Ancient DNA reveals make-up of Roman Empire’s favourite sauce

Fermented fish sauce, or garum, was an incredibly popular condiment throughout the Roman Empire. For the first time, ancient DNA – scraped from vats used to produce the sauce – has revealed exactly which fish species went into the culinary staple.

Roman fish sauce was prized for its salty and umami flavours – although the philosopher Seneca famously described one version as “the overpriced guts of rotten fish”. It came in several forms, including a liquid sauce called garum or liquamen, as well as a solid paste known as allec. To prepare the condiment, fish-salting plants crushed and fermented fish, a process that can make visual identification of the species difficult or impossible.

“Beyond the fact that bones are extremely small and fractured, the old age and the acidic conditions all contribute to degradation of DNA,” says Paula Campos at the University of Porto in Portugal.

Campos and her colleagues ran DNA sequencing tests on bony samples from roughly the 3rd century AD, extracted from a Roman fish-salting plant in north-west Spain. They were able to compare multiple overlapping DNA sequences and match them to a full fish genome, giving the team “more confidence that we identify the correct species”, says Campos.

The effort identified the fish remains as European sardines – a finding that aligns with previous visual identification of sardine remains in other Roman-era fish-salting plants. Other garum production sites have also contained remnants of additional fish species such as herring, whiting, mackerel and anchovy.

This proof that “degraded fish remains” can yield identifiable DNA “might help identify with more precision some regional variations in the main ingredients of the ancient fish sauces and pastes”, says Annalisa Marzano at the University of Bologna in Italy, who did not participate in the study.

The study also compared the DNA of ancient and modern sardines to show there was less genetic mixing of sardine populations from different ocean regions in ancient times. That insight could help “assess the effects of human-environment interaction over the centuries”, says Marzano.

For their next step, Campos and her colleagues plan to analyse other fish species from additional Roman-era garum production sites. “We are expanding the sampling locations to see if the results are consistent across the entire Roman Empire,” she says.