“Here’s another, startling way of thinking about these numbers: if you are a typical person who barely exercises, it would take you just an hour or two of walking per day to be as physically active as a hunter-gatherer. Even so, few Americans or Europeans currently manage to achieve those modest levels of activity. The average PAL of industrialized adults in the developed world is 1.67, and many sedentary individuals have even lower PALs.23 These declines, moreover, are relatively recent and largely reflect changes in how we work, especially the growth of desk jobs that glue us to our chairs. In 1960, about half of all jobs in the United States involved at least moderate levels of physical activity, but today less than 20 percent of jobs demand more than light levels of activity, an average reduction of at least a hundred calories per day.24 That modest amount of unspent energy adds up to twenty-six thousand fewer calories spent over the course of a year, enough to run about ten marathons. And outside our jobs, we walk less, drive more, and use countless energy-saving devices from shopping carts to elevators that whittle away, calorie by calorie, at how much physical activity we do. The problem, of course, is that physical activity helps slow aging and promotes fitness and health. So those of us who no longer engage in physical labor to survive must now weirdly choose to engage in unnecessary physical activity for the sake of health and fitness. In other words, exercise.”