JENA.- New ancient DNA analysis has shed light on how the black rat, blamed for spreading Black Death, dispersed across Europe – revealing that the rodent colonised the continent on two occasions in the Roman and Medieval periods. The study, led by the University of York along with the University of Oxford and the Max Planck Institutes for the Science of Human History (Jena) and Evolutionary Anthropology (Leipzig), is the first ancient genetic study of the species, also known as the ship rat.

The black rat (Rattus rattus) is one of three rodent species, along with the house mouse (Mus musculus) and brown rat (Rattus norvegicus), to have become globally distributed as a result of its ability to live around human habitations by taking advantage of food and transportation. Black rats were widespread across Europe until at least the 18th century, before their population massively declined, most likely as a result of competition with the newly arrived brown rat, the now dominant rat species in temperate Europe.

By analysing DNA from ancient black rat remains found at archaeological sites spanning from the first to the 17th centuries in Europe and North Africa, the researchers have pieced together a new understanding of how rat populations dispersed following the ebbs and flows of human trade, urbanism, and empires.

The study shows that the black rat colonised Europe at least twice, once with the Roman expansion and then again in the Medieval period – matching up with archaeological evidence for a decline or even disappearance of rats during the early Medieval period. According to the authors, this was likely related to the break-up of the Roman economic system, though climatic change and the sixth century Justinianic Plague may have played a role, too. When towns and long-range trade re-emerged in the Medieval period, so too did a new wave of black rats.

Repeated colonisation of Europe
“We’ve long known that the spread of rats is linked to human events, and we suspected that Roman expansion brought them north into Europe,”, said David Orton from the Department of Archaeology at the University of York. “But one remarkable result of our study is quite how much of a single event this seems to have been: all of our Roman rat bones from England to Serbia form a single group in genetic terms.” He adds: “When rats reappear in the Medieval period we see a completely different genetic signature – but again all of our samples from England to Hungary to Finland all group together. We couldn’t have hoped for clearer evidence of repeated colonisation of Europe.”

Greger Larson and Alex Jamieson, co-authors at the University of Oxford, said: “The modern dominance of brown rats has obscured the fascinating history of black rats in Europe. Generating genetic signatures of these ancient black rats reveals how closely black rat and human population dynamics mirror each other.”

According to the authors, the study could also provide information about human movement across continents. “This study is a great showcase how the genetic background of human commensal species like the black rat, animals which flourish around human settlements, can reflect human historical or economical events. There is much we can learn from these often neglected small animals”, said leading author He Yu, a researcher at the Max Planck Institute for Evolutionary Anthropology.