Most early mammals ate insects and worms. Carnivores or meat-eaters first lived about 35 million years ago. The largest carnivore ever lived 20 million years ago. Megistotherium was bigger than a grizzly bear. It attacked and killed huge elephants. One of the fiercest carnivores was Smilodon, the sabre-toothed tiger. It had long pointed teeth for tearing meat apart.