Human beings first ascended “the roof of the world” – the Tibetan highlands – over 3 1/2 millennia ago by relying on hardier crops and livestock to survive 11,000 feet above sea level, a new study says.

The research, published in the US journal Science, shows the human ascent all came down to one tiny thing: barley seeds.

While millet has been cultivated in East Asia for over 10,000 years, its tolerance to cold and frost made it more suitable for lower elevations. Barley, however, was far more adept at braving the elements, making it an ideal plant for the Tibetan plateau.

“Barley agriculture could provide people [with] sustained food supplies even during winter,” Science cites the three lead authors as saying in a joint e-mail. “Barley and wheat were first domesticated in [the Fertile Crescent] in West Asia around 10,500 years ago, where the environment is quite different from that in the Tibetan Plateau.” The fact that they thrived in the new, more extreme environment was “a lucky accident.”

It remains unknown just when barley migrated from the Fertile Crescent to East Asia.

Along with barley, the scientists also found examples of cold-tolerant wheat being sown around 3,600 years ago. Sheep were also being domesticated in the area around that time.

While those living at lower elevations on the plateau merely augmented their millet-reliant diets with millet, it appears that those living at higher elevations almost did away with millet, cultivating the more robust barley in its place.

The tri-national team consisting of American, British and Chinese researchers came to their results after studying animal teeth, bones and plant deposits gathered from 53 sites over the past 4 decades. Among the samples were 63 charred grains whose age could be verified via radiocarbon dating.

"Year-round survival at these altitudes must have led to some very challenging conditions indeed," lead researcher Martin Jones, from Cambridge's Department of Archaeology, told AFP.

"This poses further, interesting questions for researchers about the adaptation of humans, livestock and crops to life at such dizzying heights."

Of course, the Tibetan nomadic herders are not the first humans to have settled extreme elevations and battled the elements along the way.

According to a study published online in Science last month, Paleoindians first braved such treacherous altitudes 12,800 years ago in the Peruvian Andes – a thousand years earlier than previously believed. The Paleoindians were also living a vertigo-inducing 14,763 feet above sea level, exposing them to even harsher temperatures and doses of solar radiation.

But while the first semi-permanent settlers are believed to have arrived on the Tibetan plateau just over five millennia ago, some researchers believe older traces of human life in the area may represent a more permanent human presence than currently believed.

“I think that the 3,600-year-ago pulse [of human migration and settlement] is probably one of the very late migrations of people or ideas onto the plateau,” Mark Aldenderfer, an archaeologist at the University of California, told Science.

According to Aldenderfer, genetic studies may in fact indicate that biological changes to counter against hypoxia (lack of oxygen) may have first appeared in Tibetans at least 10,000 years ago.

Stone tools dating 15,000 years back were also found in Tibet up to 11,500 feet above sea level. It remains unclear, however, if people lived there on a permanent basis, or were merely camping out.

Meanwhile, the very first human presence was first detected on the Tibetan plateau some 21,000 years back.