Rather, they developed a different kind of defense against famine, called thrifty genes.

Simply put, ketosis is evolution's answer to the thrifty gene.

Whether humans will continue to evolve and rid themselves of the thrifty gene remains to be seen.

This idea is controversial and has been criticised by others that support the original thrifty gene hypothesis.

The thrifty gene hypothesis is an example of a nutrigenetic factor in obesity.

Researchers believe these people possessed what's known as a "thrifty gene," which helped their bodies store fat for endurance.

Fatter individuals carrying the thrifty genes would thus better survive times of food scarcity.

However, the potential "thrifty genes" that may be affected by nutritional factors have yet to be identified.

Researchers call this trait "the thrifty gene."

If that thrifty gene is present, though, the body holds onto that food as fat, even if the person has no risk of starving.

Moreover, there is some confusion among proponents of the thrifty gene hypothesis about how long famines have played a role in evolution.

Hence the name "drifty" genes, to contrast the positively selected "thrifty genes".

The thrifty gene hypothesis postulates that due to dietary scarcity during human evolution people are prone to obesity.

And because some of us are obviously predisposed to gain weight while others are not, this explanation also has a genetic component - the thrifty gene.

As for the thrifty gene, it provides the kind of evolutionary rationale for human behavior that scientists find comforting but that simply cannot be tested.

However, if selection has only been acting for the past 15,000 years then there has been insufficient time for thrifty genes to spread at all.

The "thrifty gene" is still in place, Dr. Lowenfels said, but today the stored calories show up as cholesterol and gallstones.

Thrifty genes are genes which enable individuals to efficiently collect and process food to deposit fat during periods of food abundance.

The thrifty phenotype hypothesis arose from challenges posed to the thrifty gene hypothesis.

"Humans have a thrifty gene that kept them from starving in adversity," said Dr. Blackburn of Harvard.

Speakman's critique of the thrifty gene hypothesis is based on an analysis of the pattern and level of mortality during famines.

The thrifty gene theoretically causes bearers to store high-calorie foods as body fat, a most likely as an evolved protection against starvation during famines.

Personal responsibility is all to the good, but everything else in "Fat Land" suggests it is probably no match for the thrifty gene and the Happy Meal.

Another alternative to the thrifty gene hypothesis is the drifty gene hypothesis proposed by the British biologist John Speakman.

Speakman is also well known for his work in obesity, in particular for criticising a long-established theory for obesity known as the thrifty gene hypothesis.