When earlier generations of obesity researchers discussed the storage of fat in humans and animals, they assumed that avoiding excessive fat is as important to the survival of any species as avoiding starvation. Since the average 150-pound man with a body fat percentage of only 10 percent is still carrying enough fat calories to survive one month or more of total starvation, it seems superfluous to carry around more if it might have negative consequences. “Survival of the species must have depended many times both on the ability to store adequate yet not excessive amounts of energy in the form of fat [my italics], and on the ability of being able to mobilize these stores always at a sufficient rate to meet the body’s needs,” observed George Cahill and Albert Renold, considered two of the leading authorities on the regulation of fat metabolism, in 1965. The total amount of fat stored, they suggested, “should be kept sufficiently large to allow for periods of fasting to which a given species in a given environment is customarily exposed, yet sufficiently small to preserve maximum mobility.”

The thrifty-gene hypothesis, on the other hand, implies that we (at least some of us) are evolutionarily adapted to survive extreme periods of famine, but assigns to humans the unique concession of having evolved in an environment in which excess fat accumulation would not be a burden or lead to untimely death—by inhibiting our ability to escape from predators or enemies, for instance, or our ability to hunt or perhaps even gather. It presupposes that we remain lean, or at least some of us do, only as long as we remain hungry or simply lack sufficient food to indulge our evolutionary drive to get fat—an explanation for leanness that the British metabolism researchers Nancy Rothwell and Michael Stock described in 1981 as “facile and unlikely,” a kind way of putting it. The “major objection” to the thrifty-genotype hypothesis, noted Rothwell and Stock, “must be based on the observation that most wild animals are in fact very lean” and that this leanness persists “even when adequate food is supplied,” just as we’ve seen in hunter-gatherers. If the thrifty-gene hypothesis were true of any species, it would suggest that all we had to do was put them in a cage with plentiful food available and they would fatten up and become diabetic, and this is simply not the case.

Proponents of the thrifty-gene hypothesis, however, will invoke a single laboratory model—the Israeli sand rat—to support the notion that at least some wild animals will get fat and diabetic if caged with sufficient food. “When this animal is removed from the sparse diet of its natural environment and given an abundant, high-calorie diet,” wrote Australian diabetologist Paul Zimmet in a 2001 article in Nature, “it develops all of the components of the metabolic syndrome, including diabetes and obesity.”