But on the islands of Cuba and Hispaniolashared today by modern-day Haiti and the Dominican Republicsloths survived until about 4,400 years ago.
Their subsequent extinction coincided exactly with the arrival of the first humans on the islands, Steadman says.
- Early Australians to Blame for Mass Extinctions, Study Says
- Woolly Mammoth Resurrection, "Jurassic Park" Planned
- Climate Change Caused Extinction of Big Ice Age Mammals, Scientist Says
- Extinct Giant Deer Survived Ice Age, Study Says
- Ice Age Bison Decline Not Due to Hunting, Study Says
- Ice Age Marsupial Topped Three Tons, Scientists Say
"What [this study] shows us is that there's this great big suggestive pattern that we find: Wherever human beings first appear all around the world, these large mammals pretty quickly become extinct," said Gary Haynes, an archaeologist at the University of Nevada at Reno, who was not involved in the study.
"[Some] people will say that you have to [establish the cause of extinction] species by species, and I think they're probably right," Haynes added. "But the study does create a good model that might make us think that if it worked for one big animal it's probably what we'll find for other big animals."
Overkill Vs. Climate Change
Steadman and his colleagues argue that megafauna species on the American continents, having evolved in an environment without humans, may have been particularly vulnerable to the sudden appearance of big game hunters.
The 5,000-pound (2,300-kilogram) giant ground sloth is a case in point. In addition to having no fear of humans, it was the size of a modern-day elephant, it couldn't hide, and as it name implies, it moved very slowly.
"Walking up to a ground sloth and trying to spear it to death probably wasn't one of the most macho things they [early hunters] did," Steadman said. "Any hunter could outrun one."
But other scientists maintain that climate change was the driving force in Ice Age extinctions. They argue that the retreat of ice sheets from North America caused a major change in habitat that the giant mammals couldn't adapt to.
At the peak of the Ice Age around 20,000 years ago, the ice covered much of North America.
As the sheets melted between 18,000 and 8,000 years ago, warmer temperatures and shifting rainfall patterns forced plants and animals to move out of old habitats and into new ones.
Proponents of the climate-change theory add that there's little evidence that humans hunted anything other than mammoths. Yet species like wild horses, camels, and saber-toothed cats all went extinct at about the same time.
"There are no archaeological sites for species other than mammoths, and perhaps mastodons, where you find a spear sticking out of an animal, and everyone agrees that there is evidence of human hunting," Nevada's Haynes said.
"So the lack of kill sites doesn't bother me," he added. "There's a real lack of a 'smoking gun' implicating either climate change or human hunting, but that's true for every theory."
Climate change may have been a factor in pushing the animals to extinction, Steadman says, but it took humans to push them over the edge.
"Animals like the ground sloth, which had a poor ability to regulate body temperature, should have thrived in a warmer climate," he said.
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES