Jillian Swift didn’t set out to become the “rat girl.”

Related Content How Proteins Helped Scientists Read Between the Lines of a 1630 Plague Death Registry

But as the Max Planck archaeologist studied more sites, she realized that there was a wealth of information there besides fossils and tools. “Excavation is an inherently destructive activity, so it’s best practice to collect everything we find,” Swift said by email—and that includes dirt, rocks, and yes, rat bones. “There’s a whole wealth of rat assemblages just sitting in the back of cabinets, waiting for someone to do something interesting with them.”

That fascination with making the most of the leftover bits from a dig spurred Swift and her colleagues to analyze 145 rat bones from three Polynesian island systems in the Pacific. The rodents, considered invasive, disease-carrying pests in life, proved surprisingly useful in death. By measuring the chemical composition of the rat bones, the researchers could make inferences not only about what humans were eating around 2,000 years ago, but also how their early residence on the islands—Mangareva, Tikopia and Ua Huka (also known as Marquesas)—shaped the environment.

Researchers have long used animals like dogs to study human settlements, and crystallized packrat urine for studying long-term climate change. But the results of the new study, published Monday in the journal Proceedings of the National Academy of Sciences, show that rat remains are an ideal material for measuring human-influenced changes over time. Part of the reason is that rats are what’s known as commensal species: not wild, but also not domesticated, feasting on human scraps and making themselves comfortable in whatever cultivated environments humans produce.

“I think this is a really important study,” says Torben Rick, a Smithsonian Institution archaeologist who wasn’t involved in the study. “Using rats on islands is pretty novel and tells us broadly an interesting, roundabout way to look at land-use changes.”

The new research falls in line with the mainstream view on human settlement, notes David Wright, a professor of archaeology at Seoul National University who wasn’t involved in the study: wherever humans go, the environment is inevitably transformed. For the Polynesian islands, that meant the arrival of agricultural crops like breadfruit, yams and taro, as well as domesticated animals like dogs, pigs and chicken. The early settlers also used slash-and-burn agriculture to remove forests and fertilize the soil and likely hunted many seabirds to extinction.

To get a more precise view of how human behavior impacted the islands, Swift and her colleagues used stable isotope analysis. Carbon analysis is based on the way plants process carbon dioxide: most agricultural products are classified as C3 plants, while tropical grasses are usually C4 plants. If rat bones show a higher level of C3 than C4, they were probably sneaking off with human tidbits like sweet potato and taro. Then there’s the nitrogen isotope, which increases as you move up the food chain (e.g. lions have higher nitrogen isotope levels than antelopes).

In the case of the Polynesian islands, higher nitrogen isotopes usually correlated with marine food sources, because the marine food web has a longer chain of predators eating other predators. That meant if the rats showed high nitrogen isotopes, they were feasting on seabirds, fish or other marine treats.

Swift and the other researchers traced the decline of nitrogen isotopes in the rat bones at different times on the different islands. They linked this precipitous drop to the local disappearance of seabirds and a decrease in marine resources, followed by an increase in agricultural systems. The only island that proved the exception to this rule was a steeply-hilled landmass with poor soil quality. Because of its geology, inhabitants were likely forced to rely more on fishing for subsistence—so that’s what the rats ate, too.

Archaeologists have often relied on the remains of domestic animals as well as game animals to judge human activity. They’ve used them to learn what humans were most likely to hunt (gazelle at Jebel Irhoud, but also zebras, porcupines, and snakes), the role they played in wiping out megafauna on larger islands like Zanzibar, and how farming disrupted different animals’ relationships with one another.

Studying commensal animals is a relatively new practice, but it’s growing in popularity. Rick and his colleagues used it on 7000 years’ worth of fox bones from the California Channel Islands to see how human behavior changed the environment, and another group has used Hawaiian petrels to analyze the Pacific Ocean food webs in relation to human fishing. For Wright, a fascinating new avenue of inquiry is commensal bacteria. He notes that archaeologists can now sample the soil in certain areas and analyze the microbes through metagenomics. “Certain species are unique to us and they are also indicators of the types of food we are eating and, by proxy, the broader environment,” Wright said.

The study is also a reminder that the field of archaeology has much to offer modern ecologists. “Archaeology has a lot to contribute to current discussions of environmental management and sustainability—the ecosystems we see today are a result of deep-time historical processes—but there’s definitely a bridge to be crossed between archaeological studies and modern ecology,” Swift said. Using the rat remains for quantitative analysis means that, in locations like these islands, the record can provide hard data about the deep past and show changes across longer periods of time.

And maybe with that knowledge, scientists will better be able to manage environments of the future. “This gives us a snapshot across time, of how [certain actions] might affect agricultural yields in the future, and ecosystem conservation or restoration progress,” Rick says of the study’s practical value.

On the other hand, the results also seem to point to an unavoidable conclusion: We created the rats’ world, and now we’re just living in it.