He and his co-author, Claudia Rumold, bathed the tools in a sonicator, which dislodged the starch grains from the pores using sonic waves. Then they analyzed the grains under a microscope and compared them to reference samples of other crops and wild plants from the region.

Out of 141 starch samples recovered from 14 tools, 50 “were consistent with cultivated or domesticated potatoes,” Dr. Rumold said.

Starch grain analysis, which is a relatively novel method, was key to finding evidence of potatoes because the tubers do not preserve well, Dr. Aldenderfer said. “When a seed burns, you often get something left of a seed husk. When corn cobs burn, you get something left of the cob. When potato burns, it burns up — very seldom do you get actual bits.”

The early cultivation of potatoes seems to have been part of a larger shift at Jiskairumoko, from hunting and gathering toward farming and herding, he added. Around the same time, people started to build more complex houses, and the beginnings of a social hierarchy emerged. In 2008, a team led by Dr. Aldenderfer found a gold necklace from Jiskairumoko dating back to 2000 B.C., suggesting that an elite class had formed by then.

As for how the potato spread and changed from thumbnail- to fist-sized over time, many questions remain. “We don’t have enough data to know how many times it was domesticated in this particular area, or if it was just once,” Dr. Rumold said.

Historians do know that millenniums later, after the Spaniards conquered the Incan Empire, they introduced the potato to Europe. British colonialists then brought the potato to North America, where it flourished and became a staple. Eventually, probably starting in the 1800s, the beloved spud made its way into Thanksgiving traditions. So when you eat your mashed potatoes this holiday, add ancient Andean civilization to your list of things to be thankful for.