In 2004, Dale Guthrie from the University of Alaska carbon-dated a mammoth tooth from Saint Paul and showed that its owner lived around 7,900 years ago. That was the first clear evidence that these island-dwellers substantially outlived their mainland cousins. Other researchers then dated even younger remains, showing that the mammoths persisted to at least 6,500 years ago.
To find out exactly how long they lived, Graham’s team collected and dated 14 newly identified mammoth remains from a cave on Saint Paul. They also extracted cylindrical cores of sediment from a nearby lake. Each of these cylinders is a time capsule: Its layers of mud contain pollen, plants, and microbes that had been deposited in the lake over 10,000 years of the island’s history.
Beth Shapiro, from the University of California, Santa Cruz, searched the samples for traces of mammoth DNA. Meanwhile, Yue Wang and John Williams from the University of Wisconsin, Madison, looked for spores from three fungi that grow in the dung of plant-eating animals. Large extinct beasts like mammoths produced a lot of dung, so scientists can track their disappearance by looking for sudden drops in the levels of these fungal spores.
To the team’s delight, the five lines of evidence—the mammoth remains, the DNA, and the three types of spores—all gave the same answer. They showed that mammoths survived on the island until 5,600 years ago, before finally going extinct. “We were really surprised that it all lined up well,” says Graham. “The nice thing about the cores is that they told us not just when the mammoths went extinct, but all this other information about climate. And that told us what caused the extinction.”
It wasn’t humans: The first people on Saint Paul were Russian whalers who landed there in 1787, well after the last mammoth had gone. It wasn’t polar bears: They also came later. It wasn’t volcanoes: There were no traces of volcanic sediments in the lake during the extinction window. It wasn’t a lack of space: Although Saint Paul had certainly shrunk since its isolation from the mainland, it had reached its minimum size at least 3,000 years before the mammoths disappeared. And it wasn’t a lack of food: Pollen and plant remains in the lake sediments revealed that vegetation on the island was stable when the mammoths were declining.
Instead, the final killer was probably thirst, brought about by changing climate. Saint Paul never had rivers or springs. The only sources of freshwater were shallow lakes—and these were slowly disappearing. When sea levels rise around an island, the salt water also seeps beneath it, creating a wedge that intrudes into lakes, aquifers, and other sources of freshwater. On Saint Paul, this happened between 7,850 and 5,600 years ago, as Graham’s team discovered by analyzing microbes and chemical isotopes in their sediment cores.