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Food processors, dishwashers, vacuum cleaners, and clothes-washing machines have substancially lessened the physical activity required to cook and clean. Air conditioners and central heating have decreased how much energy our bodies spend to maintain a stable body temperature. Countless other devices, such as electric can openers, remote controls, electric razors and suitcases on wheels, have reduced, calorie by calorie, the amount of energy we expend to exist.
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Daniel E. Lieberman (The Story of the Human Body: Evolution, Health, and Disease)