Researchers have uncovered the earliest known use of blue indigo dye by humans, pushing back the timeline of plant-based dyeing practices to 34,000 years ago. Traces of the dye, chemically identified as indigotin, were found on stone tools unearthed in Dzudzuana Cave in the Caucasus region of the country of Georgia.
The discovery marks the first time such a compound has been detected on artifacts this ancient, providing new insight into the complex ways early humans interacted with plants—far beyond simple food use.
The blue indigo dye originated from Isatis tinctoria L., commonly known as woad, a non-edible plant native to the Caucasus. Researchers determined that the dye forms when glycosides in the plant’s leaves react with oxygen.
The presence of indigotin on the working surfaces of the tools suggests deliberate processing, pointing to early knowledge of how to extract and apply plant-based compounds.
First detection of indigotin on Paleolithic tools
Published in the journal PLOS One, the study highlights the technological and cultural sophistication of Upper Paleolithic populations. According to archaeologist Laura Longo of Ca’ Foscari University of Venice, the findings reveal that plants were used in complex daily activities, likely including the treatment of perishable materials.
She noted that such evidence challenges the traditional view of prehistoric humans relying solely on plants for nutrition.
The discovery was made through microscopic analysis of unshaped, ground stone tools excavated from a layer dated to roughly 34,000 years ago.
An international team, including researchers from Harvard University, the Georgian National Museum, and the Hebrew University of Jerusalem, directed the excavation.
Advanced testing confirms ancient use of dye
Sampling and preliminary analysis were conducted at the Georgian National Museum in Tbilisi, led by Longo and scientist Elena Badetti.
The team used a range of advanced techniques, including optical and confocal microscopy, Raman spectroscopy, and FTIR analysis, to confirm the presence of the indigotin molecule.
Most of the blue residues, which were sometimes fibrous and accompanied by starch grains, appeared in areas of the tools that displayed heavy wear.
To understand how the residues were preserved, researchers studied the stones’ porosity using micro-CT scans at Elettra Sincrotrone Trieste. They confirmed the stones contained pores capable of trapping microscopic materials.
Controlled experiments followed, using river pebbles similar to the original tools and processing various plants, including woad. These tests, conducted in Verona over three summers during the harvest season, helped build a reference collection for identifying use-wear patterns and plant residues.
The study offers a deeper insight into how early humans utilized natural resources and introduces a new chapter in the history of blue indigo dye in human culture.
