Science Notes: Plant DNA successfully extracted from 2,900-year-old brick

It seems that every few months ‘Science Notes’ is covering another breaking story on advances in a DNA research, a testament to how quickly the field is evolving. After our reporting of the successful, non-invasive extraction of human DNA from a Palaeolithic pendant in CA 400, this month we will look at another new development: the first successful sequencing of plant DNA from a clay brick, providing new insights into ancient construction techniques as well as the flora that were present in these past landscapes.

Image: The National Museum of Denmark

The brick that formed the centre of this new analysis was discovered during British excavations in Nimrud, Iraq, which began in 1949. Due to financial support from the Danish-funded Rask-Ørsted Foundation – which facilitated the participation of several Danes on the dig – the National Museum of Denmark received a number of objects from the project, including the brick. It is inscribed with cuneiform, from a dialect of the ancient Semitic language Akkadian, saying: ‘The property of the palace of Ashurnasirpal, king of Assyria.’ This indicates that the brick was part of Ashurnasirpal II’s palace in the ancient city of Kalḫu (which is modern-day Nimrud), the construction of which began around 879 BC. This site was only the second ancient Mesopotamian city ever to be discovered when it was first excavated by Sir Austen Henry Layard in 1845, and so holds an important place in the history of Assyriology. Over the last decade, however, parts of the palace (now known as the North-West Palace) have unfortunately been destroyed due to ongoing conflict in the region.

This most recent analysis was brought about during a digitisation project in 2020, when a vertical split gave researchers access to the previously unexposed inside of the brick. Five samples were quickly taken, which then underwent DNA extraction and amplification, and were subsequently analysed by a team led by Troels Pank Arbøll and Sophie Lund Rasmussen, both at the University of Oxford at the time of the project.

Four of the samples yielded results, identifying 34 unique taxonomic groups of plants, including from seven distinct families of plant: umbellifers (Apiaceae, which includes celery, carrot, and parsley), birch (Betulaceae), cabbage (Brassicaceae), heather (Ericaceae), grasses (Poaceae or Triticeae), beech (Fagaceae), and willow (Salicaceae). DNA from the order of laurels (Laurales) was also identified, but could not be confidently defined at the family level. By using the modern-day landscape of Iraq, the team tentatively suggested that the DNA found from the birch family is possibly that of Betula pendula, or silver birch, which is the only species of Betulaceae found in Iraq today. Additionally, only two genera from the Salicaceae family are found in modern Iraq, including willow (Salix) and poplar (Populus), and one genus from the Fagaceae family: oak (Quercus).

So, what can this tell us about ancient Assyria and its natural landscape? It is known that bricks of this type were made primarily of mud collected from the nearby Tigris River, or one of its branches, and mixed with other organic materials such as chaff, straw, and/or animal dung to give it more structure before being shaped in a mould and left to dry in the sun. It is not known, however, whether wild species or cultivated plants would have been used for this organic material, so it is unknown whether the species identified in the brick were from plants naturally growing near the Tigris or from domesticated varieties. Either way, it tells us about the plants that were probably growing in and around the ancient city during the time the brick was made.

Overall, future work using this new technique presents the opportunity to help refine our understanding of ancient agriculture and manufacture. For example, the identification of plants from the Brassica family is interesting as previous research had suggested that the origin of these crops was in the northern Mediterranean during the first half of the 1st millennium BC. While, as mentioned above, the Brassica DNA identified in the brick could have been wild varieties, it opens up a new avenue of research to see if this was indeed the case. This work provides additional ways in which researchers may be able to identify and reconstruct biodiversity in the ancient past, helping to determine how it may have evolved over time and place.

Text: Kathryn Krakowka