About 5,700 years ago, on a small, isolated island about 480 kilometers off the coast of Alaska, a herd of woolly mammoths crossed a treeless tundra.

But these weren’t just any mammoths.

By this time, all of their mainland relatives had already been gone for thousands of years leaving only this population on St Paul's Island, and one other on Wrangel Island.

Which meant that these were some of the last mammoths our world ever saw.

And yet, just 200 years later, they too were gone.

Despite holding on for thousands of years, some fatal tragedy befell these mammoths, causing them to vanish basically overnight in evolutionary terms.

But what?

Whatever caused the mammoths’ disappearance on St Paul’s Island left no obvious clues behind.

Well, thanks to a weird mix of evidence from ancient DNA, tiny water fleas, and poop-eating fungi, we might finally have the answer And it may have been partly their own fault.

The first hints that mammoths had once called St Paul’s Island home emerged almost two centuries ago, when a tusk was discovered in 1836, followed by a couple of teeth several decades later.

For a while, the idea that a band of mammoths had somehow made it so far from the mainland to such a small and remote spot seemed ridiculous.

Some scientists argued that the remains probably reached the island as debris, carried inside floating blocks of ice that eventually deposited them there.

And some even suggested that the remains had actually been placed there deliberately as pranks played on credulous naturalists.

And no one likes a credulous naturalist.

But over time, as more and more remains turned up, it became clear that mammoths really had lived on the island.

And in 1999, a group of hunters stumbled upon evidence that proved that the mammoths of St Paul’s Island were even more remarkable than anyone had suspected They discovered a 12 meter-deep lava tube cave containing a trove of bones, which they reported to archaeologists.

After investigating, the archaeologists found that among the jumble of bones most of which belonged to foxes that had fallen in and died were seven pieces of mammoth.

And when they radiocarbon-dated the remains, the results came back as just 5,700 years old thousands of years younger than any other mammoths known from North America.

St Paul’s Island hadn’t just been home to mammoths, it had been home to some of the last mammoths.

So, why aren't they still there today?

They were able to outlast their mainland relatives by about 5000 years, after all, so what stopped them surviving another 5000 or so?

I, personally, want them to still be alive.

Well, when scientists started looking for the culprit, they considered the usual suspects that cause extinction, but none of them really seemed to fit.

We humans almost certainly weren’t to blame, for example, seeing as the island had no evidence of permanent human presence before 1787.

It doesn’t seem to have been predation, either.

The only animals from the island that could’ve posed a threat are polar bears, but their remains only show up more than 1000 years after the mammoths went extinct.

And we also don’t have any evidence of sudden, overnight catastrophes like volcanism during the extinction window.

Plus, the island’s overall vegetation seems to have been fairly stable during that time, too.

So whatever finally brought these mammoths down must have been something much stranger, and something that left no obvious evidence behind.

To unravel the mystery, a team of researchers collected sediment samples in 2013 from a small freshwater lake near the center of the island.

Their aim was, first, to pinpoint exactly when the mammoths went extinct, seeing as no one could be sure that the remains from the lava tube were actually the very last members of the population.

And lake sediment cores can be incredibly useful for piecing together ancient environments and tracing how they changed over time Because lake sediment captures diverse clues about the surrounding ecosystem, often keeping that evidence conveniently preserved in neat chronological layers for a long time.

Within those cores, researchers looked at a few different types of evidence.

One was the DNA of the mammoths themselves.

And they found traces of mammoth DNA in the sediment layers up until 5,650 years ago, plus or minus about 80 years.

But after that, those traces vanished, which matches up with the youngest-dated remains.

Plus, the researchers also looked for the preserved spores of fungi that grow on the dung of large mammals like mammoths.

The spores of two different poop-loving fungi species stop showing up within just centimeters of each other in the layers, right around that same mark of 5,650 years ago.

With multiple forms of evidence all converging on this exact time of death, it became one of the most precisely-dated prehistoric extinctions ever.

But as cool as it was to nail down when they vanished, we were still left with the question of why.

And, luckily, the sediment cores had clues about that, as well.

You see, of the few sources of fresh water on the island, the lake was the deepest, so it had probably been the single most important water source for the mammoths during their heyday.

So the researchers began to wonder if changes to the lake itself may have been the culprit.

Because we know that this period of time the mid Holocene Epoch saw a lot of climate change in this region that dried things out.

On the Alaskan mainland, for example, other lakes show evidence of moisture loss and fluctuations in water levels around this time.

To figure out if the lake on St.

Paul’s Island had also been experiencing major changes, the researchers searched the sediment cores for tiny organisms that had once lived in the water.

These included water fleas and single-celled algae called diatoms.

In both cases, the changes in the types of these critters over time suggested that the lake was, indeed, experiencing major shifts around the moment of the mammoths’ extinction Species of both groups that live and drift around in open water basically vanish, replaced by ones that live closer to the shoreline or attached to surfaces.

This shows that the lake suddenly became a lot shallower.

Plus, there’s a jump in the abundance of species that can tolerate higher salinity levels, which suggests that whatever freshwater remained in the lake was no longer very fresh.

And the researchers argued that the loss of this watering hole wasn’t driven just by a shift in climate but also by the mammoths themselves.

You see, modern elephants need to drink up to 200 liters of water every day.

And mammoths may have needed even more than that as the climate warmed up, seeing as they were adapted for retaining heat.

And when water sources become shallow and less drinkable, elephants often start digging up the ground around those water sources as they try to open up new wells a behavior that mammoths likely did, too.

But when there’s no more water to find, this behavior just makes things worse by increasing erosion into the water source and making it even less drinkable.

Plus, by digging up and eating the vegetation around the lake, the mammoths would have further increased the rate at which sediment filled it in.

So while a shift in climate may have kicked off the decline of their main water source, the mammoths becoming thirstier and more desperate may have been the final straw.

When the final mammoths of St.

Paul’s Island died of thirst 5,600 years ago, it marked yet another milestone in the slow, drawn out extinction of their species as a whole.

After the fall of the mainland mammoths, the only hope for the species’ survival had rested on the few isolated islanders.

And now, they were disappearing as well.

Around 1500 years after the St Paul’s Island mammoths bit the dust, the only other remaining population, on Wrangel Island, died out too, and mammoths were consigned to the prehistory books.

But the story of the penultimate group that lived and died on St Paul's shows us that extinction isn’t an event, it’s a process.

The loss of an entire species isn't always due to a single big factor dooming the species in one fell swoop Even though that’s often how we think about extinction: caused by A Thing.

A shift in climate, a new predator or competitor, a big ol’ space rock, you get the idea.

Instead, the causes of the mainland mammoths’ extinction were different from those of the St.

Paul’s mammoths, which were in turn different from the Wrangel mammoths’.

Sometimes populations gradually die out one by one, thousands of years apart, in a series of unique tragedies, until, eventually, none are left.