Near the river Tigris, outside the ancient city of Kalhu, known today as Nimrud, northern Iraq, a brickmaker once prepared a clay brick for the construction of a new palace dedicated to his king Ashurnasirpal II (approximately 883-859 BCE). Little did he know, that almost 2,900 years later, this clay brick would serve as a unique time capsule revealing details of the flora from this area and time, through the modern-day investigation of the ancient DNA hidden and preserved for thousands of years. In a new study, researchers from the University of Copenhagen, the University of Oxford and Aalborg University were able to extract ancient DNA from a recently exposed fracture surface of the 2,900-year-old clay brick and detect 34 unique taxonomic groups of plants.
The 2,900-year-old clay brick from the National Museum of Denmark. Image credit: Arbøll et al., doi: 10.1038/s41598-023-38191-w.
During a digitalization project at the National Museum of Denmark in 2020, Dr. Sophie Lund Rasmussen and her colleagues obtained samples from the inner core of the 2,900-year-old clay brick — meaning that there was a low risk of DNA contamination since the brick was created.
The researchers 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, they 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 mould 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.
“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,” Dr. Rasmussen said.
“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.”
“Because of the inscription on the brick, we can allocate the clay to a relatively specific period of time in a particular region, which means the brick serves as a biodiversity time-capsule of information regarding a single site and its surroundings,” said Dr. Troels Arbøll, a researcher at the University of Oxford.
“In this case, it provides researchers with a unique access to the ancient Assyrians.”
The results appear today in the journal Scientific Reports.
_____
T.P. Arbøll et al. 2023. Revealing the secrets of a 2900-year-old clay brick, discovering a time capsule of ancient DNA. Sci Rep 13, 13092; doi: 10.1038/s41598-023-38191-w
