One of the bizarre and little-known extinct mammals that once roamed both South and North America, the giant ground sloth or Megatherium ("Great Beast") was as big as an elephant but with all the majesty and co-ordination of a double decker bus. It grew up to 6 metres long and had large, curved claws on its front and back feet, which would have forced it to waddle along on the sides of its back paws and its front knuckles. Modern tree sloths, their closest living relatives, are adapted to life in the trees to make up for their awkwardness and lack of physical speed, but the Megatherium's size made this impossible. Standing easily on their hind legs, on which they also walked much of the time, they browsed the leaves and foliage of any trees within reach.
They flourished between 2 million and eleven thousand years ago, with their extinction coinciding roughly, probably not by coincidence, with the influx of humans into North and South America from the Bering Strait. Megatherium's natural predators up to that point had been saber-toothed cats such as Smilodon, but it didn't stand much chance against organized groups of human hunters. It's easy to imagine the joy a hunting band would have felt at discovering an animal as big and fat as a woolly mammoth and about four times as slow. It had very tough hide which protected it from the cats most of the time, but wouldn't have been as much use against stone spears.
A controversial theory exists in relation to the Megatherium's diet, based on the apparent lack of sufficient vegetation in the areas of its habitat (which tended to be arid plains already heavily populated by herbivores). Richard Farina, a paleontologist from Uruguay, believe that the Megatherium was carnivorous, a view which he supports with other evidence, such as the animal's excessively large claws. Conventional wisdom holds that these were used to strip the bark off trees, but Farina's comparison of the Megatherium's forearm bones with those of modern-day animals strongly suggests that they were used for stabbing. One contradictory piece of evidence is its teeth, which are peg-like, a shape usually associated with plant-eating creatures, but Farina hypothesizes that the teeth were primitive, not well-suited either to plant- or flesh-eating, and that the Megatherium was in the process of making an evolutionary transition when it became extinct. His thesis has so far neither been proved or disproved.