How did human settlements manage to reach high altitudes in the Tibetan Plateau, a region known as the "Roof of the World"? An international team of researchers has a simple answer: barley.
The findings, published in the journal Science, reveal a key factor that helped human settlements overcome such harsh, unforgiving environments.
Archaeologists have found traces of intermittent human presence high on the Tibetan Plateau from 20,000 years ago, including worked stone, handprints and footprints some 4,200 meters (or 2.6 miles) above sea level. In slightly more recent history, from 14,600 to 7,500 years ago, signs of encampments have been found higher than 3,000 meters (1.9 miles) above sea level, in the form of animal bones, stone artifacts and small hearths.
But these sites probably weren't permanent, the study authors wrote, equating them to "hunting camps, in most cases used for a single episode."
Those hunting humans, chasing game at higher and higher altitudes, were probably just passing through. Permanent settlements didn't come until much later, because high-altitude climates are so harsh — cruelly cold, low in oxygen, high in damaging ultraviolet radiation — that it's not easy to live in them, let alone grow crops year-round.
Over time, humans have indeed adapted to year-round living at high altitudes — but when? And how did they manage it?
To find out, a team of scientists from China, England and the United States surveyed 53 sites spanning 800 miles in the northeastern Tibetan Plateau, collecting artifacts, animal bones and a total of 63 charred (presumably cooked) cereal grains for analysis. Among the grains they found were foxtail millet and broomcorn millet (grains that are indigenous to the area), as well as barley and wheat (which was introduced from the Fertile Crescent, the "cradle of civilization" in what's now known as the Middle East). They also collected bones and teeth at 10 sites from animals commonly used as livestock, including sheep, cattle and pigs.
As time went on, the inhabitants moved upward. A group of sites dating from 5,200 to 3,600 years ago went as high as 2,527 meters above sea level; a group of sites from 3,600 to 2,300 years ago reached even higher, to 3,400 meters above sea level.
Early in this period, the vast majority of the recovered cereal grains from the sites — 98.1% — were millet, the scientists said. It makes sense that signs of millet reached only to roughly 2,500 meters, they added: Millet is sensitive to frost, and any higher altitude might have been too cold for the crops to survive.
But starting about 3,600 years ago, there was a growing shift to barley. The share of millet shrinks, and the share of barley grows over time until it accounts for nearly all the crop remains.
As the share of barley was growing, the settlements were going higher too. And that's probably no coincidence, the study authors wrote.
Barley is more frost-hardy than millet, and it also has a longer growing season (six months between sowing and harvest). To tend to these slow-growing crops, the inhabitants probably lived at these high altitudes permanently.
And it wasn't as if the frigid heights were getting warmer over time: The human settlers were able to move up even as continental temperatures were dropping, the authors pointed out.
"The key addition was barley," they wrote.
Other factors may also play into the success of humans living the high life, including genetics. But the study reveals how these imported grains from the west helped to transform this civilization.
"The importation of western cereals enabled human communities to adapt to the harsher conditions of higher altitudes in the Tibetan Plateau," the study authors wrote.