Earliest archaeological evidence of Isatis tinctoria, a plant that produces blue indigo dye, found
Published: 29 September 2025
New research has found the earliest evidence of the use of blue dye in the archaeological record, dating back over 34,000 years.
New research has found the earliest evidence of the use of blue dye in the archaeological record, dating back over 34,000 years.
The study shows that prehistoric people at Dzudzuana Cave in Georgia were using stone pebbles to grind the leaves of Isatis tinctoria, also known as dyers woad, a plant known today for producing indigo dye.
Among the international team of scientists and archaeologists was Professor Karen Hardy from the University of Glasgow, senior author of the PLOS One paper and principal investigator of the UKRI-funded Powerful Plants project, one of the major funders of the research.
The team identified traces of indigotin, the compound responsible for the indigo colour, on ancient artefacts dating back to the Upper Palaeolithic.
Professor Hardy, Professor of Prehistoric Archaeology at the University of Glasgow, said: “Blue is not a colour that commonly appears in nature, so its presence at this very early date is unexpected and very exciting. It suggests that the Palaeolithic people had a much greater understanding of the natural resources in their environment, including plants, than has been attributed to them, up to now.
“We cannot know for certain why the people were grinding this plant up. What is sure, is that once it starts to be ground up, the blue colour, that is contained within the leaf, becomes clear, although a process is required to make it into a proper dye. It had been used since very ancient times as a medicine and perhaps gradually people started to recognise its value as a beautiful blue colour.”
The research initially focused on understanding the function of the stone tool and the aim of the investigation was to identify food residues. It later revealed traces of the processing of soft and moist material, compatible with plant materials such as leaves.
Further study unexpectedly revealed blue fibres. These residues were recovered from tiny holes in the surface of the the unmodified river pebble tools that showed visible wear. This evidence shows that the plant, despite not being edible, was intentionally processed as early as 34,000 years ago.
The discovery of the indigo compound offers new insights into the complexity of early human interaction with plant resources, not only as food but also for more sophisticated uses such as dyeing and medicine.
As well as Professor Hardy, the research team includes scientists and archaeologists led by Laura Long, a post doctoral researcher based at Ca’ Foscari University of Venice, along with the University of Padua, the European University of Tbilisi and the Georgian National Museum.
To rule out contamination from modern sources as indigo is today used to colour blue jeans, this fabric was sampled. Since cotton comes originally from tropical regions, stringent testing was carried out to identify the plant that the blue came from. Results excludedcotton and the team were able to confirm the integrity of their results.
At the same time, experimental work to replicate the grinding process was carried out in Italy. Isatis tinctoria plants were processed using river pebbles similar to those found at the site. These experiments helped confirm that the plant’s blue pigment could be released through mechanical action alone.
Full Paper
Read the full paper - Longo L, Veronese M, Cagnato C, Sorrentino G, Tetruashvili A, Belfer-Cohen A, Jakeli, N., Meshveliani, T., Meneghetti, M., Zoleo, A., Marcomini, A., Artioli, 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
Powerful Plants
The Powerful Plants project is based on the premise that complex use of plants, as food, medicine and raw materials was defining in social, cultural, behavioural and technological development in human evolution. Awarded by the European Research council (ERC) and is funded by the UKRI Horizon Europe Guarantee Fund. Learn more here: https://powerfulplants.gla.ac.uk/
First published: 29 September 2025