Ancient blue: 34,000-year-old traces of indigo found in Georgian cave

Archaeologists have discovered the first evidence of indigo dyeing, indicating that humans were grinding non-edible plants for complex purposes as early as 34,000 years ago. They discovered this at the Dzudzuana Cave in Georgia’s Caucasus Mountains, where scientists found microscopic evidence of indigotin, the compound responsible for indigo’s deep blue color, on stone grinding tools.

The findings, which appear in PLOS ONE, are the first evidence of indigotin found on Paleolithic artifacts and place the earliest known use of indigo dye at more than 30,000 years ago. The residues were found on unknapped ground stone pebbles that were discovered in the 2000s, and new analysis revealed they contained residues of plant material. Included among these were blue, occasionally fibrous, particles found in worn areas of the tools, as well as starch grains.

Indigotin is formed when compounds in Isatis tinctoria L.’s leaves—woad—react with atmospheric oxygen. Woad, a plant native to the Caucasus, was traditionally cultivated for medicine and its dye. More significant than all of this, though, is that the plant is toxic and not edible, which means Paleolithic humans deliberately cultivated and processed it for another purpose, such as coloring materials, body paint, or even therapeutic use.

The discovery sheds light on what archaeologists call the “missing majority” of prehistory—perishable plant resources that only rarely survive in the archaeological record. To date, narratives of the Paleolithic have rested heavily on chipped stone tools and animal bones, items that endure for millennia. This breakthrough shows that plants played a far more diversified role in early human life than previously assumed.

The study team, led by Ca’ Foscari University of Venice in collaboration with Georgian and international partners, used a range of advanced methods to uncover the dye. They analyzed the unusual blue residues through optical and confocal microscopy, and to confirm their chemical signature, they used Raman and FTIR spectroscopy. To determine why the residues had remained so well preserved for all these years, scientists analyzed the porosity of the stones using synchrotron light at Elettra Sincrotrone in Trieste. The microscopic pores of the stones were found to have trapped and protected the organic molecules for tens of millennia.

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To further support their findings, the team conducted experimental archaeology. Nikrisi River pebbles near the cave were used to replicate grinding activity. Woad was cultivated by researchers for over three summers in Verona, Italy, harvested, and processed with replica tools. The experiments successfully demonstrated that the plants left such blue traces, confirming that Paleolithic tools could have preserved such residues.

This finding overhauls knowledge of the relationship between early Homo sapiens and their environment. Rather than being mere hunters reliant only on animals and stone tools, such groups had information about the chemical properties of plants and harnessed them in complex ways. Using a non-food plant such as woad demonstrates cultural traditions, planning by seasons, and transmission of specialized knowledge—features that suggest high cognition and societal complexity.

The find also highlights the significance of Dzudzuana Cave as one of the key sites along the ancient migration routes of the Caucasus. Previous excavations had already revealed some of the earliest human occupations in the region, and thus it is a significant place to learn about the spread and innovation of modern humans.

Regardless of the use, the 34,000-year-old blue traces demonstrate that the quest for color, symbol, and healing was an established part of human life in prehistory.