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Washington State University
CAS Connect April 2015

Seeds of the ancients: Solving a 4,000-year mystery

Poring over the pages of a twenty-first century agronomy textbook, WSU archeologist Jade Guedes found the key to solving a 4,000-year-old mystery.

Around 2000 BCE, a longstanding civilization on the fringes of the Tibetan Plateau abruptly collapsed. What caused the people’s disappearance from the mountainous margins of Western China has puzzled archeologists for years.

Jade d’Alpoim Guedes, assistant professor of anthropology, in Jiuzhaigou National Park
Jade d’Alpoim Guedes, assistant professor of anthropology, in Jiuzhaigou National Park where she conducted research on the Ashaonao archeological site.

Guedes’s review of the agronomy literature led her and an international team of researchers to use crop niche modeling—the same technique Palouse farmers employ to predict whether a plant’s growing conditions align with the climate—to determine whether the region received enough heat at the time to cultivate millet, then a primary food source.

They found that cooling global temperatures at the end of the Holocene Climatic Optimum, a 4,000-year period of warm weather, would have made it impossible for the early people to grow millet.

Their results provide a convincing explanation to why the area’s original inhabitants either left or so abruptly changed their lifestyles. They also help explain the success of farmers who practiced wheat and barley agriculture in the region 300 years later. The research recently was published online in the Proceedings of the National Academy of Sciences.

Unlike millet, wheat and barley have high frost tolerance and a low heat requirement, making them ideally suited for the high altitudes and cold weather of eastern Tibet. Guedes argues this made the two crops an important facet of subsistence immediately after their introduction around 1700 BCE.

“Wheat and barley came in at the opportune moment, right when millets were losing their ability to be grown on the Tibetan Plateau,” Guedes said. “It was a really exciting pattern to notice. The introduction of wheat and barley really enabled Tibetan culture to take the form it has today, and their unique growth patterns may have played a crucial rule in the spread of these crops as staples across the vast region of East Asia.”

An archaeological enigma

At Ashaonao, Haimenkou, and other archeological sites in the Tibetan highlands, researchers for years had noticed a growing trend. An abundance of ancient wheat and barley seeds found at the sites suggested the crops rapidly replaced millet as the staple food source of the region during the second millennium BCE.

The findings were puzzling considering the scientific consensus of the time was the region’s climate would have actually favored millet, due to its shorter growing season, rather than wheat or barley.

The conundrum intrigued Guedes so she dove into agronomy texts to investigate. She found agronomists tended to use a different measurement than archaeologists to determine whether crops can grow in cold, high-altitude environments like the Tibetan Plateau. They used growing degree days, or the accumulated amount of heat plants need over their lifetime, rather than the length of a growing season.

“My colleagues and I created a new model based off what we found in the literature,” Guedes said. “It revealed that global cooling would have made it impossible to grow millet in the Eastern Tibetan Highlands at this time but would have been amenable to growing wheat and barley. Our work turned over previous assumptions and explained why millet is no longer a staple crop in the area after 2000 BCE.”

Food for a warmer future?

Guedes’s work points to climate cooling as the culprit behind the collapse of early civilization on the Tibetan Plateau. Ironically, the region is today one of the areas experiencing the most rapid climate warming on the planet. There are some areas in the southeastern plateau where temperatures are 6 degrees celcius warmer than they were 200 years ago.

Barley cultivation in Jiuzhaigou National Park has changed little in nearly 2000 years.
Barley cultivation in Jiuzhaigou National Park has changed little in nearly 2000 years. The park is located in the Min Shan mountain range, Northern Sichuan in southwestern China.

Rapid temperature increase is making it difficult for the region’s inhabitants to practice yak pastoralism and to grow cold weather crops, once again endangering their survival. “So now we have a complete reversal and climate warming is having a big impact on the livelihood of smaller farmers on the Tibetan Plateau,” Guedes said.

In an intriguing offshoot of the research, the ancient millets that virtually ceased being cultivated on the Tibetan Plateau as the climate grew colder could regain prominence as the climate warms.

“Right now, these millets have almost become forgotten crops,” Guedes said. “But due to their heat tolerance and high nutritional value, they may once again be useful resources for a warmer future.”

Find out more about Guedes’s work »