The thrifty gene could be the answer only if diabetes was of long duration in the species—and there is no evidence of that. The disease seems to appear only after populations have access to sugar and other refined carbohydrates. In the Pima, diabetes appeared to be “a relatively recent phenomenon,” as Neel himself later noted. When Russell and Hrdlika discussed the health of the Pima in the early 1900s, they made no mention of diabetes, even while noting the presence of such “rare” diseases as lupus, epilepsy, and elephantiasis.*71 As late as 1940, when Elliott Joslin reviewed the medical records of the hospitals and physicians in Arizona, he concluded that the prevalence of diabetes was no higher among the Pima and other local tribes than anywhere else in the United States. Only in the 1950s, in studies from the Bureau of Indian Affairs, was there compelling reason to believe that diabetes had become common. When Neel tested adolescent Yanomamo for the condition known as glucose intolerance, which might indicate a predisposition to diabetes, he found none, so had no reason to believe that diabetes existed before such isolated populations began eating Western foods. The same was true of an isolated tribe of Pima, discovered living in the Sierra Madre Mountains of northern Mexico. “The high frequency of [Type 2 diabetes] in reservation Amerindians,” Neel later explained, “must predominantly reflect lifestyle changes.”

By 1982, Neel had come to side with Peter Cleave in believing that the most likely explanation for the high rates of obesity and diabetes in populations like the Pima that had only recently become Westernized was their opportunity to “overindulge in high sugar content foods.”

This left open the question of what biological factors or genes might determine who got obese and diabetic and who didn’t in the presence of such foods, but it eliminated any reason to suggest that thrifty genes had ever bestowed some evolutionary advantage. “The data on which that (rather soft) hypothesis was based has now largely collapsed,” Neel observed. He now suggested that either a tendency for the pancreas to oversecrete insulin and so cause hyperinsulinemia, or a tendency toward insulin resistance, which in turn would result in hyperinsulinemia, was the problem, which is consistent with the carbohydrate hypothesis of chronic disease. Both of these, Neel suggested, would be triggered by the “composition of the diet, and more specifically the use of highly refined carbohydrates.”