Humans definitely weren't to blame for killing off one of the last surviving woolly mammoth populations — so what is?

Scientists finally have the answer, and it's a bit of a surprise.

Thousands of years after woolly mammoths died off on the North American continent (likely due to the effects of climate change, with help from human hunters in some places), small populations survived on two northern islands that formed between North America and Asia at the end of the last Ice Age.

The mammoths of Wrangel Island in the Chukchi Sea off Siberia died out around 4,000 years ago, about the time that Stonehenge was completed. That coincides with the arrival of humans on the island, so they likely played a role.

Rising sea levels and the influx of seawater on the coast at the end of the last Ice Age left just two freshwater lakes, in the craters of volcanoes at the centre of St. Paul Island, that the mammoths were forced to rely on. (Duane Froese)

But there wasn't a single human on St. Paul Island in the Bering Strait, about halfway between Siberia and Alaska, when the mammoths there died out about 6,000 years ago, around the time of the Sumerian civilization in Mesopotamia and the first evidence of mummification in Egypt.

So what killed off the mammoths of St. Paul Island?

Based on evidence found in a lake that was once a popular mammoth watering hole, it appears the mammoths died of thirst, report U.S. and Canadian scientists in a study published this week in Proceedings of the National Academy of Sciences.

And the mammoths themselves were partly to blame for degrading their environment until they no longer had any clean water to drink.

"You can imagine a dozen mammoths sort of wallowing around a lake, what that would have done in terms of the vegetation around the lake and such," says Duane Froese, a University of Alberta professor who co-authored the study led by paleontologist Russ Graham at Pennsylvania State University.

Lauren Davies, a University of Alberta graduate student who co-authored the new paper, stands among the lush vegetation of St. Paul Island that mammoths grazed on until about 6,000 years ago. (Duane Froese)

At the end of the last Ice Age, sea levels rose, flooding the coasts of St. Paul Island until it shrank to its present size of just 100 square kilometres, about 6,000 years ago. The influx of seawater on the coast left just two freshwater lakes, in the craters of volcanoes at the centre of the foggy, grassy island, that its tiny population of mammoths were forced to rely on.

Evidence shows the shores of the lakes were heavily trampled at that time, and grew shallower, saltier and cloudier as more and more sediment got eroded into the lake. Some years, it may have dried up altogether, although scientists don't have enough evidence to tell.

What they do know is that the deterioration of the lake coincides with the disappearance of the mammoths.

In a way, that came as a surprise.

'That's a new one'

"Freshwater limitation wasn't one of the things we've ever pointed to in terms of an extinction driver. So that's a new one," Froese said.

Of course, St. Paul Island's mammoths were a vulnerable population that probably never numbered more than 30, he estimates. Pinpointing the cause of their extinction "just sort of underscores the precariousness of small island populations to what seems like fairly subtle environmental change."

Russ Graham, the Penn State University paleontologist who led the study, descends into a pit cave where the youngest mammoth remains on St. Paul Island have been found. They date to around 6,400 years ago. Evidence from lake sediments suggest mammoths survived for another 900 years after that. (Duane Froese)

Even today, the crater lake that the researchers studied is only a metre deep. The researchers drilled through the ice in winter, into the layers of sediment deposited on the bottom of the lake over thousands of years.

There they found mammoth DNA, spores of fungi that can only live in the fresh dung of large mammals like mammoths, and the remains of aquatic insects that contain chemical information about water levels over the lake's history.

Together, the data pinpoint the time of extinction at 5,600 years ago — about 900 years after the date of the youngest mammoth remains ever dug up on the island — and chronicle the deterioration of the lake during the last days of the mammoths.

The result doesn't just solve a longstanding mystery about a puzzling extinction.

It may also be a warning about the seriousness of a problem that has never been linked to extinctions in the past, but is relevant for human communities in our own age of rapid climate change, rising seas and a coastal flooding, Froese suggests:

"Declining freshwater resources as saltwater intrusion happens is something that needs to be considered and needs to be thought about."