International research team uncovers oldest evidence of indigo dye processing on paleolithic stone tools from a Georgian cave, revolutionizing understanding of early human plant use.
A latest discovery has pushed back the earliest known use of indigo blue by over 30,000 years. An international research team coordinated by Ca' Foscari University of Venice has identified traces of indigotin (the chemical compound responsible for indigo's distinctive blue color) on stone grinding tools dating back 34,000 years from Dzudzuana Cave in Georgia's Caucasus Mountains.
This remarkable find, published in the scientific journal PLOS One, represents the first time the indigotin molecule has been detected on such ancient artifacts. The discovery fundamentally challenges our understanding of early Homo sapiens' relationship with plant resources, demonstrating sophisticated botanical knowledge and processing techniques far earlier than previously imagined.
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Indigo powder. (Swapan/Adobe Stock)
Revolutionary Plant Processing in the Upper Paleolithic
The indigotin traces were discovered on stone pebbles used as grinding tools, excavated from Upper Paleolithic layers at Dzudzuana Cave. The compound is formed through a complex biochemical reaction between atmospheric oxygen and natural glycosidic precursors released from cell vacuoles in the leaves of Isatis tinctoria L., commonly known as woad, a biennial plant of the Brassicaceae family native to the Caucasus region.
"Instead of considering plants only as a food resource, as we often tend to do," explains Laura Longo, archaeologist at Ca' Foscari University of Venice, "this study highlights their role within complex operational chains, probably linked to the transformation of perishable materials for use in various phases of the daily life of Homo sapiens 34,000 years ago."
Isatis tinctoria (woad) plant showing characteristic leaves used for indigo dye production. (Alupus/CC BY-SA 3.0)
The sophisticated nature of this discovery cannot be overstated. Woad is not edible, meaning these ancient humans deliberately sought out and processed this plant specifically for its blue dye properties. This required intricate knowledge of plant chemistry, seasonal harvesting timing, and complex processing techniques, suggesting a level of botanical sophistication previously unrecognized in Upper Paleolithic populations.
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Advanced Analytical Techniques Reveal Ancient Secrets
The discovery required cutting-edge analytical methods to identify microscopic traces preserved in stone pores for millennia. The research team, led by Laura Longo and Elena Badetti from Ca' Foscari University, employed multiple sophisticated techniques including Raman spectroscopy, Infrared (FTIR) analysis, and synchrotron light analysis at Elettra Sincrotrone in Trieste.
The stone tools were initially excavated in the 2000s as part of an international project coordinated by Ofer Bar-Yosef from Harvard University, Tengiz Meshveliani and Nino Jakeli from the National Museum of Georgia, and Anna Belfer-Cohen from Hebrew University of Jerusalem. However, it was only through recent advanced microspectroscopic analysis that the blue residues were properly identified.
Microscopic examination revealed blue residues, sometimes in fibrous form, concentrated in areas showing obvious signs of wear on the grinding tools. These residues were found alongside starch granules, indicating the tools were used to process various plant materials. The team's analysis confirmed the presence of the indigotin chromophore in numerous archaeological samples.
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Three of the six stone pebbles retrieved in Unit D, showing the position of the analyzed molds indicated by blue dashed lines. (Longo et al./PLOS One)
Experimental Archaeology Confirms Ancient Techniques
To validate their findings, the research team conducted extensive experimental archaeology. They collected pebbles from the Nikrisi River, which flows directly beneath Dzudzuana Cave, to replicate the original lithic materials. Over three summers, they grew Isatis tinctoria plants at Corte Badin in Marano di Valpolicella, Verona, and processed them using replicated stone tools.
These controlled experiments were crucial for building a comparison collection that allowed researchers to recognize both the traces of use and, most importantly, the plant residues. The meticulous approach demonstrated that the blue residues could indeed be preserved in stone pores over tens of thousands of years.
The team also analyzed the porosity of the stones using synchrotron light, confirming the presence of pores with volumes suitable for retaining micrometric remains. This analysis was critical for understanding how organic materials could survive for such extended periods in the archaeological record.
Implications for Understanding Early Human Cognition
This discovery opens fascinating questions about the intended use of the indigo dye. While the exact purpose remains unclear, researchers suggest several possibilities including medicinal applications, textile dyeing, body painting, or symbolic/ritual uses.
Isatis tinctoria has been historically documented for both dyeing and medicinal purposes throughout human history. The plant contains compounds with anti-inflammatory and antimicrobial properties, suggesting possible therapeutic applications. Alternatively, the blue dye could have been used for decorating perishable materials like clothing, basketry, or wooden objects that haven't survived in the archaeological record.
"Our multi-analytical approach opens up new perspectives for understanding the technological and cultural sophistication of Upper Paleolithic populations, who knew how to skillfully exploit an inexhaustible resource, that of plants, aware of their power," explains Longo.
The discovery fundamentally challenges traditional views of Paleolithic subsistence strategies, demonstrating that early humans possessed sophisticated botanical knowledge extending far beyond basic food procurement. This suggests complex cognitive abilities including seasonal planning, chemical understanding, and cultural transmission of specialized knowledge.
Revolutionizing Paleolithic Studies
This groundbreaking research represents a paradigm shift in how archaeologists approach the study of plant use in prehistory. The identification of non-food plant processing at such an early date suggests that the cognitive and cultural foundations for complex human-plant relationships were established much earlier than previously thought.
The study also highlights the importance of advanced analytical techniques in archaeology. Without sophisticated microspectroscopic methods, these crucial traces would have remained undetected, leaving this significant chapter of human technological development unknown.
The collaborative nature of the research, involving institutions across Italy, Georgia, and Scotland, demonstrates the power of international cooperation in unlocking ancient mysteries. The project benefited from funding by the US-based Leakey Foundation and the Italian Archaeology Unit, emphasizing the global significance of this discovery.
This find from Dzudzuana Cave joins other remarkable discoveries from the site, which has yielded some of the earliest evidence of human occupation in the Caucasus region. The cave's strategic location on ancient migration routes makes it a crucial site for understanding early human dispersal and technological development.
As researchers continue to analyze archaeological materials using ever more sophisticated techniques, discoveries like this remind us that our ancestors possessed remarkable knowledge and capabilities. The 34,000-year-old indigo traces from Georgia's mountains reveal yet another facet of early human ingenuity, proving that the quest for color, beauty, and possibly healing has ancient roots in our species' history.
Top image: Left; Archaeological elongated blue micro-fragments extracted from the innermost second mold. Right; Laura Longo at the Dzudzzuana Cave entrance, Georgia. Source: Longo et al./PLOS One
By Gary Manners
References
Costa, E. 2025. Blue indigo on 34,000-year-old tools found at Paleolithic site. Available at: https://phys.org/news/2025-09-blue-indigo-year-tools-paleolithic.html
Longo, L. et al. 2025. Indigotin traces on stone tools from Upper Paleolithic Dzudzuana Cave. PLOS One. Available at: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0321262
