Around 11,000 years ago, saber tooth cats, woolly mammoths, giant ground sloths, and almost every other large mammal in North America went extinct. Scientists have been trying to solve this prehistoric whodunit for decades, and now, thanks to new radiocarbon dating, they're one step closer to fingering human hunters as the culprit.
The idea that humans caused the large mammal extinctions in North America has gained steam in recent years. The most incriminating evidence is radiocarbon dating of tools and spear points that puts humans in the right place at the right time--with weapons. But clouding the issue is the environmental upheaval that accompanied the end of the last ice age, and a few vocal scientists continue to argue that climate change was to blame. Untangling the two potential causes has proven difficult, and a dearth of archeological "kill sites" with man-made weapons and mammal bones together has forced scientists to rely on circumstantial evidence.
In the new study, biologist David Steadman of the Florida Museum of Natural History at the University of Florida in Gainesville, Paul Martin of the University of Arizona in Tucson, and colleagues studied fossilized sloth bones and dung from extinct species in North America, South America, and several West Indian islands. Using carbon dating, they found that while large sloths on the North American continent died off around 11,000 years ago, sloths in South America survived until 10,500 years ago, and some on the West Indian islands lived until 4400 years ago. Although global warming could have played a part in the North American extinctions, climate was relatively stable during the final years of the island dwelling sloths. And in each case, the sloths disappeared soon after humans first arrived, the team reports online this week in Proceedings of the National Academy of Sciences. Analysis of the sloth dung also revealed that the animals were eating plants that survive today, ruling out major vegetation changes as a factor.
The slow, ground-dwelling sloths had no previous experience with human predators and thus would have been easy prey for prehistoric hunters, Steadman argues. The few surviving sloth species are small, tree-dwelling sloths, he says, which are much harder to spot.
"Given the timing of the extinctions, the authors are right that they clearly were not caused by climate change," says paleobiologist John Alroy of the University of California, Santa Barbara, whose own computer modeling research supports the idea that humans could have hunted the mammals to extinction without realizing they were doing it.