"Motifs and patterns found in different continents share a kind of surprising similarity in terms of subjects and the way they are expressed. From this point of view, rock art can be seen as a universal language of the Stone Age," Zhang says.

For instance, images to symbolize shamans found in the area look like abstract lines of frogs. Similar patterns were also discovered in a number of other countries.

"It was common in prehistoric times to regard frogs as divine animals due to their fertility and their ability to live both on land and in water," Zhang explains.

The highlight of the rock art in the Helan Mountains is undoubtedly the human faces carved on rocks, a feature that is rare in other places. There are about 700 carvings of faces. The iconic petroglyph is the sun god. Its positioning was significant. Carved on a steep rock, it gazes down from about 20 meters high, signaling its superpower.

The eyes of the sun god are represented by two concentric circles, with rays emanating from its head.