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That was the condition of pretty much the entire human race when anatomically modern humans first arose, between 150,000 and 100,000 years ago, and things stayed that way until what some anthropologists have called humanity's worst mistake: the invention of agriculture. We now had a steady source of food, but there were downsides as well. For one thing, our ancestors began gathering in much larger population centers, where bacteria and viruses could fester. Small bands of hunter-gatherers can spread disease only so far, but the birth of cities made epidemics possible for the first time.
Nutritionally, the shift away from wild meat, fruits and vegetables to a diet mostly of cultivated grain robbed humans of many of the essential amino acids, vitamins and minerals they had thrived on. Average life span increased, thanks to the greater abundance of food, but average height diminished. Skeletons also began to show a jump in calcium deficiency, anemia, bad teeth and bacterial infections. Most meat that people ate came from domesticated animals, which have more fat than wild game. Livestock also supplied early pastoralists with milk products, which are full of artery-clogging butterfat. But obesity still wasn't a problem, because even with animals to help, physical exertion was built into just about everyone's life.
That remained the case practically up to the present. It's really only in the past 100 years that cars and other machinery have dramatically reduced the need for physical labor. And as exercise has vanished from everyday life, the technology of food production has become much more sophisticated. In the year 1700 Britain consumed 23,000 tons of sugar. That was about 7.5 lbs. of sugar per capita. The U.S. currently consumes more than 150 lbs. of sweetener per capita, nearly 50% of which is high-fructose corn syrup that is increasingly used as a sugar substitute. Farmers armed with powerful fertilizers and high-tech equipment are growing enormous quantities of corn and wheat, most of which is processed and refined to be tastier and more convenient but is less nutritious. They are raising vast herds of cattle whose meat is laden with the fat that makes it taste so good. They are producing milk, butter and cheese by the tankerload, again full of the fat that humans crave.
And thanks to mass production, all that food is relatively cheap. It's also absurdly convenient. In many areas of the U.S., if you had a craving for cookies a century ago, you had to fire up the woodstove and make the dough from scratch. If you wanted butter, you had to churn it. If you wanted a steak, you had to butcher the cow. Now you jump into the car and head for the nearest convenience store--or if that's too much effort, you pick up a phone or log on to the Internet and have the stuff delivered to your door.
Unless you make a determined effort, you'll probably choose the path of least resistance. Evolving during a time of scarcity, humans developed an instinctive desire for basic tastes--sweet, fat, salt--that they could never fully satisfy. As a result, says Rutgers University anthropologist Lionel Tiger, "we don't have a cut-off mechanism for eating. Our bodies tell us, 'Fat is good to eat but hard to get.'" The second half of that equation is no longer true, but the first remains a powerful drive.