Researchers from CA ‘Foscari University have discovered Indigotin, a blue secondary compound also known as indigo, on a non-nanoparticle ground stone tool unearthed from the Zuziana Cave at the foot of the Caucasus in Georgia. Indigotin forms through reactions with atmospheric oxygen and natural glycoside precursors from leaves of the Isatis Tinctoria. This indicates that the plant was deliberately processed as far back as 34,000 years ago, despite its non-educational applications.
Five pebbles from Zugiana Cave, Georgia. Image credit: Longo et al., doi: 10.1371/journal.pone.0321262.
Modern humans first appeared in archaeological records approximately 300,000 years ago in Africa.
Much of the evidence regarding their cognitive and technical skills is derived from the limited existence of stone artifacts and the remains of animals, as these endure far longer in the archaeological record compared to plant materials.
As a result, Paleolithic narratives predominantly emphasize animal hunting and the production of stone tools.
The so-called “majority,” particularly plants that lack significant mentions, especially those increasingly shown to be utilized for food, cordage, weaving, and medicine, create incomplete accounts.
This underscores the need to identify and illustrate the roles of plants in an array of activities, many of which may still be undiscovered.
“It’s not solely about viewing plants as food resources; that’s common. We emphasize their involvement in complex processes. It possibly involves transforming perishable materials for use at various stages of Homo sapiens 34,000 years ago.”
“While the identification of elusive plant residues not typically highlighted in traditional research continually improves, our multi-analytical approach unveils fresh perspectives on the technical and cultural sophistication of Upper Paleolithic tools, designed to skillfully exploit the vast resources of plants and fully acknowledge their significance.”
In their investigation, researchers examined stone tools dating back 34,000 years, discovered in Zugiana Cave, Georgia.
They identified signs of mechanical processing of soft, wet materials consistent with plant matter such as leaves.
Employing an array of microscopic techniques (both optical and confocal), blue residues (occasionally fibrous) alongside starch grains were unexpectedly revealed.
These residues were particularly prevalent in the areas of the tools that displayed significant wear.
To ascertain the blue residues’ nature, scientists used advanced microscopy techniques, specifically Raman and FTIR spectroscopy.
These analyses confirmed the existence of indigotin chromophores in several samples.
“Once the molecules responsible for the blue coloration were identified, new questions arose. How and why were these residues associated with the tool’s working surface?” the author remarked.
They subsequently investigated the porosity of the stone, a crucial factor for trapping and retaining biological residues.
Both microscopic fragments of the archaeological tool and larger samples of experimental replicas underwent analysis using Micro-CT tomography.
This analysis confirmed the presence of pores capable of holding trace biological remnants.
Consequently, the team devised a series of replication experiments.
Initially, raw rock materials resembling those used by prehistoric inhabitants of Zuziana were sourced.
The pebbles were collected by Nino Jakeli from the Nikrisi River, located just beneath the cave.
Following this, controlled experiments were conducted, mechanically treating a range of plants, including fiber sources (such as bast fiber) and potentially indigotin-yielding species.
“We employed a rigorous approach to pollution control and biomolecular analysis to provide insights into contemporary human behaviors and the technical and ecological knowledge likely prioritized during the Upper Paleolithic era,” the researchers stated.
“Whether this plant remains serve medicinal purposes or have other uses presents a new perspective on the intriguing potential for diverse plant utilization.”
Survey results have been published online in the journal PLOS 1.
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L. Longo et al. 2025. Direct evidence of treatment of Isatis Tinctoria L., non-nutritional factories, 32-34,000 years ago. PLOS 1 20(5): E0321262; doi: 10.1371/journal.pone.0321262
Source: www.sci.news
