A long time ago, sloths were widespread across America with quite a large family tree. They ranged in size from tiny, cat-sized sloths to massive ground sloths which could weigh as much as six tons. Now the only remaining sloth species are the two-toed and three-toed sloths, although two new studies shed light on more of the sloth family tree.
The studies have provided insight into how major groups of sloths are related to each other. For example, the results of a recent analysis suggest that the three-ted sloth is closely related to the ancient elephant-sized ground sloths, while the two-toed sloth is the last survivor of an ancient family which was previously thought to be extinct.
“The results are surprising on many levels,” co-author Graham Slater of the University of Chicago said in a statement about one of the studies. “Not only do they rewrite sloth classification, they suggest much of what we thought we knew about how sloths evolved may be wrong.”
The study co-authored by Slater was published in the journal Nature Ecology & Evolution. It followed a unique approach that analyzed proteins in fossils, which then revealed evolutionary relationships within the sloth family tree. That process enabled researchers to map a species’ entire lineage.
“All of these ancient sloths must have occupied really important roles in grazing and browsing the landscape, and so they’re important to understanding how these ecosystems worked, but getting a handle on their evolution has been difficult,” said Slater, who specializes in analyzing evolution patterns in mammals.
Researchers focused on proteins instead of DNA, which is a fragile molecule affected by certain conditions which require it to survive in a fossil. DNA is directly translated into proteins, so analyzing them seemed like a smarter idea. They used proteins to reconstruct the sequences of amino acids and then compared them one to another.
“What came out was just remarkable. It blew our minds–it’s so different from anything that’s ever been suggested,” Slater said.
Researchers thought the unau—the three-toed sloth with black lines around the eyes–was an outlier species that diverged during the early stages of the family’s evolution. However, the new evidence suggests it was nested in a large group of various ground sloths, which includes colossal elephant-sized sloths.
The protein evidence gave even more information about the sloth family tree. Researchers say the extinct Caribbean sloths were the descendants of an early branch that split from the sloth family tree 30 million years ago. This discovery has also sparked more research on whether there was a short-term land bridge which connected South America and the West Indies millions of years ago. Researchers believe curiosity and wanderlust could have driven the sloths to cross the land bridge.
“We’ve been used to thinking that today’s sloths each evolved independently for life in the trees from a ground-dwelling ancestor, but our results suggest that the ancestral sloth may have been at home in both,” Slater said.