The Black Death is infamous for killing a significant chunk of the world’s population in the 14^th century, but recent research suggests that the plague was around much earlier than that. According to a press release from Eurekalert, researchers at the University of Copenhagen have sequenced Bronze Age DNA to determine that the bacteria responsible for the plague was present almost 3,300 years before the breakout.

The study, published in the journal Cell, suggests that the plague was infecting people as early as 4,800 years ago. Researchers sequenced the DNA found in tooth samples from European and Asian Bronze Age individuals to discover evidence of the bacteria, Yersina pestis, responsible for the Black Death. The bacteria were transferred from fleas hitchhiking on the backs of rats, and were especially good at evading the body’s immune system.

According to the study’s lead author Eske Willerslev from the Center for GeoGenetics at the University of Copenhagen, “We found that the Y. pestis lineage originated and was widespread much earlier than previously thought, and we narrowed the time window as to when it developed. This study changes our view of when and how plague influenced human populations and opens new avenues for studying the evolution of diseases.”

1. pestis was made famous throughout the world during the Plague of Justinian, or the Black Death in the mid-1300s. The plague was responsible for the death of an estimated 50 percent of Europe’s population. The plague returned in the Third Pandemic, which hit China in the 1850s. The Plague of Athens hit ancient Greece 2,500 years ago, and the Antonine Plague was credited for the eventual decline of Classical Greece and the defeat of the Roman army. Despite a lack of direct genetic evidence, many researchers still believe that Y. pestis is the culprit in all of these widespread infections of the past.

Researchers believe that the plague was affecting human populations way earlier than anybody previously imagined. Just a few months ago, researchers Willerslev, Kristian Kristiansen from the University of Gothenburg, and a number of other collaborators published a genetic study of Eurasian populations in the Bronze Age, which occurred from 3000 BC to 1500 BC. The analysis showed that the Bronze Age was rife with large-scale migrations and dramatic shifts to population dynamics, many of which led to the current genetic makeup across large parts of Europe and Asia.

According to one of the study’s co-authors, Morten Allentoft of the Center for GeoGenetics at the University of Copenhagen, “One of the scenarios we discussed was the idea that large epidemics could have facilitated such dynamics. Perhaps people were migrating to get away from epidemics or re-colonizing new areas where epidemics had decimated the local populations. Could it be, for example, that plague was present in humans already in these prehistoric times?”

The research team scanned 89 billion raw DNA sequences taken from the teeth of 101 skulls from people who lived in Bronze Age Europe and Asia. The analysis revealed that Y. pestis DNA was present in seven of the individuals, who were dated to the years between 2794 BC and 951 BC. The oldest-known ancestor of Y. pestis strains is 5,783 years old.

The biggest change that led to the spread of the plague, the study found, was the addition of a gene called Yersina murine toxin, or ymt. This gene allowed the bacteria to remain in the gut of fleas without harm, which allowed for the easy transmission from rats to humans. This gene was discovered in one of the earliest Iron Age individuals, suggesting that the disease could be transmitted buy fleas as early as 3,700 years ago.

Y. pestis was also adept at evading the defenses of its mammalian hosts. The immune system in humans and rats recognizes and attacks pathogens with a protein called flagellin, which composes many bacteria’s whip-like tails that help them move around. Y. pestis strains have a mutation in the flhD gene which prevents the expression of this protein, causing our antibodies to look right past it.

This mutation was not found in the oldest DNA samples, and in the youngest sample, the flagella defense system was still evolving in Y. pestis. This suggests that the disease wasn’t fully transmissible by fleas until the start of the first millennium BC.

“The underlying evolutionary mechanisms that facilitated the evolution of Y. pestis are still present today, and learning from this will help us understand how future pathogens may arise or develop increased virulence,” says co-author Simon Rasmussen from the Technical University of Denmark. Rasmussen also states that the study confirms that the same bacteria could have been responsible for earlier plagues, like the Plague of Athens and the Antonine Plague.

Researchers will continue to scan ancient DNA samples to find out when exactly the plague made its human debut. The methods laid out by the study could also be useful for examining other ancient diseases.