As rats rely on humans for food and housing, they also moved with people through the Roman Empire — which at one point spanned 2 million square miles — and could have likewise affected by its decline, Discover adds. Based on that, researchers from Germany and the UK generated a de novo genome assembly of the black rat, Rattus rattus, and further analyzed the mitochondrial or nuclear genomes of modern black rats and ancient black rats that lived between the 1st and 17th centuries CE in Europe and North Africa.
As they report in Nature Communications this week, the researchers found that black rats were introduced into Europe from Southwest Asia first about 1000 BCE. They underwent a population turnover in parts of Europe during the 6th and 10th centuries and nearly went extinct before being reintroduced. This population turnover, the researchers note, coincides with the decline of the Roman Empire, which could have led to a collapse in trade and decreased grain shipments the rats relied upon.
This, in addition to other archaeological data, lends "support to the idea that the economic system did change," the study's co-senior author David Orton from the University of York tells Discover.