A sloth at rest

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Ancient molecules reveal surprising details on origins of ‘bizarre’ sloths

From elephant-size animals that browsed North American grasslands to moose-size swimmers that plied the Pacific coast of South America, sloths have roamed Earth for more than 50 million years. Yet scientists know little about how the dozens of known species are related to each other. Now, two new analyses of ancient sloth DNA and proteins—some of which are more than 100,000 years old—are rewriting the sloth family tree. The studies even suggest a land bridge connected the West Indies with South America 30 million years ago, allowing the slow-moving animals to reach the islands.

“It’s a remarkable achievement,” says Timothy Gaudin, a paleontologist at the University of Tennessee in Chattanooga, who was not involved in the work.

Of the more than 100 sloth species identified, all but six are extinct. So scientists have had to compare the shapes of fossil bones to piece together how the animals evolved. Such comparisons are not clear-cut, however, and new techniques for isolating DNA and proteins from fossils have made it possible to compare the genetics of long-extinct animals. Ancient DNA allows scientists to compare genes directly, but proteins last longer. So although they provide less precise information, paleontologists are increasingly using them to study even older fossils.

In one of the new studies, paleoprotein expert Samantha Presslee of the University of York in the United Kingdom and her colleagues sampled more than 100 sloth fossils from across North and South America for traces of collagen. This protein is prevalent in bones, and can stick around for more than 1 million years. In 17 samples the researchers analyzed, the collagen was preserved well enough that they were able to piece together the amino acid sequences that form the building blocks of proteins. That allowed them to compare the various collagens—one of which was more than 130,000 years old—and build likely family trees, which they describe today in Nature Ecology & Evolution.

Genetic analysis suggests today’s three-toed sloths (top) are related to the giant ground sloths Megatherium (right) and Megalonyx (center), whereas modern two-toed sloths (upper right) are cousins of the South American Mylodon (left).

Jorge Blanco

Working independently, evolutionary biologist Frédéric Delsuc of the University of Montpellier in France and colleagues analyzed nearly full mitochondrial DNA sequences—the genetic material found in a cell’s energy-producing machinery—from 10 sloth fossils, ranging in age from 10,000 to 45,000 years old. They, too, used the data to draw likely sloth family trees, which the group describes today in Current Biology.

The two teams came to strikingly similar conclusions: Today’s three-toed sloths don’t form their own branch on the tree as previously thought, but are related to the giant ground sloth, Megalonyx, which lived in North America until about 15,000 years ago. And today’s two-toed sloths are distant cousins of the giant South American Mylodon, believed to be the last ground sloth to go extinct, less than 10,000 years ago.

Perhaps most surprising, the wide variety of now-extinct sloths that lived on the islands of the West Indies until about 5000 years ago all seem to have evolved from a common ancestor that lived about 30 million years ago. “Nobody had ever suggested that,” Gaudin says. That means a single population of sloths likely reached the islands just once. That fits with a theory that, instead of swimming or drifting, many animals reached the islands by walking over a land bridge that appeared about 30 million years ago and later was submerged.

“The fact that the [two studies] agree with one another is really interesting,” Gaudin says. But, he cautions, the analysis only includes a fraction of the known species. “There are loads of different extinct sloths that we could add to the tree,” Presslee says. “That’s the next step.”

Combining data from fossil shapes with the genetic data could produce even better trees, says Gerardo De Iuliis, a paleontologist at the University of Toronto in Canada. That might reveal how certain sloth traits—like the long, powerful forearms that allow today’s sloths to move while hanging from branches—arose independently multiple times. “They are bizarre animals that are bizarre in similar ways,” Gaudin says.