Meat-hungry humanity has long been suspected of hunting North American megafauna to extinction, but new research suggests that Homo sapiens may have gotten a bum rap -- at least when it comes to the demise of the mighty American mastodon.
Research published Monday in the journal PNAS argues that the ancient beast started to disappear from what is now Alaska and Canada's Yukon Territory long before humans ever set foot on the continent.
One key reason for the confusion, authors say, is that specimen contamination has rendered decades of carbon-14 dating research potentially inaccurate.
"It's a concern," said Yukon paleontologist Grant Zazula, lead author of the study. "A lot of the radiocarbon dates that are out there in the literature are probably problematic."
Although it is still possible that intensive hunting by humans may have played a role in the extinction of Mammut americanum south of the continental ice sheets about 10,000 years ago, study authors say the mastodon's disappearance from the Arctic and subarctic was a matter of "overchill," and not overkill.
"There was a massive die-off of a good part of their population in the northern part of the continent around 75,000 years ago," Zazula said.
"We suspect that once the northern group died off, the species was already heading toward trouble," Zazula said. "What ultimately pushed them over the edge, though -- hunters picking off the last of them or climate change at the end of the ice age being just too much for them -- is an unanswered question. There isn't a smoking gun."
The mastodon belongs to the same Proboscidean family as the woolly mammoth, but the two species differ greatly in their diets and preferred habitat.
Shaggy mammoths grazed on the grasses and flowering plants commonly found on tree-less steppe tundras. The mastodon, however, preferred warmer mixed woodlands and lowland swamps where it munched on trees and shrubs, authors say.
The mastodon's favorite plants were likely abundant in the Arctic and subarctic between 125,000 and 75,000 years ago, when Earth was as warm as it is today.
However, when things began to cool during the last glacial period, the forests gradually disappeared -- along with the mastodon, authors argue.
Zazula, along with UC Irvine physicist and carbon-14 dating expert John Southon, initially set out to reconcile conflicting data regarding the mastodon's disappearance in the north.
Although vegetation reconstructions suggested that the animal's habitat had changed drastically 75,000 years ago, previous research suggested the mammoth was still roaming the Arctic and subarctic as recently as 18,000 years ago -- almost the height of the last ice age.
Zazula, Southon and their colleagues radiocarbon-dated 36 mastodon fossils but were careful to filter the samples to eliminate potential contamination.
Preservative varnishes and glues painted onto fossils by well-meaning curators, as well as soil acids that have interacted with remains over thousands of years, can confuse dating results and make specimens appear much younger, Zazula said.
After eliminating these sources of contamination, the researchers found that all of the high-latitude fossils were older than the limits of carbon-14 dating -- or more than 50,000 years old.
Zazula said that although the researchers were unable to directly date the fossils, it made the most ecological sense that they were at least 75,000 years old.
And though the disappearance of mastodons in the warmer south may have overlapped with the presence of humans, Zazula said there was still no clear evidence that they were hunted to extinction.
"There really isn't good evidence for hunting of mastodons," Zazula said. "There's mastodons at archaeological sites in the Midwest and in the northeast United States, but it's not like there's piles of dead mastodons everywhere with spear points and arrows in them. There's no evidence of slaughter, no evidence of overkill."