Changing climate probably played the decisive role in drawing hominids out of the trees, up on their hind legs, and off in search of food whose supply had been dispersed by the replacement of rainforests by grasslands. Migrations were motivated by the search for food; during ice ages, when sea levels dropped, new areas became accessible and populations spread. Links between climatic changes and emerging civilizations have also been postulated; the collapse of civilizations confronted with climatic challenges to which they could not or would not adapt is clearly documented: the Indus River civilization, the Viking settlement in Greenland, even the French Revolution. And Hitler’s disastrous decision to invade Russia in 1941 was apparently based on inaccurate weather forecasting! (26 minutes)