https://phys.org/news/2025-09-blue-indigo-year-tools-paleolithic.htmlAn international research team coordinated by Ca' Foscari University of Venice has identified the presence of indigotin—a blue dye compound—on stone pebbles dating back to the Upper Paleolithic. This molecule, derived from the leaves of Isatis tinctoria L., a biennial plant in the Brassicaceae family native to the Caucasus and commonly known as woad, was found on ground stone tools.
This is the first time indigotin—a blue secondary compound, also known as indigo—has been identified on such ancient artifacts. The molecule forms through a reaction between atmospheric oxygen and the natural glycoside precursors in Isatis tinctoria L. leaves, released from the cellular vacuoles. This proves that the plant, despite not being edible, was intentionally processed as early as 34,000 years ago.