Two groups of sloths are left on Earth now — two- and three-toed sloths — but there are more that have existed in the Americas throughout history.

Some are small creatures that lived up in the trees, while others are lumbering giants on the ground that are believed to weigh as much as six tons.

Classifying all these sloths has been a challenge for decades. Now, findings from two different papers reveal that some of the longstanding consensus on the sloth family tree may not be accurate.

In both papers, the researchers concluded that the three-toed sloth isn't an outlier that diverged from the family tree. Instead, it's part of a branch that includes the elephant-sized ground sloths Megalonyx, which lived in North America until around 15,000 years ago.

On the other hand, the two-toed sloth appears to be the last surviving species of a lineage that was previously believed to be extinct.

Using Proteins To Solve The Mysteries Of Sloths

In the first paper published in the journal Nature Ecology and Evolution on Thursday, June 6, researchers conducted an analysis of proteins in fossils, which is an emerging field known as paleoproteomics.

While DNA are delicate with a need for specific conditions to survive, proteins are sturdier with much of the same information as DNA. So, the researchers extracted collagen from various fossils and compared them with each other in an attempt to piece together the relationships of the different sloth species.

Graham Slater, an assistant professor of geophysical sciences at the University of Chicago who co-authored the paper, explained in a statement that the findings not only rewrite the classifications of two- and three-toed sloths, but they also suggest that many of the widely accepted beliefs about sloth evolution may be wrong.

The study discovered that many extinct sloths living in West Indies until 5,000 years ago were descendants of an early branch that diverged from other sloths about 30 million years ago. This means that there was likely a land bridge connecting South America and the West Indies, which a group of sloths crossed many millions of years ago before the bridge was submerged.

Additionally, the technique the team used in studying protein opens up endless potential in exploring ancient species.

"The very oldest DNA you can get is 800,000 years old, but in theory we should be able to get protein data from specimens that are millions of years old," Slater said. "A whole bunch of questions suddenly come into reach. It opens doors that we were only dreaming of."

Second Study Uses Mitochondria

The second study, which was done independently from the first, focused on the analysis of almost full mitochondrial DNA sequences that were pulled from 10 sloth fossils. These fossils ranged from 10,000 to 45,000 years old.

The study published in the journal Current Biology collected the data from the genetic material and pieced it together to create a cohesive sloth family tree.

Incredibly, the team led by evolutionary biologist Frédéric Delsuc of the University of Montpellier in France reached the same conclusions as Slater's team.