A multidisciplinary team led by researchers from the Interdisciplinary Center for Marine and Environmental Research (CIIMAR) has succeeded, for the first time, in extracting and sequencing ancient DNA found in the salting factory of the Roman village of Adro Vello, in Spain, allowing a glimpse into fish-based human diets during the Roman Empire. The study, now published in the journal Antiquity, shows that the salting tanks used to ferment fish into pastes and sauces popular throughout the Roman Empire predominantly used European sardines as the main ingredient.

This discovery was possible because the team, which included researchers Paula F. Campos and Gonçalo Themudo from CIIMAR, developed an innovative method that can identify animal remains when they are already damaged beyond recognition.

Fish was a vital source of protein for the Romans and was often preserved through salting and fermentation to produce fish pastes and sauces. The most famous was garum - a sauce popular as a seasoning and condiment with an umami flavor. Garum was prepared both for local consumption and to be sold over long distances, and its long shelf life facilitated trade throughout the empire. Large-scale fish salting factories, known as cetariae, were therefore built in coastal areas, particularly on the Atlantic coasts of Hispania (today's Iberia) and Tingitana (today's Morocco), which were famous for their fish sauces.


DNA degraded by processing

Due to the fragility of fish remains, they are often found damaged and cannot be identified by observation alone. “Fish bones are a frequent find in the archaeological record of the Roman period, but their use in archaeogenomic studies is residual, probably due to the fragmentary nature of the remains, which makes it difficult to identify the species or even the genus,” says Paula F. Campos, leader of the study “Roman Atlantic garum: DNA confirms sardine use and population continuity in north-western Iberia”. This is especially true for those that come from salting tanks, since they were intentionally crushed during the production of the sauce.

To find out more about the species used in Roman diets to make foods such as garum, it is necessary to use ancient DNA analysis. However, this is a difficult process: the main techniques used to carry out this type of analysis are not sufficient since, in the case of fish pastes and sauces, the processes of grinding and fermentation accelerate the degradation of DNA.


An innovative method

It was in this context that this multidisciplinary team developed an innovative method to analyze the DNA of fish remains from the Roman fish fermentation vats in the salting factory at the archaeological site of the Roman villa of Adro Vello, in Grove, Spain. This important archaeological site is the focus of a multidisciplinary project funded by the Xunta de Galicia and led by the Department of Archaeology at the University of Vigo.


Main ingredient: the sardine

The main results of this work have now been published in the international scientific journal Antiquity and have shown that the new method was effective in identifying the species corresponding to the remains found in the salting factory at the archaeological site of the Roman village of Adro Vello. Furthermore, by comparing the DNA sequences of the archaeological remains with the current species, it was possible to see that these vats were used to salt sardines in Roman times and that they are closely related to those found in the same area today, confirming genetic continuity despite the high mobility of the species.
Overall, these results confirm the value of ancient DNA studies for identifying animal remains that have been damaged over time: “in this study, we have shown that usable DNA can survive in fermentation environments, such as the brines used by the Romans to make garum,” concludes Paula Campos. “Despite enduring conditions that promote DNA degradation, the methods we have described allow species to be identified from these processed bones.” This new methodology has significant implications for the study of archaeological fish remains in the future, allowing us to understand not only the evolution of these species, but also the history of their connection to humans.

In addition to CIIMAR researchers, researchers from the University of Vigo and the University of Léon in Spain and the University of Copenhagen in Denmark took part in this study. This research was financed through national funds from the Foundation for Science and Technology and European Regional Development Funds, under the PT2020 Program.