[patrickmn]

In paleolithic times, the average human diet included a lot of fatty meats. When the fatty game went extinct, and protein-heavy diets failed (protein isn't a fuel source except through gluconeogenesis,) carbohydrate-heavy diets became more common. It's not clear that a carbohydrate-heavy diet is the ideal human diet, just one we're compatible with. The human body can make almost anything work, for a time.

At the same time, tribes in Papua New Guinea who eat almost exclusively carbohydrates have been shown to be virtually free of heart disease.

Zooko and Amber Wilcox-O'Hearn maintain a fascinating blog on the science of ketogenic (very low carb) diets that I can highly recommend: http://www.ketotic.org/