How We Grew So Big — page 2

Why is this happening? The obvious, almost trivial answer is that we eat too much high-calorie food and don't burn it off with enough exercise. Richer salaries have led to a richer diet; according to the U.N.'s Food and Agriculture Organization, meat consumption has more than doubled in China and India since the 1970s, while fat and sugar in the diet has also more than doubled since the 1980s. If only we could change those habits, the problem would go away. But clearly it isn't that easy. The growing diet and slimming industries in Asia have had no more success than their Western counterparts in checking the rise of obesity. Food and drug companies around the globe spend ever greater amounts of money trying to find a magic food or drug that will melt the weight away. Yet the world's collective waistline just keeps growing.

It's natural to try to find something to blame—fast-food joints or food manufacturers, or even ourselves for having too little willpower. But the ultimate reason for obesity may be rooted deep within our genes. Obedient to the inexorable laws of evolution, the human race adapted over millions of years to living in a world of scarcity, where it made biological sense to eat every good-tasting thing in sight when you could find it.

Although our physiology has stayed pretty much the same for the past 50,000 years or so, we have utterly transformed our environment. Over the past half-century especially, technology has largely removed physical exercise from day-to-day urban life. At the same time, it has filled store shelves with cheap, mass-produced, tasty food that is packed with calories. And technology has allowed advertisers to deliver constant, virtually irresistible messages that say "Eat this now" to everyone old enough to watch TV.

This artificial environment has increasingly spread from ground zero of the global fat crisis—the U.S.—to less developed countries. As Asian nations catch up economically and adopt more Western lifestyles, their problems with obesity have caught up, too. By contrast, among people who still live in conditions most like those of our distant Stone Age ancestors—such as the Maku or the Yanomami of Brazil—there is virtually no obesity at all.

And that's almost certainly the way it was during 99.9% of human evolution. For most of the 7 million years or so since we parted ways with chimps, life was very harsh—"poor, nasty, brutish and short," in Thomas Hobbes' memorable phrase. The average life expectancy was probably well under 30. But much of that dismal brevity could be chalked up to accidents, infections, traumatic childbirth and unfortunate encounters with saber-toothed cats and other such predators. If a Cro-Magnon, say, could get past these formidable obstacles, he might conceivably live into his 60s or even longer, with none of the obesity-related illnesses that plague modern humans.

Our earliest ancestors probably ate much as their cousins the apes did, foraging for fruits, shoots, nuts, tubers and other vegetation in the forests and savannas of Africa. Because most wild plants are relatively low in calories, it took constant work just to stay alive. Fruits, full of natural sugars like fructose and glucose, were an unusually concentrated source of energy, and the instinct to seek out and consume them evolved in many mammals long before humans ever arose. Fruit wasn't always available, but those who ate as much of it as possible when they could find it were more likely to survive and pass on their sweet tooth to their progeny.

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