Sloths once roamed the Americas, ranging from cat-sized animals that lived in trees all the way up to giant ground sloths. The only species we know today, however, are the two- and three-fingered sloths. They seem remarkably similar, but new molecular (DNA and protein) studies estimate they last shared a common ancestor more than 30 million years ago. The authors unexpectedly discovered that the three-fingered sloth is related to two giant ground sloths, the elephant-sized Megatherium and the pony-sized Megalonyx, which last roamed the Earth around 10,000 years ago. The results were published in the journal Current Biology and the journal Nature Ecology & Evolution.

Sloths are represented today by six living species, distributed in tropical forests and conventionally placed in two genera: Choloepus, the two-fingered sloths (two species), and Bradypus, the three-fingered sloths (four species).

However, the living species represent only a small fraction of the past diversity of sloths.

More than 100 genera of sloths are known, including the large-bodied species of the Pliocene and Pleistocene. This includes the giant ground sloth (Megatherium americanum) with an estimated body mass of more than 4,000 kg and Darwin’s ground sloth (Mylodon darwinii).

Scientists have long believed that modern tree-dwelling sloths evolved independently from ancient ground-dwelling ones.

But by using different tools, the researchers discovered the ancient sloths may have been at home on both land and in the trees.

“Mining the fossil record for these organic residues helps us write new histories about sloth evolution and biogeography,” said McMaster University’s Dr. Hendrik Poinar, co-lead author of the Current Biology paper.

Dr. Poinar and colleagues used state-of-the-art analysis of ancient DNA to sequence 10 extinct sloth mitogenomes, which included the iconic continental ground sloths Megatherium, Megalonyx, Mylodon and Nothrotheriops, the recently extinct Caribbean sloths Parocnus and Acratocnus, and all living tree sloths.

The results have forced to rethink the entire evolutionary relationships.

For example, the three-fingered sloth, has long been regarded as being so anatomically different from other sloths that it was classified on its own, as a completely separate, evolutionary lineage.

But both the mitochondrial and protein evidence suggest that this is incorrect, and that amazingly, the three-fingered sloth fits within Megatherioidea, a group that also includes the largest of all sloths, the elephant-sized ground sloth Megatherium.

Similarly, the two-fingered sloth, was found to belong to another major group of sloths called the mylodontoids, whereas it had previously been placed within a very different group called Megalonychidae, a family which also includes the extinct North American ground sloth Megalonyx as well as a number of island species that lived in the West Indies until a few thousand years ago.

“Our work demonstrates the power of ancient DNA to decipher evolutionary radiations whose species have been decimated by the latest megafaunal extinctions,” said University of Montpellier’s Dr. Frederic Delsulc, first author of the Current Biology paper.

“This study reveals how little we know about morphological evolution and the new molecular phylogenetic framework provides an opportunity to better understand the underlying processes.”

The combined molecular evidence suggests that the West Indian sloths diverged from the ancestor of both megatherioids and mylodontoids more than 30 million years ago and this means that the first sloths to reach the Greater Antilles or what we know today as the Caribbean islands may have entered them over a temporary land connection between these islands and South America.

In taxonomic terms, that makes these recently extinct species the closest relative or sister group of all other sloths (giant and small), whereas they had previously been regarded as a late-evolving group.

“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 University of Chicago’s Dr. Graham Slater, co-author of the Nature Ecology & Evolution paper.

“Although the molecular results conflict with current paleontological views based on anatomical features, there can be only one history of life,” said American Museum of Natural History researcher Dr. Ross MacPhee, senior author of the Nature Ecology & Evolution paper.

“The job now is to reconcile these differing methods of inference, which means a lot more work on everyone’s part. We are going to learn a lot, and that’s exciting.”

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Frédéric Delsuc et al. Ancient Mitogenomes Reveal the Evolutionary History and Biogeography of Sloths. Current Biology, published online June 6, 2019; doi: 10.1016/j.cub.2019.05.043

Samantha Presslee et al. Palaeoproteomics resolves sloth relationships. Nature Ecology & Evolution, published online June 6, 2019; doi: 10.1038/s41559-019-0909-z