A museum worker in Victoria, Canada, repairs a woolly mammoth diorama. (Jonathan S. Blair/National Geographic/Getty Images)

The year is 3,600 B.C. The planet is warming, humans are multiplying, the megafauna have all been killed off. In Egypt, people are just beginning to mummify their dead. The first cities have sprung up in Mesopotamia. Popcorn had been invented in Peru, and smelters in the Middle East have just created bronze. And still, civilization barrels forward — toward the invention of writing, the rise of empires, the domestication of horses, the discovery of chocolate.

Even on St. Paul Island, an isolated spot in the Bering Sea that has stayed frozen in the Pleistocene for thousands of years, time is taking its toll. Here, some of the world's last woolly mammoths — holdouts from a colder, simpler time — are starting to die off.

And scientists think they may finally know why.

It wasn't because of predation by polar bears — they don't appear in the island's fossil record until 1,000 years after the last mammoths were gone. Nor was it humans, who are often blamed for the demise of mammoths and other megafauna on the mainland — humans wouldn't step foot on St. Paul until 1787.

It was thirst, the researchers write in the Proceedings of the National Academy of Sciences. Five distinct fossil indicators suggest that the warming climate and rising seas caused St. Paul Island — and more importantly, its fresh water ponds — to shrink.

But it was also the mammoths themselves.

"The mammoths were contributing to their own demise," Russell Graham, a paleontologist at Penn State University who is the lead author on the study, told the BBC. As the massive mammoths crowded around smaller and smaller pools of drinking water, they would have trampled the vegetation at the water's edge. The loss of greenery would cause the underlying soil to erode, sending sediments cascading into the same lake their lives depended on. Without rain or snowmelt to refill the basins, St. Paul Island would have quickly changed from a remote refuge to a resource-less trap.

"It wouldn't have taken long if the water hole had dried up," Graham said. "If it had only dried up for a month, it could have been fatal."

Graham and his colleagues amassed a substantial body of evidence to support their conclusions about the timing and cause of the mammoths' demise. Using radiocarbon dating, they determined the age of the remains of the youngest mammoths as well as the last traces of mammoth DNA found on the island and three types of fungus that live in mammoth dung — all five turned out to be roughly 5,600 years old.

The scientists say that this is one of the most accurately timed extinctions from the prehistoric world.

“It’s a really tight story, with multiple lines of proxy evidence supporting the conclusions,” University of Maine paleoecologist Jacquelyn Gill, who was not involved in the study, told the Atlantic. And it's not necessarily a thing of the past: Earlier this summer, conservation biologists reported that the rats of Bramble Bay Island in Australia had died out after rising sea levels and worsening storms made life on the island untenable. It's said to be the first mammal extinction due to climate change.

The mammoth extinction "is a likely model for extinction in the near future,” Gill said.

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