Our ancestors for many millions of years ate a diet based on starchy foods, fruit and vegetables, with the occasional meal of protein to provide for growth and repair of tissue. They were rarely exposed to fat in the diet that wasn't closely linked to the plant or animal from which it came. As a result our appetite mechanisms evolved to work on starch meals or on protein meals to ensure the delicate balance between the food we need and the food we eat.
Our ancestors' digestive system evolved to work optimally with either the starch, fruit and vegetables that could be gathered each day, or the protein that was occasionally found as a nest of eggs, or caught. To this day our digestive system works most efficiently when protein and starch meals are kept separate. There have been too few generations of humans since our diets changed for our digestive systems to change to cope well with the mixtures that are now available.