"You may think exercise is normal, but it’s a very modern behaviour. Instead, for millions of years, humans were physically active for only two reasons..."

"... when it was necessary or rewarding. Necessary physical activities included getting food and doing other things to survive. Rewarding activities included playing, dancing or training to have fun or to develop skills. But no one in the stone age ever went for a five-mile jog to stave off decrepitude, or lifted weights whose sole purpose was to be lifted." 

From "Just don’t do it: 10 exercise myths/We all believe we should exercise more. So why is it so hard to keep it up? Daniel E Lieberman, Harvard professor of evolutionary biology, explodes the most common and unhelpful workout myths" (The Guardian).