Hunter-gatherers had to live in small groups and move around because "once you've caught all the bunnies you have to go someplace else and catch bunnies there," he said. "So we evolved to function in small groups that moved about the landscape to make a living hunting and gathering."
"As a consequence some of our predilections are actually detrimental. We really like the taste of fat and salt because those were rare and valuable for our ancestors but they kill us today because we have them in abundance."
Obesity, he says, is the simplest example of how some of our evolutionary traits have come back to haunt us. Our ancestors didn't live in a world in which they could "drive up and supersize," he said.
If you are a hunter-gatherer, he says, "you have to go out and catch that mammal and that's a lot of work. That means we are designed for a world in which you have to expend calories in order to get calories."
Sometimes it must have been really difficult.
"If you have to work hard to get calories and there are times when calories are extremely scarce, then individuals who have a set of preferences that lead them to maximize the correct food intake when food is available will have done better," he said. "They had fat reserves when times were lean that the other folks didn't have. So we are descended from the people who liked the taste of fat. The ones who didn't like the taste of fat didn't make it."
Thus we are evolutionarily predisposed to pig out.
Fat didn't kill our ancestors in their challenging environment, he says, and today we have no "mechanism that says you live in a world where food is cheap and easy to come by, there's more fat than you can use, so don't like it anymore. That's not the way it works."
The consequence can be deadly.
"Very soon dietary causes of death will overtake all other preventable causes of death in the United States," Fessler said. "Right now smoking still leads by a little, but probably within the next couple of years the rates of death due to dietary-related behavior, primarily obesity, will overtake smoking."
"So we're dying at an enormous rate basically from having the dietary preferences of ancestors who lived in a world where there was no McDonald's."