It’s 3,600 B.C. Humanity is ascendant. We’re a few centuries out from inventing writing and entering the Bronze Age, but we are already smelting copper and lead, fashioning silk, and making wheels. In the Middle East, the great civilizations of Egypt and Mesopotamia are poised to take off.
Meanwhile, 10,000 kilometers away, the last, lonely woolly mammoth of Alaska’s Saint Paul Island is dying of thirst.
Saint Paul lies halfway between Russia and the United States. It used to be part of a land bridge that connected Asia and North America. But rising sea levels cut it off from both continents and turned it into one of the world’s most remote islands. The encircling waters also trapped a population of woolly mammoths. While their kin went extinct on the mainland sometime between 14,000 and 13,200 years ago, the Saint Paul mammoths persisted for millennia more.
They didn’t have a lot of space. At just 110 square kilometers in area, Saint Paul is roughly the size of Paris or Walt Disney World. And yet, this tiny nubbin of land sustained a population of giant creatures for at least 8,000 years. How they survived is a mystery. But a team of scientists led by Russell Graham from Pennsylvania State University have at least worked out exactly when they finally died—and, more importantly, why. In a tragic twist, it turns out that they probably played a role in their own downfall.
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.
This, combined with a drying climate, meant that Saint Paul’s water supplies were getting smaller, shallower, and saltier. That was disastrous for the mammoths. Modern elephants need to drink between 70 and 200 liters of water every day, and mammoths probably needed more to keep cool. An elephant can get rid of heat by sweating, relying on the evaporating moisture to cool its skin. But a mammoth’s dense, waterproof fur would have wicked sweat away from the skin before it could evaporate. To compensate, they must have sweated a lot more, which in turn gave them a truly mammoth thirst.
Ironically, the mammoths probably made things worse for themselves. As they were forced into a dwindling number of waterholes, they would have destroyed the surrounding vegetation, eroded the banks, stirred up the sediments, and slowly filled up the lakes. “They sort of hastened their own demise,” says Graham.
“It’s a really tight story, with multiple lines of proxy evidence supporting the conclusions,” says Jacquelyn Gill from the University of Maine, who has also studied mega-beast extinctions. And it’s still relevant today. Conservation biologists recently showed that the Bramble Cay melomys—a small island rat—went extinct in a similar way: Rising sea levels corralled it into an ever-smaller space, making it extremely vulnerable to other changes. “It’s a likely model for extinction in the near future,” says Gill.
Graham adds that we should pay close attention to the threat of rising saltwater wedges. “These haven’t been discussed a lot, but we’re seeing their effects in the South Pacific now,” he says. “There are islands where people are having a hard time getting enough fresh water. If the sea level continues to rise, it will continue to get worse. Florida is in the same boat. People are standing on the beach and waiting for big waves to come in and flood the state, but really the salt water is coming in from below their feet. It will really limit the availability of freshwater.”
“The Saint Paul mammoth story is also important in a different way, because it reminds us that the age of mammoths is also the age of modern humans,” adds Gill. The last mammoths on Saint Paul and the nearby Wrangel Island were “just as modern as writing, the wheel, and celestial navigation. That's a blink of an eye for the ecosystems that lost mammoths and other mega-beasts, and the consequence of those losses are still playing out today.”