Brian Switek notes a new study on the locomotor dynamics of sloths. I perked up when reading this passage...
Superficial appearances to the contrary, two-toed and three-toed sloths are not very closely related to each other and last shared a common ancestor over 21 million years ago. Because both lineages became adapted in similar ways to living in trees, however, it is likely both types of sloths co-opted some of the anatomy of their ancestors to allow them to make that move into the trees.
...because it reflects an example of convergence to arboreal suspension. The Ardipithecus skeletal reconstruction raised the specter that many locomotor specializations in chimpanzees, gorillas and orangutans were not retentions from the great ape ancestor, they may instead have evolved convergently. Convergent evolution of arboreal locomotor adaptations may not be so unusual.