The average American adult is now 23 pounds overweight. That’s according to Thomas Frieden, director of the Centers for Disease Control and Prevention, who spoke in July at a CDC conference on obesity in Washington, D.C. Obese Americans—those who have a body mass index of 30 or greater, a number calculated from an individual’s weight in relation to height—spend about 40 percent more on health care annually (nearly $1,500 per person) than people of normal weight. Obesity increases the risk of developing heart disease and stroke, type II diabetes, certain cancers, and other diseases.

The prevalence of obesity among Americans has more than doubled in the past 30 years; one in three adults is now deemed obese. The problem is not peculiar to the United States: Human beings as a species are the fattest they have ever been, Michael L. Power and Jay Schulkin report in The Evolution of Obesity. While obesity may lead to pathology, and even be viewed as pathology, its biological underpinnings likely helped enable human evolution, say Power, a nutrition and metabolism researcher at the Smithsonian National Zoological Park, and Schulkin, a behavioral neuroscientist with the American College of Obstetricians and Gynecologists. To thrive in a world where food often was scarce and hard to acquire, the earliest humans stored fat on their bodies. Since they rarely had enough to eat, they probably stayed lean.

“We evolved on the savannas of Africa; we now live in Candyland,” the authors declare. We eat out more than our parents did. We supersize: Twenty years ago, a typical bagel was three inches in diameter and had 140 calories; today’s bagel is six inches wide and contains 350 calories. We eat highly processed foods: The added sugars and fats they contain may activate reward circuits in the brain, stimulating further eating. These foods are often cheaper than healthful alternatives, such as fresh fruits and vegetables. The result is one of the great ironies of our time: Obesity often coexists with poverty and malnutrition.

Most jobs now involve little physical labor. Exercise has become a leisure-time pursuit that often requires special clothing and equipment, health club fees, and child-care arrangements. Our built environment also discourages physical activity, with transportation routes favoring cars over bicyclists or pedestrians. At the typical two-story shopping mall, it is a lot easier to find escalators than stairs.

Lack of sleep also may contribute to weight gain. In 1960, most adults in the United States reported that they slept eight or more hours each day. Recent surveys show that adults sleep less than seven hours on average, and nearly one in three reports habitually sleeping less than six hours. Laboratory studies show that sleep loss increases appetite, particularly for high-carbohydrate foods. It also raises levels of ghrelin, a hunger-stimulating hormone, and lowers levels of leptin, a hormone thought to help suppress hunger and regulate reproduction and development.

So why isn’t everyone fat? The answer is that humans vary. Even people with a genetic predisposition to obesity do not inevitably become obese. Power and Schulkin say that their aim in writing this book was to understand the how and why of human obesity, not to suggest how to prevent or “cure” it. Their scholarly review, with 51 pages of references, left me feeling overstuffed yet also hungry for solutions.

The CDC also has an appetite for answers, and after convening experts and reviewing hundreds of obesity reduction programs, in July it recommended 24 strategies by which communities can promote healthy eating and active living. Among them: make healthy foods and beverages available and affordable by, for example, boosting the number of full-service supermarkets and farmers’ markets in areas that lack them; encourage breastfeeding; require physical activity programs in schools; build more sidewalks; and encourage active outdoor recreation.

Is the war on fatness winnable? We all know we should watch what we eat and exercise more. Many of us intend to do that . . . starting tomorrow. Maybe we haven’t evolved quite enough.