Caribbean extinctions: climate change probably wasn't the culprit.

Remains of extinct sloths unearthed in Cuba and Haiti indicate that the creatures persisted in Caribbean enclaves until about 4,200 years ago. Such a recent demise practically absolves post-ice age global warming as the cause of die-offs among these mammals and could undermine climate change as the trigger for extinctions throughout the Western Hemisphere since the last ice age ended some 10,000 years ago.

As many as 150 species of large mammals disappeared from North America and South America soon after the ice receded. The cause or causes for those extinctions have been debated for decades, says Ross D.E. MacPhee of the American Museum of Natural History in New York.

While some scientists blame widespread changes in ecosystems that occurred when the world's climate warmed several degrees, others suggest that the arrival and spread of humans in the hemisphere played a major role in the die-offs (SN: 12/4/99, p. 360). Because both phenomena were unfolding simultaneously, paleontologists have had difficulty identifying the true culprit, MacPhee notes.

Now, analyses of sloth fossils from continental landmasses and from Caribbean islands apparently disqualify climate change as a factor in the extinctions of those creatures, which ranged in weight from 20 kilograms to 200 kg. MacPhee and his colleagues carbon-dated the fossil dung and bones of extinct sloths. Before the last ice age, large and small sloths inhabited regions from northern Canada to the tip of South America.

Archaeological evidence suggests that people showed up in the Americas about 13,500 years ago, says MacPhee. The last members of several extinct sloth species, according to analyses of dozens of ancient dung samples, roamed the hemisphere about 10,200 years ago, he reported last week at a meeting in Mesa, Ariz., of the Society of Vertebrate Paleontology.

However, analyses of sloth bones from Cuba and Haiti indicate that the creatures survived much longer than their continental cousins did. Of nearly a dozen fossils of such Caribbean holdouts, the most recent come from a Haitian sloth that lived about 4,400 years ago and a Cuban counterpart that lived about 4,200 years ago. At these times, the climate in the region was stable.

The disappearance of the Caribbean sloths came about 1,300 years after the first reliable records of human presence on the islands, says MacPhee. Although there isn't any evidence that the sloths were hunted to extinction, the timing of their demise implicates human influence more than it does climate change.

The appearance of people and subsequent loss of the sloths is a "powerful correlation" that mirrors what's happened elsewhere, says Trevor H. Worthy, a paleontologist at the University of Adelaide in North Terrace, Australia. For example, people arrived on Madagascar about 2,200 years ago--also a time of worldwide climate stability--and within a millennium, the island's hippopotamuses, elephant birds, and many species of lemurs went extinct.

MacPhee and his colleagues are "building a strong case that humans [caused the extinctions] on the islands" says Alan B. Shabel of the University of California, Berkeley. Nevertheless, he notes, some paleontologists may find it hard to believe that people alone could have caused the post-ice age die-offs on continent-size landmasses.