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ONE of the iconic scientific images of our age is the saw-tooth curve that shows how global temperatures have changed over the last half-million years. Based on Antarctic ice cores, it reveals troughs of cold and peaks of warmth that succeeded each other in an approximately 100,000-year cycle, as ice sheets at high latitudes grew and then collapsed again. It was during those periodic ice ages that the genus Homo evolved in Africa. When modern humans dispersed over the globe from around 200,000 years ago on, that marked the beginning of the end for other types of human.…