides, others of which don’t.

Louderback and her team compared the patterns in the grains from North Creek Shelter to those of similar modern tubers and found that nine matched all of the characteristics of an extant species of wild potato called Solanum jamesii, also known as the Four Corners potato.

Another 61 grains had most of those same traits and were likely bits of S. jamesii, too, they noted.

The small, white-flowered S. jamesii plant is found in shady spots around the Southwest, particularly in New Mexico.

In south-central Utah, however, it’s mainly found near archaeological sites, including a patch that’s still growing some 150 meters (about 500 feet) from North Creek Shelter.

By contrast, the next-closest population of S. jamesii is found 150 kilometers away (about 93 miles), near the spectacular rock art panel known as Newspaper Rock.

(Learn more about Newspaper Rock: “What Is ‘The Rock That Tells Stories’?“)

More historically recent inhabitants of the region, like the Ancestral Puebloans, framed their diet on the famous triumvirate of corn, beans, and squash, as well as game and foraged plants.

But both ethnographic records and historic accounts show that the Four Corners potato was a familiar food of native groups from the Zia and Zuni Pueblo to the Southern Paiute to the Hopi.

“The use of Solanum jamesii by the Hopi people is not so much now, but in the past, as the farmers would be out in their fields down below the mesas, they would dig the wild potatoes and bring it home to add to their diet,” said Max Taylor, botanist at the Hopi Tribe Water Resources Program, in the press statement.

“There were many ways to prepare the potatoes, but one way for sure was to boil the potatoes in a white clay to draw out the toxins from the potato to make it edible.

“The particular clay is called the potato clay for that reason,” he noted.

“It’s a clay similar to the one from which the potteries are made.”

The discovery of the ancient potato particles is not only a fascinating find for the record books, the researchers say; it also highlights the prevalence — and persistence — of an overlooked plant that helped shape many cultures of the Southwest over the millennia.

(Read about a related find: “Earliest Use of Chocolate in America Discovered, Suggesting Ancient Immigrants“)

“It’s hard to persuade the general public to care about rare plants,” said Bruce Pavlik, director of conservation at the University of Utah’s arboretum in Salt Lake City, and a co-author of the paper.

“But this one has a real history associated with native people, with pioneers, with folks living though the Depression and with the residents in [the area] today.

“Across the range, it should be treated as an antiquity, in a sense.”

The team reports its findings in the Proceedings of the National Academy of Sciences.