Human Blood Helped Preserve 2,000-Year-Old Rock Art in Southern China, Study Finds

An extensive gallery of ancient rock paintings in southern China, known as the Zuojiang Huashan Rock Art Cultural Landscape, has intrigued archaeologists for surviving centuries of tropical heat and monsoon rain. A new study led by researchers at Shandong University, published in the Journal of Archaeological Science, has identified components of human blood as a binding agent in the red pigments, which may explain why the paintings still look fresh after generations of exposure. The UNESCO-protected site, overlooking the Zuo River, contains thousands of bright red human and animal figures on limestone cliffs.

Archaeologists believe the paintings are the work of the Lac Viet, or Luoyue, a matrilineal culture that lived near today's China-Vietnam border until roughly 100 CE; radiocarbon dating suggests some of the earliest paintings could be nearly 2,400 years old. To identify what kept the pigment bound to the crumbling limestone through centuries of weathering, researchers collected samples from fallen rock fragments at three sites and analyzed them using techniques that excite chemical components to reveal identifying signatures, plus a protein-fragment analysis that built a library of organic materials in the paintings.

The results showed the bright red pigment came from a clay rich in haematite, a form of iron oxide. To bind it to the rock, the artists used a mortar made from a mix of slaked lime, or calcium hydroxide, and blood. The amount of blood required was fairly small, the study found, and was probably mixed with blood from at least two species of herbivorous animals. Proteins from milk and eggs have been used as paint binders elsewhere in history, and human blood has previously been identified in Paleolithic rock art in Australia's Northern Territory dated to around 20,000 years ago.

The researchers suggest the blood may have been collected during childbirth, a fitting symbol of fertility in a society led by women. The findings do not explain why the artists climbed to such heights to create the paintings, or what the images mean.