Sloths once came in a variety of sizes and lived in multiple settings in many parts of the world. A study in the journal Science examined sloth evolution over the past 35 million years, investigated multiple factors driving their growth and expansion throughout the world, and concluded that human hunting starting around 15,000 years ago drove their dramatic decline.Today, only six species within two genera remain. All are relatively small (especially compared to their largest ancestors) tree-dwellers that primarily live in the tropical rainforests of South and Central America.“These species are a tiny remnant of a once diverse American clade that was mostly made up of large-bodied species,” according to an editorial summary that accompanied the paper. Ancient Sloths Were Once WidespreadThat’s a huge contrast to sloth life during the late Cenozoic. During that period, more than 100 genera of sloths lived in a wide range of habitats and a variety of sizes, topping out at nearly 20 feet tall and weighing several tons.To investigate this diversity — and to track where, when, and why it collapsed — a team of scientists examined fossil measurements, DNA and protein sequences, and advanced evolutionary modeling. In doing so, they reconstructed sloth evolutionary history across 67 genera. They then investigated whether evolutionary changes in size were linked to habitat, diet, climate, predation, or other ecological pressures.Habitat Drove Sloth SizeThe findings show that habitat appeared to be a major driver in shaping their body size evolution. The earliest sloths were large and grazed on the ground. Some species adapted to tree dwelling and developed smaller body sizes. However shifts in both sloth size and dwelling didn’t happen in a straight line. The species size grew or shrunk as the climate warmed and cooled, and as ecosystems shifted from grasslands to woodlands.The species thrived for tens of millions of years, exhibiting the most variety in body sizes in the Pleistocene, which began about 2.6 million years ago.Ancient Humans Caused Dramatic DeclineThen, starting about 15,000 years ago, the creature experienced “a sudden and dramatic decline,” according to a press release.The researchers report that decline doesn’t mesh with any major known climate events. "Size disparity increased during the late Cenozoic climatic cooling, but paleoclimatic changes do not explain the rapid extinction of ground sloths that started approximately 15,000 years ago,” according to the paper. However, it does coincide with the expansion of humans into the Americas. The likely conclusion is that human hunting drove the extinction of the larger, ground-based sloths, while the smaller ones related to today’s creatures escaped by taking to the trees.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.