Humans to Blame for Ice Age Extinctions, Study Says

Hillary Mayell
for National Geographic News
August 10, 2005

Humans are likely responsible for the extinction of Ice Age megafauna—large mammals like giant sloths, short-faced bears, mammoths, and saber-toothed cats—that occurred in the Americas around 11,000 years ago, a new study says.

Scientists have long debated whether giant pre-historic mammals disappeared because of climate change or because humans hunted them to extinction.

The mass extinctions coincided with both the end of the last Ice Age and the arrival of humans in the Americas around 11,000 years ago. This timing has made it difficult for scientists to isolate the cause of the species' disappearance.

But a study comparing the extinction of giant ground sloths in North and South America with the disappearance of their smaller relatives in West Indian islands has helped clear up the picture, scientists say.

The researchers say archaeological and fossil evidence strongly suggests that ancient hunters pushed the animals to extinction.

Giant ground sloths "cruised through" at least 22 major climate cycles as the continental ice sheets in North America advanced and retreated over the last two million years, said David Steadman, a paleobiologist at the University of Florida.

Steadman is a co-author of the new study published in the Proceedings of the National Academy of Sciences.

"The only thing that's different [at the end of the Ice Age] is the arrival of people," he said.

Giant Sloth: A Case Study

Until about 11,000 years ago, at least 19 different sloth species lived in North and South America in a variety of ecosystems. Only a few small, tree-dwelling sloth species survive today.

Steadman and his colleagues argue that if ecosystem shifts resulting from climate change caused the sloths' demise, then all extinctions—on both islands and the mainland—should have taken place at the same time, as the last Ice Age ended between 15,000 to 9,000 years ago.

Radiocarbon dates of bones, dung, and other tissue of extinct sloths place their last appearance in North America at around 11,000 years ago and at about 10,500 years ago in South America, Steadman says.

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