A biggest-picture history tour, from the Big Bang to the KT extinction event that starved, suffocated, froze, and basically snuffed every four-legged animal heavier than 25 kilos (55 pounds) out of existence. Oh, except for two or three. Life, never easy, got a lot harder 5 different times in our planet's inhospitable past. So hard that it almost ceased to exist entirely once or twice. Graphic: "Oz" by Ian Armstrong