I don't think it's a case of the species changing, and if you don't believe species change (which is really a separate argument) I'm curious why you think a sudden emergent epidemic of sugar addiction accounts for widespread obesity. I think surroundings, at least in the industrialized world, have changed a lot; in particular, generations of habit has conditioned Americans to subsist on "farmers diets" grossly inappropriate when most of them no longer rise before dawn and begin hard physical labor that continues past sunset. Where else in the world, except possibly Europe, would the Atkins diet even be POSSIBLE, let alone popular? Imagine telling people in Sudan or India, "Stop eating grains and live off meat, even though it takes many times as much land to get the same caloric intake from the latter as the former".

Additionally, what makes stay at home moms less common also makes cheap fast food more popular; people working longer hours for less pay don't have time to shop for expensive raw ingredients and prepare their own food. Subsidies to the remaining (mostly corporate agribusiness) farms, especially for the corn from which high fructose corn syrup comes, ensures the healthy food IS often the most expensive.

The flip side of people buying unhealthy fast food to save time and money is people with enough disposable income to buy and consume a lot more food than they need. In this respect, the slow rate at which species change works against us, because animals have been physically as well as psychologically conditioned for centuries not to miss a cheap meal since another may not soon be forthcoming. Cheap plentiful food naturally encourages overconsumption.

All of these factors have long been well documented; I'm actually quoting some of the knowledge on American nutrition that's supposedly so lacking. It's not that people know no better or can't control themselves, it's a combination of habit, convenience, indulgence and the greater availability of cheap unhealthy foods over healthy ones.