ABSTRACT
Fossil evidence indicates that the globally-distributed brown rat (Rattus norvegicus) originated in northern China and Mongolia. Historical records report the human-mediated invasion of rats into Europe in the 1500s, followed by global spread due to European imperialist activity during the1600s-1800s. We analyzed 12 genomes representing seven previously identified evolutionary clusters and tested alternative demographic models to infer patterns of range expansion, divergence times, and changes in effective population (Ne) size. We observed three range expansions from the ancestral population that produced the eastern China (diverged ˜326kya), Pacific (˜42.8kya), and Southeast Asia (˜0.68kya) lineages. Our model shows a rapid range expansion from Southeast Asia into Europe 535 years ago (1480AD). We observed declining Ne within all brown rat lineages from 100-2kya, reflecting population contractions during glacial cycles. Ne increased since 1kya in Asian and European, but not Pacific, evolutionary clusters. Our results support the hypothesis that northern Asia was the ancestral range for brown rats. We suggest that southward human migration across China between 800-1550s AD, resulted in the introduction of rats to Southeast Asia, from which they rapidly expanded into the Middle East then Europe via existing maritime trade routes. Finally, we discovered that North America was colonized separately on both the Atlantic and Pacific seaboards, yet with evolutionary clusters of vastly different ages and genomic diversity levels. Our hypotheses should stimulate discussions among historians and zooarcheologists regarding the relationship between humans and rats.
