If a meteorite had not fallen on the earth sixty-five million years ago, we would not be where we are now; or more exactly we would not be here at all. If icebergs had not covered a third of the globe's surface three hundred million years ago, this collision some two hundred and thirty-five million years later would have been of no benefit to us. If drought had not swept over the Eurasian continent some ten million years ago, we would still be in the trees, and nothing would encourage us to come down from them.