Over 80,000 years ago, somewhere on a southern California beach, a mammoth wandered across the sand. The beast didn't stop at the water’s edge. Step by step, its trunk held high, the towering proboscidean walked into the waves until its feet no longer touched the bottom. One, two, three, kick, the elephant kept paddling, the air-filled sinuses of its great skull helping to keep the mammoth’s head near the surface as it moved further and further out from the coastline. It had a long swim ahead. In the distance, rising from the surface 12 miles to the west, were a small set of islands. That’s where the mammoth emerged onto its new island home.

We know that such an event must have happened, and likely happened more than once. The evidence doesn’t come from swim tracks. Those would be nice, but haven’t been found yet (if they were ever preserved in the first place). The clues are in skeletons. On California’s Channel Islands, in sediments left from the last Ice Age, paleontologists have found the remains of mammoths that could have only gotten there thanks to some intrepid pachyderms that made the journey from the mainland.

The form that braved the surf were Columbian mammoths – a species only found in North America that ranged over much of the continent, and was presumably less shaggy than its famous woolly relative. These mammoths weren’t just visitors to the Channel Islands, though. They stayed, and, thanks to a phenomenon called insular dwarfism, their population eventually evolved into a smaller species – the pygmy mammoth Mammuthus exilis. These little mammoths stood only about five and a half feet at the shoulder, less than half the stature of their ancestors.

Swimming was the only way the mammoths could have arrived. There was no landbridge between the continent and the Channel Islands for them to cross. That’s why other parts of California’s Ice Age fauna – like sloths and sabercats – haven’t been found along the chain. Only aquatically-adept species could make it, and, if the skills of today’s Asian elephants are any indication, Columbian mammoths were likely strong swimmers. And they got a little help from ice. When the world’s glaciers crept over the land, sucking up water to expand their reach, the sea level off the California coast fell and vastly reduced the distance to the Channel Islands. During these times, when the Ice Age was in full effect, the mammoths could swim out to the islands.

It was thought that the Columbian mammoths made this trek during the Last Glacial Maximum, sometime around 26,000 years ago or so. Most mammoth remains on the Channel Islands fall within the window of 22,000 to 12,000 years ago. But now there’s an even older date. Thanks to a tusk found along a Channel Islands beach, U.S. Geological Survey researcher Daniel Muhs and colleagues have now pushed back the oldest known mammoth on the islands by tens of thousands of years.

Which species the tusk belonged to isn’t clear. It’s small enough that it could either by a pygmy mammoth or a juvenile Columbian mammoth. But dates derived from prehistoric corals from levels beneath the tusk indicate that this mammoth died around 80,000 years ago.

This newly-discovered mammoth probably wasn’t the first to stomp across the archipelago. At the time the beast inhabited the Channel Islands, Muhs and coauthors report, the world was in an interglacial. The local sea level was high, and, even if mammoths were accomplished swimmers, the distance was likely too far for them to cross. Therefore, the paleontologists hypothesize, the mammoths must have arrived during even earlier times when glaciers created a shallower sea.

Falling sea levels allowed mammoths to cross the Santa Barbara Channel. Credit: Muhs et al. 2015

The two earlier periods when mammoths could have swum across the channel were 150,000 and 250,000 years ago. With luck, the bones of the earlier arrivals will help refine the date and help fill in this new gap in the mammoth story. It could be that the pygmy mammoths evolved from their Columbian ancestors earlier than thought. Then again, perhaps there were multiple waves of mammoth habitation, flourishing and declining as the seas rose and fell. This isn’t just a story of evolution, though. It’s also one of extinction.

No one knows what wiped out California’s mini-mammoths. The traditional Pleistocene culprits have been invoked, and both have their problems.

While hunting by humans is often cited as an extinction trigger, especially on islands, there was relatively little overlap in time between the Channel Islands mammoths and humans. Not to mention that no one has yet found unequivocal evidence of hunting or butchery. Extinction by overkill can’t be taken as a default explanation.

Swift ecological change is the other popular option. Rising sea levels could have put the squeeze on mammoth populations, and changing vegetation may have limited the beasts’ food supply. But if mammoths arrived on the islands over 80,000 years ago and maintained stable populations from that time on, Muhs and colleagues argue, then they would have previously experienced even more extreme shifts in sea level, climate, and vegetation. If they survived these changes in the past, why not at the end of the last Ice Age?

These two explanations aren't mutually exclusive. Perhaps both changing habitat and humans had roles to play. We often confuse the reason the last members of a species died for the singular extinction trigger. The truth may be that multiple causes made populations vulnerable to a particular stress which eventually eradicated any hope of recovery. Extinction is a process, not a singular event. How that process played out amongst California’s lost mammoths, however, is a secret still held tight in tusk and bone.

References:

Muhs, D., Simmons, K., Groves, L., McGeehin, J., Schumann, R., Agenbroad, L. 2015. Late Quaternary sea-level history and the antiquity of mammoths (Mammuthus exilis and Mammuthus columbi), Channel Islands National Park, California, USA. Quaternary Research. 83: 502-521. doi: 10.1016/j.yqres.2015.03.001

[This post was originally published at National Geographic.]