North America once hosted a spectacular diversity of megafauna, including giant sloths, giant condors, and giant beavers (can you spot the theme?). But perhaps the most iconic species of the Pleistocene continent was the American mastodon, an elephant-like animal weighing about five tons, which suddenly went extinct about 11,000 years ago. But what drove this species, which was otherwise incredibly successful, to drop off face of the planet?
A paper published today in the Proceedings of the National Academy of Sciences sheds new light on this ongoing paleontological mystery, and suggests that humans, at least, are off the hook for the mastodon's extinction.
"There was a provocative idea that came out back in the 1960s, called overkill, suggesting that the first people that came to North America across the Bering Land Bridge went on this rampaging killing spree," lead author Grant Zazula, a paleontologist at the Yukon Palaeontology Program, told me over the phone.
"When people first entered North America at the end of the Ice Age, it was a small population, pretty sparse groups of people," he continued. "To think of them just mowing everything down in front of them using spears is not a very satisfactory explanation."
Zazula and his colleagues, including American Museum of Natural History curator and paleontologist Ross MacPhee, were able to rule out humans by revisiting and updating the timeline of the mastodon's migration south from the Arctic.
They used two different methods of radiocarbon dating on 36 fossilized mastodon teeth, and discovered that the specimens were much older than originally estimated.
"In a nutshell, what we found that there isn't really a basis for thinking that mastodons lived anywhere further north than say, the Great Lakes, except when conditions were especially warm like they were in the last interglaciation [about 125,000 years ago]," MacPhee told me over the phone.
The results suggest that mastodons were long gone by the time the last full glacial period was in swing 75,000 years ago, contrary to previous research. Humans didn't make their way to North America until around 14,000 years ago, which means they could not have sparked the mastodon's mass exodus to the south.
That's not to say that humans didn't contribute to the animal's extinction, or to the decrease in megafauna more generally. But the species was already in deep trouble long before humans were on the scene—a finding that's backed up by genetic analysis.
"You can use genetic diversity as a proxy for population diversity," explained Zazula. "It's been done with a number of Ice Age species, and what's actually provocative about a lot of the data is that their populations are in steady decline leading up to the final extinction. These are populations that are going downhill anyway, and something happened at the end of the Ice Age that pushed them over the edge."
"What's causing that overall decline in diversity has to be something other than humans because for the most part, humans weren't around," he added.
So if humans didn't kill off the mastodons, then what did? That's the million dollar question. "There really isn't a good answer, and that's what makes the question interesting still," said Zazula, "because the answer isn't really clear and there's still lots to be learned."
Climate change definitely played a role by limiting the animal's range, but that doesn't explain the extremely sudden drop-off 11,000 years ago. "We know that these mammals went through numerous periods of rapid climate change in the past and our evidence shows that they were always able to pull through," said Zazula. "So what makes the Ice Age so different that they just weren't able to pull through?"
Some scientists have speculated that an extraterrestrial impact—from either an asteroid or a comet—was behind the swift disappearance of megafauna, but Zazula says the evidence for that is dodgy. Meanwhile, MacPhee has theorized that a pandemic may have raged through the mastodon population, effectively wiping them from the face of the Earth.
"Infectious diseases, particularly ones for which the species in question have no innate capacity to deal with—no genetically based immunity—could have caused essentially an instantaneous drop in population size, which is really what you need to do to provoke an extinction," he told me. There is not a lot of evidence to back that up at the moment, but the last decade has produced major advances in detecting pathogens in fossils, so the theory may yet bear fruit.
For the moment, however, the death of the mastodon remains one of paleontology's most alluring unsolved mysteries. "We're really missing something," said Zazula. "Something just doesn't make sense. It really defies all explanation right now."
But regardless of the reasons behind their beleaguered end, MacPhee thinks the species are entitled to more respect for what they did with their time on the planet.
"Mastodons never get good press and I never understood why," he told me. "They were very successful not only in the old world but also in the new world. They got over most of the habitable part of the world at one time or another."
Point taken. While it's great to hear that humans weren't responsible for edging out a species for once, the team's paper is also a welcome opportunity to reflect on how truly awesome mastodons were—in the original sense of the adjective.