There’s no question that regular exercise is essential to health. For the vast majority of our evolutionary history, we’ve had to exert ourselves — often quite strenuously — to get food, find shelter and simply survive. We naturally spent a lot of time outdoors in the sun, walking, hunting, gathering, and performing various other physically-oriented tasks. We had no concept of this as “exercise” or “working out.” It was just life.
Things are different today. Most people in modern societies spend the majority of their time indoors, sitting on their butts (like you’re probably doing right now).