Sloths once roamed the Americas, ranging from tiny, cat-sized animals that lived in trees all the way up to massive ground sloths that may have weighed up to six tons. The only species we know and love today, however, are the two-toed and three-toed sloths -- but paleontologists have been arguing how to classify them, and their ancestors, for decades. A pair of recently published studies have shaken up the sloth family tree, overturning a longstanding consensus on how the major groups of sloths are related. According to the results, the three-toed sloth is more closely related to a large family that included ancient elephant-sized ground sloths; meanwhile, the two-toed sloth appears to be the last survivor of an ancient lineage previously thought extinct. The study uses a pioneering approach that uses proteins in fossils to discover evolutionary relationships -- marking the first time an entire lineage has been mapped with the method.
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