Goswami and his colleagues calculated how sloth bodies would have changed over time. They found that every million years, members within some sloth family trees increased by more than 200 pounds. That's one of the quickest rates of morphological change in the history of mammalian evolution.
The key was looking back at old bones. "If we ignore the fossil record and limit our studies to living sloths, as previous studies have done, there's a good chance that we'll miss out on the real story and maybe underestimate the extraordinarily complex evolution that produced the species that inhabit our world," Goswami says.