It wasn’t until the late 1970s, just a few years before Neel himself publicly rejected his hypothesis, that obesity researchers began invoking thrifty genes as the reason why putting on weight seems so much easier than losing it. Jules Hirsch of Rockefeller University was among the first to do so, and his logic is noteworthy, because his primary goal was to establish that humans, like every other species of animal, had apparently evolved a homeostatic system to regulate weight, and one that would do so successfully against fluctuations in food availability. We eat during the day, and yet have to supply nutrients to our cells all night long, while we sleep, for example, so we must have evolved a fuel storage system that takes this into account. “To me, it would be most unthinkable if we did not have a complex, integrated system to assure that a fraction of what we eat is put aside and stored,” Hirsch wrote in 1977. To explain why these components might cause obesity so often in modern societies, he assumed as fact something that Neel had never considered more than speculation. “The biggest segment of man’s history is covered by times when food was scarce and was acquired in unpredictable amounts and by dint of tremendous caloric expenditure,” Hirsch suggested. “The long history of food scarcity and its persistence in much of the world could not have gone unnoticed by such an adaptive organism as man. Hoarding and caloric miserliness are built into our fabric.”

This was one of the first public statements of the notion that would evolve into the kind of unconditional proclamation made by Kelly Brownell a quarter century later, that the human body is an “exquisitely efficient calorie conservation machine.” But it depended now on an assumption about human evolution that was contradicted by the anthropologic evidence itself—that human history was dominated by what Jared Diamond had called the “conditions of unpredictably alternating feast and famine that characterized the traditional human lifestyle.” Reasonable as this may seem, we have no evidence that food was ever any harder to come by for humans than for any other organisms on the planet, at least not until our ancestors began radically reshaping their environment ten thousand years ago, with the invention of agriculture.

Both the anthropological remains and the eyewitness testimony of early European explorers suggest that much of the planet, prior to the last century or two, was a “paradise for hunting,” in the words of the Emory University anthropologist Melvin Konner and his collaborators, with a diversity of game, both large and small, “present in almost unimaginable numbers.”*72 Though famines have certainly been documented among hunter-gatherer populations more recently, there’s little reason to believe that this happened prior to the industrial revolution. Those isolated populations that managed to survive as hunter-gatherers well into the twentieth century, as the anthropologist Mark Nathan Cohen has written, were “conspicuously well-nourished in qualitative terms and at least adequately nourished in quantitative terms.”