S1, E1: Professor Alice Roberts begins her Ice Age odyssey in North America, the hunting ground of the saber-toothed cat. The continent was half covered by ice that was up to two miles thick. Yet this frozen land also boasted the most impressive cast of Ice Age giants in the world, such as the grizzly bear-sized Shasta ground sloth and the glyptodon, surely the weirdest mammal that ever lived.