Deep in a Georgian cave, archaeologists have uncovered a 34,000-year-old mystery that's rewriting our understanding of early human behavior. Stone tools from Dzudzuana Cave contain traces of indigotin, a brilliant blue compound that forms the basis of indigo dye. According to the study, this is the oldest known evidence of humans deliberately processing plants for non-food purposes.
But this has left the archaeologists with an even larger question; what were these ancient humans doing grinding up bitter, inedible leaves to create this blue dye?
Laura Longo, an archaeologist at Ca' Foscari University of Venice, was studying six stone pebbles excavated from Dzudzuana Cave in Georgia's Caucasus foothills. The tools, dating back 32,000 to 34,000 years, were river stones that showed clear signs of use. They had polished surfaces, scratches, and wear patterns indicating they had been used for pounding and grinding.
According to the study, published in PLOS One, Longo's team used advanced microscopy techniques and discovered something unexpected: blue residues trapped deep within the stone's porous surface.
"Rather than viewing plants solely as food resources, as is often the case," Longo explained in a press release, "this study highlights their role in complex operations, likely involving the transformation of perishable materials for use in different phases of daily life."
The mysterious blue compound, identified through sophisticated chemical analysis, is indigotin, the same substance that gives blue jeans their color today. Today, we use synthetic indigo, but this ancient version came from Isatis tinctoria, commonly known as woad, a plant native to the region that belongs to the mustard family.
But here's where it gets interesting.
Indigotin doesn't naturally exist in the plant. On the surface, woad leaves are green. They contain chemical precursors that only transform into the blue compound when the plant is crushed in an alkaline rich environment and exposed to oxygen.
This process requires deliberate mechanical action, and the mere presence of indigotin proves these early humans were actively processing it.
To verify their findings, the research team conducted extensive contamination tests and even replicated the ancient process using similar stone tools and fresh woad plants. The results were clear: the blue residues were authentic prehistoric artifacts, not modern contamination.
So what were these ancient humans using it for? The plant's bitter taste makes it virtually inedible, ruling out nutrition as a motive. This leaves two primary possibilities, both fascinating in their implications.
Isatis tinctoria has well-documented medicinal properties, containing compounds with anti-inflammatory, antimicrobial, and antiviral effects. Traditional medicine has long used various parts of the plant to treat wounds, infections, and other ailments. Even today, animals in the wild are known to self-medicate using plants, so it's entirely plausible that early humans had discovered and utilized these healing properties.
Perhaps more interestingly, blue is rare in nature, and the ability to create a vibrant blue colorant would have been remarkable. While we know Upper Paleolithic humans used mineral pigments for cave paintings and body decoration, this would represent the first evidence of extracting color from plants which is a much more complex process requiring sophisticated knowledge of plant chemistry.
The truth might be both. Many traditional cultures use the same plants for multiple purposes, and early humans may have discovered that their "medicine" also produced a striking blue color suitable for decorating objects, skin, or textiles.
"Each time a new piece of secure information emerges, it provides an extraordinary insight into our prehistoric human past," the research team notes in their study. The sophisticated plant processing evidenced at Dzudzuana suggests these early humans possessed detailed ecological knowledge.
The research team utilized some pretty advanced spectroscopy techniques to identify the molecular signature of indigotin in microscopic plant fragments, as well as synchrotron radiation, which is a powerful form of X-ray imaging, to understand how the stone's porous structure trapped and preserved these ancient residues.
This method could be used in other instances to study our ancient past. As Longo explained, this technology and their research "opens new perspectives on the technological and cultural sophistication of Upper Paleolithic populations, who skillfully exploited the inexhaustible resource of plants."