Modern diets fade in and out in a few short years -- think Atkins, South Beach or Jenny Craig -- but one of the best ways to lose weight and stay healthy may have existed since the dawn of man. Move over, NutriSystem ( NTRI): It's called the Paleolithic diet, and the theory goes that our digestive systems have had roughly two million years to adapt to a hunter-gatherer lifestyle. In the mere 10,000 years since agriculture took hold, our guts haven't had a chance to adapt well to grains, legumes, refined sugar or other trappings of civilization, advocates say. Eliminate these foods, and many diseases of modernity, including heart disease, diabetes and obesity, could disappear along with them. The idea gained medical recognition in 1985, when Drs. S. Boyd Eaton and Melvin Konner published "Paleolithic Nutrition: A Consideration of Its Nature and Current Implications," in the New England Journal of Medicine. It inspired Staffan Lindeberg of the University of Lund in Sweden, an advocate of Paleolithic-style eating, to continue to study evolutionary nutrition. To find evidence that humans should eat like cavemen, Lindeberg took a trip to the Trobriand Islands in Papua New Guinea during the late 1980s and again in the 1990s. There, according to his Web site, he found one of the last societies to follow the Paleolithic diet strictly.