The rapid growth of the Silk Road helped bring to China a group of men whose roots are the subject of debate

The young man - or per-haps an old adolescent - is visibly tired: sitting on a stool with head buried in his lap and his forehead rested on one arm, he confronts the viewer with a full crop of exuberant curls. Invisible to us are his weary eyes.

But his story, which can be fully told only by drawing on the imagination, still intrigues, even after 1,500 years.

During his lifetime he was known as a Kunlun slave.

"Kunlun in this context means black," says Ge Chengyong, one of China's leading historians.