These days, rats are rarely a welcome sight but some 3,000 years ago, the ancestors of modern-day Polynesians brought the creatures along as a source of food as they migrated from other regions. By analyzing genetic changes in ancient and modern rats, scientists have tracked the paths of this human diaspora.

Elizabeth Matisoo-Smith and Judith H. Robins of the University of Auckland in New Zealand studied mitochondrial DNA (mtDNA) sequences from both ancient and extant Pacific rat populations throughout the Pacific and Southeast Asia. Rats may seem like an unlikely source of information about ancient human movement, but because Rattus exulans cant swim, scientists know they reached new islands solely because people transported them on ancient outrigger canoes. The researchers identified three genetically distinct populations of rats as well as clear geographic patterns in the mtDNA recovered from various locales, including Papua New Guinea, Thailand and Polynesia. (In total the team analyzed 131 samples, which included rat remains found in archaeological digs that were more plentiful and available for study than were human remains.)

According to a report published online this week by the Proceedings of the National Academy of Sciences, the findings rule out two simplistic models of how the ancient Lapita people dispersed into the islands of Oceania. Instead, the scientists suggest a more complex view of Lapita movement from Southeast Asia to Polynesia, with significant interaction with other cultures along the way. The authors conclude that "integrating these results with those from other fields such as archaeology, comparative linguistics and molecular biology of human populations will be the only way we can fully understand the complex prehistory of this region."