[EvaH (OP)]

Dan wants to know:

Recently one of your contributors suggested that defective genes play a part in causing obesity. This seems like a very 21st century-centric view. Wouldn't these genes actually be of benefit to our prehistoric and primate ancestors whereas 'skinny genes' would be a detriment? It could be that the obesity causing genes are the natural ones and the 'skinny genes' are the defective ones.

What do you think?

[evan_au]

The tendency of some Pacific Islanders towards obesity when fed an abundant "western" diet probably has a genetic component.

Their ancestors traveled into the unknown on journeys of thousands of kilometres without GPS; the person who was still alive when they reached land is likely to be the one who had greater stores of body fat when they left.

The same could be said for hunter-gatherer societies where there were long and irregular gaps between big kills (and no refrigeration). You pack on fat when you can, to store energy for the future.

So, yes, there is likely some evolutionary advantage to occasional obesity.
But chronic obesity is a disadvantage - it's better to keep the excess calories in the freezer, rather than carrying them around with you everywhere you go.

[Colin2B]

The same could be said for hunter-gatherer societies where there were long and irregular gaps between big kills (and no refrigeration). You pack on fat when you can, to store energy for the future.

So, yes, there is likely some evolutionary advantage to occasional obesity.
But chronic obesity is a disadvantage - it's better to keep the excess calories in the freezer, rather than carrying them around with you everywhere you go.

I think this would be the main advantage, the ability to store fat quickly and efficiently, it becomes less important with the growth of of agriculture and food storage and so alternative, thin genes can survive.
Interestingly, people who go on diets (because they are overweight) find the return to normal diet difficult. It’s as if the body thinks it has experienced a famine and tries to set the fat store level higher than before the diet.
Although obesity is a disadvantage you have usually gone through prime breeding age before it gets you.

[Bored chemist]

it's better to keep the excess calories in the freezer, rather than carrying them around with you everywhere you go.

True, but, since your (great)  grandparents  didn't have a freezer, evolution wasn't looking at that choice.
It was looking at the choice between carrying those spare calories around, or losing them forever.

Evolution picked the better of the two options that were actually available to it.