Someone figured this out a long time agoThe metabolic defect in the Fatten-Easilies (their tendency to store carbohydrate as fat instead of promptly turning it into energy) is probably hereditary and may be regarded as a failure to make the adaptation to a diet based on agriculture which the Constant-Weights have achieved.
Before the cultivation of cereal crops, our ancestors, as has been said before, lived by hunting animals and subsisted on an all-meat diet of fat and protein. If they lived near woods or in forests, they may have taken a few berries, fruits and roots as well, but on the open plains they lived on meat alone and there is evidence that on this diet they were never corpulent.
With the gradual introduction of starch and sugar which followed the cultivation of the land, some people found that they could adapt themselves to the new foods and stay slim, while others must have failed to develop the biochemical mechanism for getting energy from carbohydrate and became fat instead.
These were the Fatten-Easilies.
Eat Fat And Grow Slim by Richard Mackarness (1958)