The Evolution of Obesity

1 Humanity on the Fat Track

21 ONE Humanity on the Fat Track Humanity is getting fat. Not everyone, but many of us in every quarter of the globe: men and women, young and old, rich and poor, from every race and ethnicity. There is a worldwide obesity epidemic that shows little sign of slowing, let alone reversing. What is amazing, and frightening, is how quickly this change in human body weight is occurring. Within a few generations the bell curve of human weight distribution has shifted and skewed toward greater weight. The median-weight person of today would have been considered to be significantly heavier than average only a short time ago. And this trend appears to be continuing. In the United States, the good news is that the proportion of the population that is underweight has decreased over the last 20 years. Morbidity and mortality due to hunger has mostly been eliminated. Unfortunately this has not translated into an increased proportion of the U.S. population being at a healthy weight. Instead, the proportions of overweight and obese people have increased—and continue to increase at an alarming rate. The proportion of the population considered extremely obese has proportionately increased the most (Freedman et al., 2002). The number of people considered extremely obese in the United States has more than tripled since 1960. Why is this happening? Or perhaps more to the point, why is it happening now? Why are so many human beings susceptible to sustained weight gain in the modern environment? There have been obese people throughout history, but for most of that time obesity has been rare and unusual. In the past, being fat was a mark of wealth and status in many 22 THE EVOLUTION OF OBESITY cultures (Figure 1.1). It was difficult to become obese. Nowadays, being thin is rare and unusual, and the mark of celebrity and wealth. The rapidity with which the incidence of obesity has increased worldwide suggests that genetic change on a population level is an unlikely cause. We haven’t suddenly become genetically more obesity prone. The underlying genetic and biological factors that are contributing to the large proportion of people that become obese in the modern environment likely FIGURE 1.1. The Tuscan general Alessandro del Borro painted by Charles Mellin (ca. 1635). This gentleman was successful and proud of it. He made sure his portrait displayed his stout build. Photo: Bildarchiv Preussischer Kulturbesitz /Art Resource, NY. HUMANITY ON THE FAT TRACK 23 have been extant in our species for a considerable time. Our evolved genome is interacting with our dramatically changed environment; for many of us the result is sustained weight gain. Although technological, economic, and cultural factors have created the circumstances allowing the increased incidence of overweight and obese people in the world, to understand why these factors lead to obesity requires understanding the underlying biology. And in our opinion, a complete understanding of our biology relevant to obesity requires a careful consideration of the evolutionary events and pressures that have shaped our adaptive responses to hunger, food, exertion, and energy stores that, in today’s world, may not be as appropriate as in the past. All living things carry the past with them. What they are depends on who their ancestors were. We don’t live like our ancestors did, but we carry their biology. The evolutionary past both enables and constrains. To understand ourselves now we need to examine our evolutionary past. People who follow “traditional ways of life” are becoming rare. Most people do not live like their ancestors did. There are, of course, many good things associated with this change: longer life spans and lower infant mortalities, for example. Let there be no mythology about the past. Traditional lifestyles represent successful strategies for the human species but can be brutally hard on individuals. We are not interested in making value judgments regarding lifestyles in this book. Rather, we are exploring the value of understanding the adaptations to past life in order to understand the challenges and consequences of modern circumstances. We carry biology that evolved to solve challenges of our ancestors 5,000 years, 50,000 years, 500,000 years, and even longer ago. We see the value of understanding this past in order to understand the present. Neither of us wishes to go back to living as our ancestors did. In this book we explore human biology as it pertains to...