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  • Researchers propose that ancient artists in southern China used a blood-based emulsion to bind red pigment to limestone cliffs, explaining the art's two-millennia survival.
  • Protein analysis detected markers often associated with pregnancy, leading some scientists to speculate about ritual connections to fertility and matriarchal society structures.
  • Independent experts caution that the small sample size and faint protein signals raise concerns about potential laboratory contamination or overinterpretation of the data.

Vivid red murals adorning the karst limestone cliffs of southern China have withstood nearly two thousand years of wind, rain, heat, and humidity. These images, which depict drums, bells, weapons, animals, and human figures, are part of the Zuojiang Huashan Rock Art Cultural Landscape. Located in the Zuojiang River Valley near the border with Vietnam, the artwork is spread across more than eighty sites spanning roughly one hundred sixty miles. The durability of these ruby-colored paintings has long puzzled scientists, but a new study suggests that blood may have been the key ingredient responsible for their preservation.

Archaeologists believe the Luoyue people created the art between the fifth century B.C.E. and the second century C.E. This society was characterized by matrilineality, tracing descent and property through the mother’s side, and operated as a women-led fisher-hunter-gatherer community. To understand how the images survived such harsh environmental conditions, researchers collected five fallen fragments from three different sites. They employed multiple analytical techniques, including infrared light interaction studies, high-powered microscopy, and protein makeup analysis, to determine the composition of the paint layers.

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The findings indicate a two-step creation process. First, artists gathered materials such as animal bones, bird droppings, limestone, or shells. These were heated to produce a mixture of calcium phosphate and quicklime. Water and blood were then added to create what researchers describe as a milky emulsion of mortar. This mixture served as a primer, painted directly onto the limestone cliffs. Once this base layer partially dried, artists applied a pigment solution made from water and local red clay rich in iron oxide.

The chemical reaction played a crucial role in preservation. As slaked lime reacted with carbon dioxide in the air, blood proteins helped form calcium carbonate, securing the pigment firmly to the rock. Lead author Meng Wu, a chemist at Shandong University, noted that the bond is so strong that when peeling occurs, it is often the limestone layer itself that detaches along with the pigments. In some cases, researchers had to use electric drills to separate the paint layers from the base rock, whereas natural sediments without paint were only weakly bonded.

Protein analysis revealed that approximately ninety-seven percent of the detected proteins in samples from one site came from humans, with the remainder originating from cows, antelopes, sheep, goats, or other herbivores. A sample from a different site contained roughly equal amounts of human and animal blood proteins. Notably, the human blood contained prolactin-inducible protein and lactotransferrin, two proteins found in high concentrations in the blood of pregnant women. The researchers estimate that only about eight milliliters of blood were needed for every square meter of rock painted.

The presence of these specific proteins has led to speculation about ritual significance. The rock art itself depicts pregnant women, sexual intercourse between men and women, and men with prominently featured genitalia. This visual evidence supports the hypothesis that the blood may have been collected during or after childbirth, reflecting the matriarchal society’s reverence for women and fertility. However, this interpretation remains a subject of debate among experts who were not involved in the study.

Matthew Collins, an archaeologist at the University of Cambridge, described the research as elegant but expressed skepticism regarding the certainty of the conclusions. He noted that the signals of human blood were faint and did not appear to be affected by millennia in a harsh environment, raising the possibility that the samples may have been contaminated in the laboratory. Collins suggested that while the authors might be correct, they could also be overinterpreting their data.

Jose Miguel Perez-Gomez, an archaeologist at Simon Bolivar University in Venezuela, offered similar cautions. He pointed out that prolactin-inducible protein and lactotransferrin can also be found in people who are not pregnant, meaning the blood may not have been related to childbirth. Additionally, he highlighted that the study analyzed only a small number of samples and did not recover ancient DNA, making it difficult to definitively determine whose blood was used or its exact purpose.

Despite these uncertainties, the study provides new insights into the technological sophistication of the Luoyue people. Earlier this year, researchers reported recovering ancient human and animal DNA from rock art for the first time, confirming that such biological materials can survive for at least two thousand years. Future studies may need to expand sample sizes and incorporate DNA analysis to clarify the origins of the blood and its role in the creation of these enduring cultural artifacts.

The Zuojiang Huashan Rock Art Cultural Landscape remains a significant historical resource, offering a window into the lives and beliefs of ancient societies in southern China. While the exact rituals and motivations behind the use of blood as a binder remain unclear, the chemical resilience of the paint underscores the ingenuity of its creators. As scientific techniques continue to evolve, further analysis may resolve current debates and provide a more comprehensive understanding of this remarkable archaeological site.

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  • Smithsonian Magazine↗This Rock Art in China Has Remained Bright Red for Nearly 2,000 Years. How? The Secret Ingredient Might Be Blood