Chinese scientists have found through genetic studies that modern humans had successfully colonized the Qinghai-Tibet Plateau in the Late Paleolithic Age, at least 21,000 years ago.

The plateau, with an average altitude above 4,000 meters and known as "the Roof of the World" in southwestern China, is one of the most challenging areas in the world for human settlement due to its environmental extremes, such as extreme cold and low oxygen levels.

"Through Paleolithic era stone tools excavated from the plateau years ago, archaeologists believed human beings possibly inhabited the plateau 30,000 years ago," said Zhao Mian, a researcher from the Kunming Institute of Zoology with the Chinese Academy of Sciences.

But, with the drastic drop of temperature on the Earth in the Last Glacial Maximum of the Late Paleolithic Age, about 23,000 years ago, many species could not adapt to the changes and died out, she said.

"Scientists have been debating heatedly whether modern humans on the plateau had survived the adverse conditions," she said.

From the perspective of genetic continuity studies, geneticists had also attempted to find out when modern humans settled on the plateau by determining the age of the ancient components found in the genes of modern-day Tibetans.

"But due to few DNA samples of Tibetans, especially from Tibet, and lack of distinguish ability in previous studies, geneticists found it hard to judge whether Tibetans have ancient components in their genes," she said.

Led by her tutor Zhang Yaping, director of the the Kunming Institute of Zoology, Zhao and 14 other geneticists, including a German scientist, set up a research group three years ago.

They collected 680 genome samples of genetic structure from Tibetans in several major Tibetan-populated areas, including Tibet,Qinghai, Gansu, Sichuan and Yunnan. Of those, 388 were sampled from Tibet.

"Based on studies of their mitochondrial DNA genome variation, our results confirm the vast majority of Tibetan matrilineal components can trace their ancestry to Epipaleolithic and Neolithic immigrants from what is now northern China, or about 10,000 years ago, which accords with previous studies," Zhao said.

In genetic studies, mitochondrial DNA is a tool for tracking ancestry through females and has been used in this role to track the ancestry of many species back hundreds of generations.

"Another significant finding was that researchers identified an infrequent novel haplo group, M16," she said.

In human genetics, haplo groups can be used to define human beings' ancestral groups and genetic populations.

"Unlike Tibetan matrilineal components inherited from north China immigrants, M16 branched off directly from the genetic components of the ancestors of modern Eurasians," Zhao said.

"M16 has an ancient age of at least 21,000 years, based on calculations through various dating methods in genetics," she said.

"Its nearly exclusive distribution in Tibetan populations and ancient age suggest that M16 may represent the genetic relics of the Late Paleolithic inhabitants on the plateau," she said.

"We believe the research results give a relatively clear answer to the debate on when modern humans settled down on the Qinghai-Tibet Plateau successfully," she said.

Archaeologists have discovered Paleolithic human hand and footprints near Lhasa, the heart of the plateau, and reckoned they dated back about 20,600 years to 21,700 years, she said.

"The age of the relics is close to that of M16, so we believe that supports our research results to some extent," she added.

Zhao and her group's article about the research will be published on a leading science periodical, the Proceedings of the National Academy of Sciences (PNAS) of the United States of America.

PNAS articles are usually published on-line before print, and Zhao's article, "Mitochondrial genome evidence reveals successful Late Paleolithic settlement on the Tibetan Plateau", can be found in the on-line edition of PNAS.

(Xinhua News Agency December 14, 2009)