34,000-year-old indigo plant residues found in Georgia’s Dzudzuana Cave reveal that prehistoric humans processed plants for more than just food.
In a remarkable breakthrough, archaeologists have found the world’s earliest direct evidence of indigo plant processing at the Paleolithic site of Dzudzuana Cave in Georgia. Traces of Isatis tinctoria, also known as dyer’s woad, were detected on stone grinding tools dating back 32,000 to 34,000 years, proving that early Homo sapiens used plants not only for food but also for medicine and dye production.
The study, published in PLOS ONE and led by researchers from Ca’ Foscari University of Venice in collaboration with international partners, employed a cutting-edge, multi-analytical approach.
Scientists combined microscopy, Raman and FTIR spectroscopy, and synchrotron-based micro-CT scanning to identify tiny blue residues trapped within the pores of stone pebbles. These residues were confirmed to contain indigotin, the chromophore responsible for the iconic blue color of indigo dye.
Plants Beyond Food: A Forgotten Chapter of Prehistoric Life
For decades, narratives of the Paleolithic have focused primarily on stone tools and animal bones, since these materials survive longer in the archaeological record. This has often left plants in the shadows, creating what researchers call the “missing majority” of prehistory. Yet, plants were essential for survival and innovation, providing not only food but also fibers, medicine, poisons, and dyes.
The discovery at Dzudzuana offers a rare glimpse into this hidden world. Isatis tinctoria, while bitter and essentially inedible, was valued for its medicinal properties and as a source of indigo dye. Its leaves contain indoxyl glycosides, which release indigotin through oxidation—a process that may have been intentionally harnessed by early humans.
“Rather than viewing plants solely as food resources, we highlight their role in complex operations,” said Dr. Laura Longo of Ca’ Foscari University, lead author of the study. “Our findings demonstrate the technological and cultural sophistication of Homo sapiens 34,000 years ago.”
How the Evidence Was Found
Archaeologists examined six stone pebbles excavated from Dzudzuana Cave, five of which showed clear signs of use. These tools bore microscopic wear patterns consistent with grinding soft plant matter. When researchers analyzed the residues, they discovered blue-colored fragments, some fibrous, embedded deep within the stone surfaces.
To rule out modern contamination—such as indigo from denim—scientists ran extensive control tests, even analyzing cotton fibers from blue jeans. The results confirmed that the residues were not modern pollutants but authentic Paleolithic traces of Isatis tinctoria.
Replicative experiments further supported the findings. Researchers ground fresh leaves of indigo-bearing plants using similar river pebbles, reproducing both the microscopic residues and the chemical signatures seen in the ancient tools
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Why Did Paleolithic Humans Grind Indigo Plants?
The exact purpose of this activity remains open to interpretation. The leaves may have been processed to extract blue dye, to prepare medicinal remedies, or perhaps for both. Isatis tinctoria has long been recognized for its antimicrobial, antiseptic, and protective properties, and ethnographic records confirm its widespread historical use as both medicine and colorant.
What makes this discovery so striking is that it represents the earliest evidence for processing a non-food plant. Until now, Paleolithic plant use was thought to focus on edibles or mineral pigments like ochre. The identification of indigotin pushes back the timeline for plant-based dye use by tens of thousands of years and challenges long-held assumptions about prehistoric technology.
Broader Implications for Human History
Dzudzuana Cave, located in Georgia’s Caucasus region, has already yielded an impressive record of Upper Paleolithic artifacts, including bone needles, ornamental beads, and animal remains. The discovery of indigo plant processing adds a new dimension, showing that humans in this region engaged in sophisticated ecological knowledge and experimentation with plants.
“Plants were an inexhaustible resource,” the researchers conclude. “Their deliberate processing at Dzudzuana reveals the ingenuity of early Homo sapiens, who recognized the power of plants not only for survival but also for cultural expression.”
This finding forces scholars to reconsider long-standing narratives of prehistory. Far from being simple hunters and gatherers, early humans were innovators who explored the potential of their environment in ways strikingly similar to our own curiosity and creativity today.
Longo, L., Veronese, M., Cagnato, C., Sorrentino, G., Tetruashvili, A., Belfer-Cohen, A., Jakeli, N., Meshveliani, T., Meneghetti, M., Zoleo, A., Marcomini, A., Artioli, G., Badetti, E., & Hardy, K. (2025). Direct evidence for processing Isatis tinctoria L., a non-nutritional plant, 32–34,000 years ago. PLOS ONE, 20(5), e0321262. https://doi.org/10.1371/journal.pone.0321262
Cover Image Credit: Indigofera tinctoria Family: Fabaceae. Public Domain
