The team from the Washington State University led by Jade D'Alpoim Guedes 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.

The results provide the first convincing explanation for 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.

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 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 role in the spread of these crops as staples across the vast region of East Asia," she explained.

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.

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

The new work points to climate cooling as the culprit behind the collapse of early civilisation 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 six degrees Celsius higher than they were 200 years ago.

"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 concluded.

The research appeared in the journal Proceedings of the National Academy of Sciences.

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