When we are young, physical activity prompts us to develop capacities like strong bones and improved memory; as we grow older, physical activity triggers many key maintenance and repair mechanisms that help us stay vigorous into middle and old age. And so, for countless generations, our ancestors rested as much as possible but also spent many hours a day walking, carrying, and digging, and occasionally they also ran, climbed, threw, danced, and fought. Their lives were challenging, and plenty of them died young, but physical activity helped many of those who survived childhood to become
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