Assyrian brick reveals plant DNA

The inscription identifies this mudbrick as the building block of the palace of the Neo-Assyrian king Ashurnasirpal II (until 869 BC). © Arnold Mikkelsen and Jens Lauridsen.

Time capsule with cuneiform signature: Researchers have isolated DNA traces from a 2,900-year-old mud brick of a Neo-Assyrian royal palace. They provide clues to wild and cultivated plants in the center of the mysterious empire in the Middle East. Above all, however, the study is groundbreaking, say the researchers: The method could now also be used with other archaeological finds made of clay materials in order to track down living beings in the environment of the inhabitants of old cultural sites.

Modern genetics has also revolutionized the study of human history: DNA traces that have survived in bones or in certain materials can provide insights that traditional archeological techniques cannot offer. Researchers are also discovering more and more ways to obtain genetic material from lifeworlds long past. A Danish-British research team is now focusing on materials that are often found during excavations in different contexts: objects and structures made of loam or clay.

The object of your study is a mud brick from the collection of the Danish National Museum in Copenhagen. It comes from an excavation in the ruins of the former Neo-Assyrian capital of Kalhu on the Tigris River in what is now northern Iraq. The building block can be precisely assigned, because after its completion it was attributed to a specific use: cuneiform writing in Akkadian says that it belonged to the palace of King Ashurnasirpal II. Thus, the mud brick can be dated to his reign, which lasted until 869 BC. was enough.

Clay objects as time capsules

As the scientists report, the idea for the investigation developed through a minor accident: the mud brick broke during an inspection. This revealed material that had been locked up for around 2,900 years and thus also protected from more recent genetic contamination. So the researchers decided to find out to what extent old DNA traces could be extracted from the clay. Potential was obvious, because clay bricks were once made from a mixture of clayey soil and organic materials such as straw or manure and air-dried. To obtain genetic material from samples taken from the brick's inner fracture region, the researchers modified protocols previously used for other porous materials, such as bone.

The effort was successful: "We were thrilled to find that it was actually possible to extract DNA protected from contamination from this 2,900-year-old brick," says co-first author Sophie Lund Rasmussen from the University of Oxford. The scientists were then able to use the genetic material for a metagenomic analysis. With this method, the DNA fragments found are sequenced and then compared with reference databases. In this way they can be assigned to known living beings or at least groups of organisms. As part of the study, the researchers have so far limited themselves to evaluating plant DNA, as it delivered the best results.

Notes on biodiversity and cultivated plants

As they report, they were able to identify genetic traces from 34 different plant groups. They probably got into the material mainly through the addition of hay during the manufacture of the brick. It is obvious that these are the remains of wild and cultivated plants that grew or were cultivated in the city area at the time. In addition to groups of wild plants, the researchers also found the genetic traces of representatives of cultivated grasses (Triticeae). These include wheat, barley and rye. DNA sequences from umbelliferae (Selineae) were also found. Well-known representatives of this group are, for example, carrots.

The researchers particularly emphasized DNA from the Brassica genus, which includes the various cabbage cultivars. So far, their history of domestication is unclear. It is possible that a form of these plants was cultivated in the region 2900 years ago. "Because of the inscription on the brick, we can assign the material to a specific time period, which means that the brick represents a kind of time capsule with biological information. In this case, there is a special connection to the ancient Assyrians,” says co-first author Troels Arbøll from the University of Copenhagen.

In addition to the concrete findings, the scientists see further potential in their results: clay materials are found in almost all archaeological sites around the world and, due to their context, they can often be dated quite precisely. Thus, the method could have many possible applications. In addition to plants, it will also be possible to identify animals in the future, and further developments in the technology could possibly also enable more precise species classifications, according to the researchers.

Source: University of Oxford, professional article: Nature Scientific Reports, doi:

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