A woman exercises in a gym in central London, Thursday, Feb. 25, 2010. Credit: Sang Tan/AP/File photo Exercise is a relatively recent phenomenon. After all, it’s difficult to imagine a caveman on a treadmill. And it’s safe to say that paleolithic humans never pumped iron. But something changed as we moved from the plow to the Peloton. Exercise — physical exertion for the purpose of improving health or fitness — became a huge part of modern life, and a nearly $100 billion global industry.