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...