How did prehistoric humans express their feelings? The answer can be found in more than 20,000 rock paintings at Helan Mountain in northwest China.
One of the rocks in Ningxia Hui Autonomous Region is carved with two goats beside an enclosure by a road.
"The rock painting sends us a message that our ancestors started phasing out the nomadic lifestyle and settled down to keep animals," said Wu Lige, a guide at Helan Mountain rock painting scenic spot, which boasts around 6,000 rock paintings.
On a mountain wall 20 meters above the ground, you can see the cliff painting known as "God of the Sun."
With a round face, radial lines surrounding the head, with piercing eyes, the "God" looks mighty and penetrating.
"In ancient times, people owed their bumper harvests to the gods. The importance of the sun outweighed all the others. Thus, they worshiped the sun and personified it as the way it looks on the rock," said Li Xiangshi, the honorary president of the Chinese rock art society.
Rock carvings appeared before the invention of writing systems. Helan Mountain has more than 20,000 cliff paintings carved by the nomads that once lived in northern China.
The paintings depict herding, hunting, sacrificial rites and the life of the people who lived 3,000 to 10,000 years ago. Images of animals dominate the paintings, followed by humans, vehicles, plants, planets, fingerprints and abstract signs.
One of abstract paintings shows two hand prints. Below the smaller one on the left, a goat faces the larger hand print on the right and lowers its head. A peach-shaped portrait with big round eyes appears above the right hand print. It is the image of a god.
Researchers believed that the painting is actually a contract. The hand prints represent two tribes. The right tribe conquers the left one, therefore the cattle and sheep belong to the tribe on the right. In the presence of god, the contract lasts forever.
"Rock paintings are encyclopaedia carved in the hills, sowing seeds for the development of natural and social sciences," Li said.
Chinese rock paintings are found in the northwest Altai and Helan mountains, and in other provincial regions such as Guangxi, Sichuan, Yunnan, Guizhou and Fujian.
Although many paintings have already been put under state or local protection, it is still difficult to keep these ancient relics of human civilization intact.
They can be eroded by wind and rain, faded by sunlight, or damaged by spread of plants and human activity, or even tinier invaders: bacteria, fungi and algae.
"The major challenge facing rock paintings in Helan Mountain is wind erosion that may abrade or crack rocks," said Zhang Jianguo, deputy director with Helan Mountain cliff painting administration.
So the administration has strengthened the rock surface, sent guards for 24-hour patrol, installed video cameras for monitoring and set up information database for paintings on the stones.
Last year, a rainstorm-triggered flood washed many individual rocks on which paintings were carved away. "Thanks to our protection efforts, the rocks were all retrieved after the rain because they had been registered in the database," Zhang said.
He added that another challenge for rock painting protection was lack of talent and high technology. Thus, several cooperation projects on rock painting protection were launched during the international rock art forum in Yinchuan, Ningxia's capital, this week.
Paintings on the stones are cultural codes left by prehistoric people to their modern descendants.
"But if we want to decode and preserve them, the first thing is to have an audience who really know their value," said Zhang Yasha, professor with School of Ethnology and Sociology, Minzu University of China.