Uncovering the Brick from Kalhu: Unlocking the Secrets of Ancient DNA trapped in Clay

Friday - August 25 2023, 20:28 UTC - 1 year ago

The National Museum of Denmark is currently home to a clay brick from the palace of the Neo-Assyrian king Ashurnasirpal II. An interdisciplinary team was able to extract DNA from the brick and identify 34 distinct taxonomic groups of plants. The research serves as a proof of concept and method that can be applied to other archaeological sources of clay around the world, to identify past flora and fauna.

The National Museum of Denmark is currently home to a clay brick from the palace of the Neo-Assyrian king Ashurnasirpal II, situated in the ancient city of Kalhu. This palace is now recognized as the North-West Palace in Nimrud, located in what is today northern Iraq. Its foundations were laid around 879 BCE. The brick features a cuneiform inscription in Akkadian, a now-extinct Semitic language, which reads: "The property of the palace of Ashurnasirpal, king of Assyria." This inscription allows us to accurately date the brick to between 879 BCE and 869 BCE.

During a digitalization project at the Museum in 2020, the group of researchers was able to obtain samples from the inner core of the brick – meaning that there was a low risk of DNA contamination since the brick was created. The team extracted DNA from the samples by adapting a protocol previously used for other porous materials, such as bone.

After the extracted DNA had been sequenced, the researchers identified 34 distinct taxonomic groups of plants. The plant families with the most abundant sequences were Brassicaceae (cabbage) and Ericaceae (heather). Other represented families were Betulaceae (birch), Lauraceae (laurels), Selineae (umbellifiers), and Triticeae (cultivated grasses).

With the interdisciplinary team comprising assyriologists, archaeologists, biologists, and geneticists, they were able to compare their findings with modern-day botanical records from Iraq as well as ancient Assyrian plant descriptions.

The brick would have been made primarily of mud collected near the local Tigris River, mixed with material such as chaff or straw, or animal dung. It would have been shaped in a mold before being inscribed with cuneiform script, then left in the sun to dry. The fact that the brick was never burned, but left to dry naturally, would have helped to preserve the genetic material trapped within the clay.

Dr Sophie Lund Rasmussen (Wildlife Conservation Research Unit, Department of Biology, University of Oxford), joint first author of the paper, said: "We were absolutely thrilled to discover that ancient DNA, effectively protected from contamination inside a mass of clay, can successfully be extracted from a 2,900-year-old clay brick. This research project is a perfect example of the importance of interdisciplinary collaboration in science, as the diverse expertise included in this study provided a holistic approach to the investigation of this material and the results it yielded." .

In addition to the fascinating insight this individual brick revealed, the research serves as a proof of concept and method that could be applied to many other archaeological sources of clay from different places and time periods around the world, to identify past flora and fauna. Clay materials are nearly always present in any archaeological site around the world, and their context means they can often be dated with high precision.

This study only described the plant DNA extracted, as these were the most prevalent and best-preserved specimens. However, depending on the sample, all taxa could potentially be identified, including vertebrates and invertebrates.