The Tibetan Plateau, the harsh Asian domain known as the 'roof of the world,' would not seem an ideal place for people to call home thanks to its extreme altitude, frigid temperatures, relentless winds and low-oxygen conditions.

When people did succeed in colonizing this remote land, it was only after they discovered how to feed themselves year-round with cold-hardy crops like barley brought to the region from far away, scientists said on Thursday.

They described 53 archeological sites in Qinghai province in northwestern  where they found remnants of rustic structures, hearths, pottery, animal bones, cereal  and other evidence of human habitation from 5,600 feet to 11,000 feet above sea level (1,700 to 3,400 meters).

They found signs of periodic human presence dating to the Ice Age at least 20,000 years ago. There were settlements at lower altitudes by 5,200 years ago, mainly in the valleys of the upper Yellow River, with inhabitants relying on millet, a frost-sensitive crop unsuited for the higher altitudes.

Permanent settlements with agriculture and  were established about 3,600 years ago at high altitudes - above 9,800 feet (3,000 meters) - after barley was introduced to the region.

"They could quite plausibly be the earliest sustained settlements in the world at this altitude," said University of Cambridge archaeologist Martin Jones, one of the researchers.

Unlike millet, barley flourishes even in the conditions of the Tibetan Plateau's high altitudes.

"As barley is frost hardy and cold tolerant, it grows very well on the Tibetan Plateau even today. Therefore, barley agriculture could provide people enough - and sustained - food supplies even during wintertime," added archaeologist Dongju Zhang of Lanzhou University, another of the researchers.

Barley was the main high-elevation crop, with wheat grown as well. Neither was native crop to the region. They were domesticated in the so-called Fertile Crescent of the ancient Near East thousands of years earlier and were introduced to this area about 4,000 years ago, the researchers said.

 also were important in sustaining the settlements. The researchers said domesticated sheep arrived at about the same time as barley and wheat.

Jones said these people not only conquered the extreme altitude raising livestock and growing crops, but their expansion into the higher, colder heights occurred as the temperatures on continent were becoming colder.

The research was published in the journal Science