When humans made the move into higher altitudes around 5,000 years ago, for example up onto the Tibetan plateau, they had some help -- from a humble but frost-resistant source, namely barley.

While there's evidence of humans being present on the plateau beginning around 20,000 years ago, signs of them settling down and living there in permanent homes only go back to about 5,200 years, researchers say.

A study that analyzed charred seeds left by early full-time residents of the plateau who moved to heights of 8,200 feet and above suggests the key to living there was a crop new to them -- barley.

"The key to their movement is that crops from very different parts of Asia were coming together at that time," says Cambridge University archaeologist and study author Martin Jones. "There was a sort of reshuffling of old crops with alien ones. They added a new ingredient to their farming tool kit."

People had previously lived on millet, a crop that doesn't deal with frost very well.

That kept people tied to lower altitudes until the arrival of barley, still an important cereal crop in Tibet today, which probably originated in the fertile crescent of the Middle East and then slowly made its way eastward.

"It's taking a novel crop and using it in a different way -- exploring these high altitudes in a way that wasn't possible before," says Jones.

It was just one of many crops that were spreading among different peoples at the time, as wheat and barley moved east, African crops began to arrive in India, and rice farming spread almost universally.

"It's a global phenomenon of farmers taking on exotic crops," Jones explains. "It's basically an expansionist period where people were looking for new options in new, extreme environments."

Other experts not involved with the study agree with that conclusion.

"People spread almost everywhere as hunter-gatherers, but they were mostly few and far between, and mobile, especially in places like Tibet," says Dorian Fuller of University College London. "Agriculture allowed populations to settle into [new] environments and grow in numbers."

This expansion and spreading of crops was a result of the beginnings of the first Central Asian trade routes from 4500 to 4000 years ago, says Jones, routes that would become the celebrated Silk Road linking China and the West by 2,500 to 2,000 years ago.