Of nature's many guises, winter at the Arctic Circle would have to be one of the least subtle. It's hard to imagine that humans would have survived generations of frigid climate without some adaptation giving them a way to cope. Scientists have in fact postulated a "thrifty genotype" that some humans acquired 30,000 or so years ago during their migration from Asia, across a land bridge at what's now the Bering Strait, to North America. These genes may have given cold warriors an ability to store fat and metabolize it sparingly, a handy trait for the dark, cold months when food is scarce.

Now that the land bridge is long gone, the descendants of these first North Americans are stuck with genes optimized for life in the Ice Age. The same traits that allowed their ancestors to thrive in the Arctic wilderness may be making them uniquely vulnerable to the high-fat, high-cholesterol, sedentary American lifestyle. Members of the Pima tribe of Arizona, for instance, suffer one of the world's highest rates of diabetes--50 percent among adults over the age of 35--and 95 percent of the diabetes sufferers are overweight. The problem with evolution, as the Pimas know firsthand, is that it can't keep pace with the modern world.

What makes the Pima tribe so interesting is that they're like the rest of us, only more so. Asians are thought to possess many of the Pimas' thrifty-genome traits, which may explain why the number of obese Chinese doubled between 1992 and 2002 to 60 million, according to China's Health Ministry. Some Mediterraneans and Africans may not have acquired the thrifty genes of Arctic peoples, but their hunting-and-gathering ancestors didn't leave them a whole lot better equipped. Half of Brazil is now overweight, and one in eight is obese. In France and Italy, about one in three is overweight, and the proportion is rising. All told, about 1.2 billion people in the world are fat, and another 350 million are obese. Obesity-related illnesses, such as heart disease and diabetes, are rising.

Scientists are beginning to appreciate the variations in how different people respond to diet. For most people--particularly Asians--eating food rich in saturated fats will generally increase the level of "bad" cholesterol and decrease the "good" cholesterol. "When [Asians] move from their traditional environment to the West"--or when they start eating at their local McDonald's in Tokyo or Beijing--"they immediately get into trouble with obesity and heart disease--more than Caucasians," says Jose Ordovas, director of the Nutrition and Genomics Laboratory at Tufts University. By the same token, Northern Europeans and Celts, and some Mediterranean populations, tend to have the same cholesterol levels no matter what they eat--the work of a gene inherited from Viking ancestors.

A person's vulnerability to the diseases associated with obesity depends not just on diet but on his level of activity as well. And there's some evidence that activity is a product of biology as well as culture. In a paper published last week in the journal Science, Dr. James Levine, a nutritionist and endocrinologist at the Mayo Clinic in Rochester, Minnesota, reports that a genetic predisposition to obesity may turn on how much a person fidgets. Levine outfitted 10 lean people and 10 overweight people with special underwear that measured and tracked their body postures. (All the participants described themselves as "couch potatoes," he says, and were instructed not to undertake any exercise during the 10-day study.) In the end, the fitter subjects turned out to expend 350 calories a day more than the fatter ones on average--the equivalent of a weight gain of 30 or 40 pounds in a year--merely by getting up and moving around more. To rule out the possibility that lean people fidgeted more because they happened to be lean, Levine then had them gain weight by eating more. When measured, they still fidgeted just as much. Likewise, the fat people didn't fidget more when they gained weight.

Earlier studies suggest a neurochemical basis for the propensity to fidget. For instance, rats injected with the neuropeptide orexin began to "run around their cages like crazy," says Levine. But scientists are only beginning to get a handle on the problem. There's a possibility that fat in the body slows down the metabolism over the long term--a dynamic that Levine's short-term study would have missed. What's certain is that thrifty genes work in more complex ways than scientists appreciate at present. "There's a profound interplay going on between the amount of energy people take in and their level of activity," he says.

The question scientists would ultimately like to answer is how to compensate for the obsolete genes we've inherited from our primitive ancestors. Identifying the hundreds of genes involved--let alone figuring out how to neutralize their ill effects--won't be easy. Undoing thousands of years of evolution never is.