Solving a 4,000-year-old mystery, scientists have found that climate change may be responsible for the abrupt collapse of civilisation on the fringes of the Tibetan Plateau around 2000 BC.
Washington State University archaeologist Jade D'Alpoim Guedes and an international team of researchers 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 ancient people on the Tibetan Plateau to cultivate millet, their primary food source.
Guedes' team's research provides the first convincing explanation for why the area's original inhabitants either left or so abruptly changed their lifestyles.
The findings also explain the success of farmers who practiced wheat and barley agriculture in the region 300 years later.
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 said.
She said this made the two crops an important facet of subsistence immediately after their introduction around 1700 BC.
"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," she said.
Researchers also said that the ancient millet seeds that fell out of cultivation on the Tibetan Plateau as the climate got colder might soon be useful again as the climate warms up.
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 BC.
The findings were puzzling considering that the scientific consensus of the time was the region's climate would have actually favoured millet, due to its shorter growing season, over wheat or barley.
Guedes dove into the agronomy literature 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 total 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," she said.
