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. But why do we spend so much time and money at the gym or on the track and does it actually help our well-being? And why is exercise, at least for some of us, such a miserable experience? Daniel Lieberman, professor of human evolutionary biology at Harvard University and author of the book “Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding,” has some fascinating answers.